Earth 2020: An Insider’s Guide to a Rapidly Changing Planet

Wri� en by world-leading thinkers on the front-lines of global change research and policy, this mul� -disciplinary collec� on maintains a dual focus: some essays inves� gate specifi c facets of the physical Earth system, while others explore the social, legal and poli� cal dimensions shaping the human environmental footprint. In doing so, the essays collec� vely highlight the urgent need for collabora� on and diverse exper� se in addressing one of the most signifi cant environmental challenges facing us today.

Kopp) and extreme weather (Neville Nicholls). And as these impacts become ever clearer, there is increasing discussion of potential geoengineering to limit the worst potential consequences, as discussed in one essay (Douglas G. MacMartin and Katharine L. Ricke).
These technological approaches represent a case of fighting fire with fire, but perhaps there are other ways to imagine the problem and its potential solutions. In this respect, long-held wisdom of Indigenous knowledge systems (Deborah McGregor) has much to teach us. At Introduction 7 planet look like in 2070, and how will our current understanding of Earth's trajectory map onto the reality that unfolds over the next half-century? Few of the authors in Earth 2020 will be able to answer this question; many are at, or approaching, the end of their careers, and few will even be around to see 2070. Nor will they be the ones most burdened by the environmental consequences of our collective actions over the past fifty years. For this reason, the last word must be given to our newest generation of leaders (Zoe Craig-Sparrow and Grace Nosek), those who have stepped up to demand systemic change, and who will drive the way, with our support and encouragement, to a better future. A s we look to the uncertain future ahead, it is clear that our path forward will not resemble the road we have traveled to get here. As the essays in this book demonstrate, planet Earth has changed in profound ways, and these changes will be with us for generations to come. In the face of this transformation, we must not be paralyzed by fear and anxiety.
Rather, we must harness new tools and understanding, working collectively to develop innovative approaches to address many of our most challenging environmental and social problems. In that respect, free and open exchange of ideas and information is critical; we must be able to learn from each other, drawing inspiration from past successes, while avoiding previous mistakes. It seemed only natural, therefore, to use an open access publishing model for Earth 2020, making it freely available to anyone in the world. But wide distribution is not enough. We must also explore other multimodal approaches to engage broad audiences who feel increasingly overwhelmed in the age of information overload, where ideas compete for relevance in a crowded digital landscape. To this end, two examples of multimodality are offered as part of this volume, in the section directly following this introduction. These take the form of musical compositions drawn from a range of Earth System data; sonic representations of our rapidly evolving planet.
For much of the past year, as I have worked on this book, my own outlook on planet Earth has fundamentally shifted. For one thing, I have come to a much deeper understanding of the historical and political context that has driven humanity's impact on the planet. Through the words and ideas of the book's authors, the events that have unfolded around me over the past five decades have come into sharper focus as part of a 8 Earth 2020 larger emergent narrative. And what stands out most, perhaps, is the notion of possibility.
It is true that things look grim, but they also did in 1970. Our history has shown that we have the capability to address daunting global challenges if we have the will and the fortitude. In the words of the young climate activist, Greta Thunberg, delivered to the US Congress, in September, 2019: 'You must take action. You must do the impossible. Because giving up can never ever be an option'. It is my great hope that you, the reader, will find both knowledge and inspiration in this book, and that it will mobilize you to take action in pushing society towards a more just and sustainable future.

1.
Available at https://www.ipcc.ch/site/assets/uploads/2018/06/2nd-assessment-en.pdf 14 Earth 2020 a frontier; now, it is mainstream. We did have an essay by Paul Sears that looked back nearly fifty years to the Dust Bowl calamity of the 1930s, and considered 'the inseparable tie between the good earth and human destiny'. 2 We paired that essay with another, by Jeremy Sabloff, that looked even further back, to the collapse of the Maya civilization. 3 The word 'sustainability' hardly existed in 1970, but these two essays did call attention to risks to the continuity of civilization.
In our introduction to those two 'Lessons from the Past', we noted that the Dust Bowl tragedies resulted from farmers, ranchers and land developers ignoring the warnings of soil scientists and agronomists. The Maya, we suggested, did not see the consequences of their population growth under limited land resources, and lacked the knowledge to make the metal tools that might have extended their farmland. We wrote: 'Every society has its blind spots and from a distance one's reactions to them are instinctively charitable. But to the deaf spots in a society, how should one respond?' 4 Let us turn that judgmental spotlight upon ourselves, and assess our choices of topics in Patient Earth. Which warnings did we hear, which could we have heard if we had paid attention, and which did we not hear because they did not yet exist? Such analysis can provide insight, more generally, into how society can learn to open its ears.
In 1970, environmentalism was deeply intertwined with three other contemporary concerns: wilderness and the non-human environment, militarism and population. We were determined to address all three. Notably, they are scarcely present in the collection of topics addressed in Earth 2020.
To emphasize wilderness and the non-human environment, we recruited an essay by Albert Hill and Michael McCloskey about how the High Sierras in California were about to be invaded by a ski resort, 5 and another by Kent Shifferd about how the remote woods of northern Wisconsin were threatened by an immense transmitter for submarine communications. 6 We also wrote our own essay on the menace to the Florida Everglades presented by a proposed international jetport west of Miami. 7 Activists battled all three, and none were built. Today, environmental organizations present the need to protect the environment in largely instrumental terms, stressing the direct benefits to humans (clean air and water, and carbon storage, for example). We straddled this breech ourselves. In our essay on the Everglades, we highlighted the negative human impacts resulting from the degradation of nature and noted how 'the well-being of man (sic) and the park, in quite direct and material ways, are critically linked', 8 a notion now referred to as 'ecosystem services.' But we could not have guessed then that fifty years later, there would be mounting evidence for declines in the numbers and diversity of insects, including the pollinators that sustain our food supply.
The second concern, militarism, was very much alive in 1970. At the time, the US was still prosecuting the Vietnam War. There is an essay in Patient Earth by Arthur Galston on the use of defoliating herbicides in Vietnam to open up its forests to US bombers, 9 and a primer on radioactivity, addressing both nuclear weapons and nuclear power, which we wrote with Joseph Ginocchio. 10 At the time, avoiding nuclear war was the primary objective among physicists like us who engaged with public affairs. It still ought to be. We had blind spots, of course. We never made the connection between climate refugees and war, nor did we consider oil fields as potential military targets.
The third concern -population -was discussed in practically every environmental critical question with environmental significance is whether a similar downward trend will emerge worldwide. If that happens, the global population will decrease, and our species will have an easier time accommodating to this small, shared planet. 16 Earth 2020 I n 2020, these three previous concerns have been replaced by two new ones: planetaryscale thinking and environmental justice. We emphasized the first in Patient Earth, but to the second we were deaf.
Although Patient Earth deliberately focused on US issues in its case studies, again and again it zoomed outward to treat the planet as a whole. We presented the Earth as a single system that could be overwhelmed by human activity in ways that resemble anthropogenic impacts on lakes and airsheds. We taught the reader to perform calculations relevant to global warming, and observed that 'it is ominous that our capacity to change our planet has outrun our understanding of what is happening'. 13 We couldn't have anticipated an ozone hole driven by chlorofluorocarbons (CFCs), but we could have come close; the effect of supersonic airplane emissions on stratospheric ozone was already a live issue.
We did not deal with ocean acidification adequately. We described how the oceans had taken up a portion of anthropogenic CO 2 up to 1970, and commented, briefly, on the increasing acidity of surface ocean waters. We explained chemical buffering, and how increasing the ocean's acidity reduces its capacity to take up more CO 2 . But we utterly failed to point out that an increase in acidity was a threat to the ecological integrity of the oceans. We didn't ignore warnings about ocean acidification because there were none then, but we also didn't listen to our own words and pursue their consequences.
The essay about resource scarcity by Charlotte Alber Price -on helium conservation programs -adopted an entirely US perspective. 14 We wrote nothing about world hunger, or ice, or sea level or the world's forests and fisheries -all treated in Earth 2020, which is globally-focused throughout. Both books are silent on the overuse of antibiotics, and uncontrollable epidemics -topics that must also be brought into the discussion.
Much of the planetary thinking in Patient Earth is at the societal level. Herman Daly, at our invitation, contributed an essay that was the first publication of his path-breaking ideas about 'the Equilibrium Society,' where material flows through an economy reach a plateau. 15 Such zero-growth arguments remain unfashionable (and incomprehensible to economists) today, in about the same way as they did fifty years ago. Meanwhile, in my current home in Los Angeles, the average number of days with temperatures exceeding 32°C has increased from 53 in 1970 to 67 today. This is two extra weeks' worth of very hot days that desiccate California's landscape, priming it for ferocious wildfires, and days that put vulnerable populations at risk -days when children cannot play sports or have school recess outside, when heat-related emergency room visits by outdoor workers soar, and when deaths among the elderly spike because they are susceptible to heat stroke and heat stress-induced heart attacks.
From 1970 to now, global warming has gone from an abstract threat discussed by scientists to a fact that cannot be ignored. It is here. We feel it. We see it.
The global warming we experience now was predicted long ago. In an 1896 paper that marked the birth of modern climate science, Swedish chemist and Nobel Laureate Svante Arrhenius connected rising and falling CO 2 levels to global warming and cooling in an attempt to explain the waxing and waning of ice ages over Earth's history. 1 From earlier Earth 2020 measurements by others, such as the American astronomer Samuel Langley, Arrhenius knew that CO 2 and water vapor are what we now call greenhouse gases: gases that selectively absorb the infrared radiation emitted by heated bodies (the radiation that warms your hand next to a stove or radiator). Arrhenius demonstrated how rising CO 2 levels would lead to warming by trapping heat near Earth's surface. He also recognized that water vapor exerts an important amplifying feedback, since a warmer atmosphere holds more water vapor, which itself is a greenhouse gas that traps heat.
Arrhenius' model was simple, and the measurements he used were inaccurate.
Fortuitously, errors from the simplification and in the measurements largely canceled each other, and he was able to get what is now considered not far from the correct result.
Arrhenius predicted that doubling atmospheric CO 2 levels would raise Earth's temperature by 5-6°C. But more important than the precise degree of warming Arrhenius predicted was the fundamental physical insight he delivered: there is a close link between greenhouse gas concentrations and global temperatures. In later work, he observed that burning coal could lead to a significant rise in atmospheric CO 2 levels and appreciable global warming within a few centuries to millennia, a prospect entirely desirable from his Nordic vantage point: 'We would then have some right to indulge in the pleasant belief that our descendants, albeit after many generations, might live under a milder sky and in less barren natural surroundings than is our lot at present'. 2 Arrhenius' insights proved prescient about what the future would hold, though he and generations of scientists after him severely underestimated the rate at which CO 2 would accumulate in the atmosphere and change the climate.
W e now know from historic air preserved in bubbles in the ice sheets of Antarctica and Greenland that atmospheric CO 2 levels hovered around 270 ppm for 10,000 years, following the end of the last ice age. By the late 1800s, however, industrial activities began to increase atmospheric CO 2 levels, which reached 295 ppm by the turn of the twentieth century. Modern measurements of atmospheric CO 2 levels were started in the late 1950s by Charles David Keeling from the Scripps Institution of Oceanography and brought an almost immediate surprise: concentrations were rising more rapidly than anticipated, implying that the oceans were taking up less of the CO 2 emitted by human activities than scientists had previously believed.
By the first Earth Day in 1970, CO 2 levels had reached 320 ppm, 20% above preindustrial levels. The current value, half a century later, is around 415 ppm, more than 50% more than pre-industrial levels. 3 These values imply that we have added about twice as much carbon dioxide to the atmosphere since 1970 as in all of previous human history before. Worldwide emissions of carbon dioxide from all human sources, including fossil fuels and deforestation, have steadily climbed from 20 billion metric tons per year in 1970 to 42 billion tons now, with no peak in sight. Today, the average North American loads the atmosphere every year with an amount of carbon dioxide weighing about the same as ten midsize-passenger cars. We are releasing CO 2 into the atmosphere far more rapidly than Arrhenius could have possibly imagined.
Along with a growing global network of CO 2 measurements, we have also amassed a large instrumental record of temperature measurements from the nineteenth into the twentieth centuries. In the late 1930s, English engineer Guy Callendar first demonstrated a global warming trend, which he linked to the 10% rise in CO 2 levels that had already occurred by that date. Modern temperature data compiled from all over the world have demonstrated that the average land temperature has increased by 1.4°C since 1900. 4 The vast majority of this increase (1.2°C) has happened since 1970, with a rate of increase in the Arctic (2°C since 1970) that is almost twice the global average. These seemingly small temperature increases hide large changes, leading, in the case of the Arctic, to thawing permafrost and the collapse of structures built on formerly frozen ground.
In response to this warming, the Arctic's summer sea ice cover has plummeted 40% and is approaching its demise. 5 Arctic summers without sea ice will soon be a reality, with enormous implications for human livelihoods and regional ecology. 6 Across the globe, increasing temperatures are associated with a wide range of climate concerns, including stronger rain storms, prolonged droughts and sea level rise. 7 Even worse, we have yet to see the full extent of the warming to which we have already committed our planet. At least some of the warming associated with increased greenhouse gas levels is masked by air pollution. Over much of the middle to late twentieth century, 26 Earth 2020 smog blanketed industrialized areas such as London, Los Angeles and Central and Eastern Europe. 8 Smog consists of tiny aerosol particles, which reflect sunlight back to space, shading and cooling Earth. The added aerosol particles can also increase the number of droplets and ice crystals in clouds, which increases their reflectivity and adds to the cooling effect of air pollution.
Although air quality in the west has improved over the past fifty years (thanks to amendments to the Clean Air Act in the US in 1970, and similar legislation in other western countries that followed), air pollution has worsened in much of the rapidly industrializing world, especially in China and India. The persistence of smog in Earth's atmosphere has thus masked some of the warming that rising greenhouse gas levels otherwise would have caused. As countries improve their air quality, the cooling effects of smog will be reduced, leading to more warming.
T oday, we know there's more to climate change and the ways it affects humans than how greenhouse gases regulate the transfer of radiation through the atmosphere. Other To approach the monumental challenge of simulating a coupled Earth system, climate models break down the complexity of the system into coarser chunks. This is achieved by dividing the globe into a grid and then performing computations separately for each box of the grid. The size of the grid's boxes -the resolution at which the model can view Earth -controls the accuracy of its calculations. Early climate models in the 1970s had a grid size of about a thousand kilometers, meaning that a slice across the Atlantic Ocean might span just four or five boxes. Current models with much smaller grid sizes can resolve processes down to scales of tens of kilometers. The most sophisticated models today capture radiative processes and larger-scale turbulence in the atmosphere and oceans, and they include models of the land and ocean biosphere. They have allowed us to explore complex processes, such as the link between global warming and intensification of rainstorms.
But despite significant advances in climate models since the 1970s, some critical processes remain difficult to resolve. The small-scale turbulence that sustains clouds, and processes occurring on tiny scales, such as the microphysical processes shaping droplets and ice crystals in clouds, cannot be accurately represented in current models. Yet even these small-scale processes matter for climate. A cloudy night is warmer than a clear night because clouds are good absorbers of Earth's emitted infrared radiation. Clouds can also make for a cool day at the beach because they reflect sunlight back to space, shading Earth.
These small-scale processes affect the trajectory of longer-term climate change, and therein lies the rub -without resolving these processes in climate models, it is difficult to predict precisely how much more warming, extreme storms and sea level rise we should plan for, even if we know how much greenhouse gases will be emitted.

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Earth 2020 Despite the uncertainty of climate predictions, some things are clear. If greenhouse gas emissions were immediately cut to zero, the level of these gases in the atmosphere would stabilize, before starting a slow decline to a new baseline level over centuries to millennia. But the air would also be cleared of the polluting and cooling aerosols produced by fossil-fuel burning. The result would be more warming in the short term, despite stabilization of greenhouse gas levels. The climate effects of air pollution have not been precisely quantified, but current models suggest that we would see an additional global average warming of 0.4-1.7°C within years of eliminating all greenhouse gas emissions. 9 We cannot stop CO 2 emissions suddenly; our energy economy has the agility of an oil tanker. Over the past fifty years, growth in global energy demand has outpaced growth in energy production from renewables. Greenhouse gas emissions are growing with no peak in sight, much less a reduction to zero. There is virtually no chance that we can avoid the 1.5°C global-average warming above pre-industrial temperatures aimed for by the Paris Agreement in 2015 (signed in 2016). 10 If we consider the 1.1°C global-average warming that has already occurred since the nineteenth century, and the time-delays in our energy economy and in the climate system, the inescapable conclusion is that we are on track to exceed 1.5°C and perhaps even 2°C global-average warming above pre-industrial temperatures.
While not physically impossible, limiting global warming to 1.5°C requires an implausibly short-term turnaround of greenhouse gas emissions, and staying within a 2°C warming target requires an economic restructuring at a pace not previously seen in history. Just to have a fighting chance of avoiding more than 2°C warming, we would have to drop greenhouse gas emissions down to zero within about 30-40 years -the lifetime of today's fossil-fuel power plants. Even achieving zero emissions in that timeframe would give us only a two-thirds chance of limiting global warming to 2°C above pre-industrial levels, according to the generation of climate models that came out in the early 2010s. 11 Worse still, many of the most recent climate models are running hotter, indicating a higher sensitivity of the climate system to greenhouse gases than previously considered likely. This result stems in part from recent findings that the cooling effect of polluting aerosols may be stronger than previously thought. But if cooling by air pollution in the past Climate 1970Climate -2020 was stronger than previously estimated, the sensitivity of the climate system to increases in greenhouse gases must be larger than previously estimated, or else we would not be able to account for the twentieth-century temperature rise. If the new models are more accurate than the previous generation -which is unclear at present -we may have underestimated the warming response to greenhouse gases. In that case, limiting global warming to 2°C above pre-industrial levels will be extremely challenging, if not impossible.
F rom the first Earth Day in 1970 to today, global warming has moved from an abstract scientific prediction to a reality we must contend with. At the same time, the discussion of global warming has moved from an exclusive focus on mitigation to the deepening realization that adaption is also critical. Mitigation was the focus of the 1992 United Nations Framework Convention on Climate Change, in which countries around the world committed to 'stabilize greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system'. 12 Follow-on treaties such as the Kyoto Protocol (1997) and the Paris Agreement were attempts to make this specific and enforceable. What 'dangerous anthropogenic interference with the climate system' means remains unclear. Nonetheless, we do have an idea of where we are headed.
The last time carbon dioxide levels were sustained at today's levels (around 415 ppm) was three million years ago, during the mid-Pliocene. At that time, Earth's global mean temperature was 2-3°C warmer than today, and sea level was about 17 m higher. The Greenland ice sheet was ephemeral and the Antarctic ice sheets were smaller; the water locked up in them now was part of the oceans. Mammalian life on Earth was thriving, but Homo sapiens did not yet exist, and neither did currently low-lying cities such as Alexandria, Amsterdam, Cape Town, Guangzhou, London, Miami, Mumbai, New York, Osaka, Rio de Janeiro or Shanghai. Even today's greenhouse gas levels, if sustained for centuries, must be considered dangerous for human civilizations that are adapted to the relatively stable climate and coast lines that existed for the 10,000 years before the industrial revolution.
Mitigating global warming to the greatest extent possible remains essential to prevent the cataclysms that await when current greenhouse gas levels are sustained for centuries, or increase even further. After decades of failures, efforts to stem rising tides and temperatures 30 Earth 2020 are much more urgent now than in 1970 or 1992, when snow and skiing were still common in the Harz Mountains of my childhood. But mitigation alone no longer suffices. Climate change will leave no one untouched. We have no choice but to adapt.

1.
For a reprint of Arrhenius' paper and a discussion of its context and reception, see D.  11. These estimates are based on the carbon budgets in  House South Lawn. Coca-Cola and the chemical manufacturing giant DuPont also signed up to celebrate the Earth. Over the past half-century, Earth Day has become a ritualized, 'safe' event, layered with hypocrisy and opportunism. Yet, this event still persists in raising awareness and action, and its first celebration, in 1970, was a landmark in many ways. Looking back over the five decades of environmental law and policy since Earth Day 1970, what stands out? There are two threads to follow in the emerging challenge of environmental governance: the global North-South divide ) and the emergence of global corporate rule (1990 onwards). The latter is particularly important, as it threatens to undo hard-won progress in preserving Earth's natural systems.
F or the first two decades after Earth Day, all industrialized countries started down the path of controlling pollution with a focus on science-based regulations and policy. Issues confronted in this period were largely solved at local and regional scales: eutrophication, acid rain, local air quality, visible water pollution from factories and sewers, and so on.
These problems were both created and solved within the national context of wealthy industrialized countries. There was no need for diplomacy or multi-lateral negotiation. 1 Nor was there an apparent need to recognize the uneven burdens and responsibilities of global environmental degradation.

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The vast majority of developing countries were absent from the 1972 UN Stockholm  3 Stratospheric ozone plays a critical role in screening out the sun's most harmful ultraviolet rays, and the loss of this protective layer was a cause for significant concern. The massive ozone hole that developed over Antarctica became emblematic of this threat, and helped spur the world to action.

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Earth 2020 As industrialized countries mobilized to develop a protocol leading to an effective ban on ozone-depleting substances, the developing world expressed concern that, once again, this was an issue for the rich countries of the world. In the Global South, rotting food was a bigger issue than thinning ozone, and developing countries wanted to expand their use of CFCs, particularly a class of these compounds known as freons for refrigeration. Even more uncomfortable was the reality that the skin cancer threat was highest for the pale to the atmosphere. 6 The future of the land carbon pool depends significantly on human 46 Earth 2020 land use practices, but also on the stability of huge deposits of frozen organic carbon in northern permafrost soils. Warming tundra and Arctic soils are accelerating the melting of these frozen deposits, which will likely release more CO 2 than any other part of the land surface could match. 7 Taken together, these human perturbations of the global carbon cycle are analogous to the volcanic CO 2 releases in the 'greenhouse extinctions' of the geological past. The total quantities of CO 2 liberated naturally by volcanos were probably larger than humans could muster by burning fossil fuels, but the rate of our CO 2 emissions are likely unprecedented in Earth history.
What happens to all of the CO 2 released by human activities? About half of it is still in the atmosphere, with the concentration rising from around 320 parts per million (ppm) in 1970 to around 415 ppm today (a roughly 30% increase). The rest of the anthropogenic carbon has mostly been absorbed into a giant oceanic pool, which has helped to stabilize both the atmospheric CO 2 concentration and the temperature of Earth's surface (and thus global climate). Over the past fifty years alone, the oceans have absorbed about 150 billion metric tons of CO 2 from the atmosphere, while also absorbing significant amounts of heat.
In the short-term, oceanic uptake of CO 2 and heat are mitigating the greenhouse effect.
Over the longer-term, however, CO 2 and heat pollution stored in the ocean will eventually be re-released to the atmosphere, slowing down any future recovery. In addition, CO 2 uptake by the oceans has a significant effect on seawater chemistry, resulting in increasing acidity (decreased pH) as hydrated CO 2 becomes carbonic acid. The global-scale response of the ocean carbon cycle to a shift toward greater acidity is difficult to predict, but we do know that ocean acidification was a prominent feature of previous mass extinction events on Earth.
The time it takes for ocean pH to recover from an abrupt increase in CO 2 concentrations is on the order of thousands of years -long by human standards, but short geologically.
And herein lies a critical distinction between human-derived fossil fuel carbon and natural volcanic CO 2 sources. Whereas volcanic CO 2 was released into the atmosphere over millions of years, fossil-fuel carbon has been released over the last couple of centuries at a rate that overwhelms the capacity of natural chemical buffering processes. Our rapid CO 2 emissions will thus lead to larger spike in ocean acidity than any previous disturbance of the carbon cycle. 8

Carbon 47
Political negotiation on the climate issue has focused on trying to limit peak global temperature rise to 1.5°C. 9 This much warming would make the planet warmer than it has been in millions of years, since long before the development of humans as a civilized species. Warming of 2°C or more would almost certainly be worse, but the choice of 1.5°C itself is somewhat arbitrary. A true 'safety' boundary could be defined in terms of the of the consumer population followed by collapse when the nourishment is gone. However, of all of the climate episodes and extinctions in the history of the carbon cycle, this is the first in which the agent of the event is at least beginning to understand the consequences of its actions.
Our perturbation of the carbon cycle is primarily an energy problem, so fundamental to our lives that it is challenging to imagine changing it quickly enough. But there is plenty of energy all around us, from the sun, and in the wind. If we were simply running out of fossil fuel now, would our civilization really collapse? Much of the human activity on planet Earth is driven and guided by our financial system; when there is immediate money to be made, we are extremely clever and adaptable.
Fossil CO 2 can be seen as a waste-management problem, like that of Shel Silverstein's Sarah Cynthia Sylvia Stout, who would not take the garbage out. 12  But many of these pollinators are in trouble. There has been a rapid increase in morality in managed honeybee hives, accompanied by widespread reductions in native pollinator abundance and massive declines in insect abundance generally. 3 The reasons for these declines are complex and not fully known, but likely include land-use and climate changes, pesticide use and other forms of pollution.
The global decline of pollinators is symptomatic of a much larger global trend. Since the dawn of the industrial era, species extinction rates have accelerated dramatically.
Today, we are losing an estimated 1,000 to 10,000 times more species per year than would be natural under pre-human conditions. 4 And the surviving species are dwindling rapidly, with about 60% of wild vertebrate populations -amphibians, reptiles, mammals, fish and birds -shown to be in decline. 5  1970 to today will surely guide the next half-century of conservation efforts to stave off a looming mass extinction. But one thing is certain; the sooner we act to prevent species 54 Earth 2020 declines, the more successful we will be. Moreover, swift action now is likely to save us an immense amount of resources, financial and otherwise, that we would need to invest down the road to achieve the same results.
The loss of biodiversity is not just a matter of disappearing species, but also of radical landscape transformation. There is perhaps no starker example of such transformation than the rise of urban areas around the globe. While accounting for less than 5% of Earth's area, cities now house almost 60% of the human population. 7  Yet protected areas will never be enough -increasingly, they are islands, too small, too few, and too remote to support the biodiversity upon which human society depends. This was perhaps one of the most visionary turns of the Environmental Movement of the 1970s.
No longer was US conservation focused only upon protected areas; rather, the passage of the Endangered Species Act, the Clean Water Act and the US Clean Air Act underscored the need for biodiversity to be protected in the sea of humanity. This was done not only for the inherent value of biodiversity, but the realization that our own species depends on functioning ecosystems to provide vital life-support services. In cities, for example, green spaces and street trees reduce temperatures in urban heat-islands, purify urban air and attenuate city noise. Moreover, daily exposure to such natural elements has been shown to have manifold benefits to mood, attention span, and memory retention over standard urban or suburban landscapes. 8 And across sweeping landscapes and seascapes, ecosystems produce important goods (such as timber and seafood), essential life-support processes (such as natural pollination and water purification), life-fulfilling conditions (such as beauty, serenity and inspiration) and preservation of potential future benefits (for example, the conservation of genetic diversity for future use in agriculture or medicine).
A lthough government action plays a major role in biodiversity conservation, we must also employ other tools going forward. Increasingly, this means engaging with the economic system to create more ecologically-sustainable goods. Consider the refreshing beer you might drink at the end of your long workday. The global beer industry has been long dominated by a few key players. However, with the recent insurgence of small craft microbreweries, the rules of the game are changing quickly. Many consumers are now willing to pay a premium for beer that boasts both a greater flavor profile and a greater corporate sustainability ethos. This sustainability is achieved through a variety of approaches, including the use of spent hops as agricultural feed (rather than sending them to landfill), and partnerships with local conservation groups to secure forests situated upstream of key water supplies. While it is true that some of these actions are being taken to improve corporate 'green' image of the company ('greenwashing'), 9 sustainable business practices are now not only possible, but increasingly profitable. At the same time, we are seeing increasing public scrutiny and boycotts of companies and industries that refuse to incorporate the value of biodiversity into their decision-making. One of the most prominent examples is the refusal of many consumers to buy products containing palm oil -a crop whose rapid proliferation is endangering tropical rain forests around the globe.
Governments at various levels are also increasingly taking the economic value of nature into account. New York City became a posterchild for this movement in 1997 when it opted to secure its drinking water quality by investing in natural capital rather than building a physical treatment plant. The decision was based on economic analysis, showing a capital cost of $6-8 billion for building a water treatment plant, plus annual operating expenses of $300 million, as compared to an estimated $1-1.5 billion, in perpetuity, for habitat protection in the source watersheds about 100 miles north of the city. 10 Twenty years of experience show that the natural capital investment is working, yielding a triple win -safe water for the ten million people living in New York, compensation for a public service long supplied by farmers and foresters upstream, and protection of many other benefits under the umbrella of safe drinking water. Over the past two decades, this case inspired adoption of similar projects by over fifty major cities in Latin America, a rapidly growing number in Asia, and some now in Africa. Globally, an estimated 25% of major cities stand to benefit from this approach. 11 Also in 1997, Costa Rica adopted national economic incentives for biodiversity conversation, pioneering a payments for services (PES) scheme that incentivized local farmers to conserve or restore their forests in recognition of the economic returns from increased eco-tourism, carbon storage and water purification (for hydropower -a major export -as well as for irrigation and drinking). 12 This proved to be the beginning of a global trend, with many countries soon establishing similar programs. For example, China launched their own PES program in 1999, enrolling 120 million households in restoring steeply sloping lands for flood protection and water purification. 13 Today, there are over 550 such programs around the globe, with total annual payments of nearly $40 billion. 14  There can be no doubt that modern energy has transformed how humans move, eat, live and play, while also radically altering our impact on Planet Earth. Over the past 10,000 years, new power-producing technologies have been the foundation of modern societies.
In human history, today is the energy anomaly. Supported by energy, more people live longer now than in any other time in the history of our species, with access to vastly improved healthcare, sanitation and seemingly limitless opportunities for travel. Energy has also benefited social mobility; no longer are 70-90% of humans serving as serfs and slaves needed to farm and transport goods; in many countries, traditional 'women's work' Earth 2020 of cooking, laundry and cleaning has been drastically cut by energy-driven appliances (and with men sharing the work!).
But whenever there is progress, there is also regress and unanticipated consequences.
Both coal mining and natural gas and oil production, including refining and combustion, affect land use and pollute water and air. Uranium is mined to produce nuclear-powered electricity, creating long-lived radio-active waste. Hydropower entails the damming  in 2020 likely coming close to 415 ppm). The trend appears to be continuing unabated; in 2018, the world used 3% more energy than the previous year, with accompanying annual CO 2 emissions increasing at 2% to 37 billion metric tons of carbon dioxide. 2 The fear of a climate tipping point looms large, and energy system decarbonization is now more critical than ever.
The good news is that there has been significant progress on the transition to alternative energy sources. Economic incentives, including tax credits, feed-in-tariffs and other subsidies, have stimulated research, development, deployment and investment, and helped reduce the costs of renewable energy globally. As a result, the addition of low carbon energy production capacity has surpassed expert predictions in scale and speed.
Innovation and global investment in renewables like solar and wind power topped $300 billion per year for the fifth year running, and 2018 saw near record numbers of renewables, and lower carbon natural gas dominate new energy installations.
The bad news is that our progress has not been nearly enough. Overall global growth in energy demand is outstripping decarbonization efforts, and fossil fuel consumption continues to increase as more people are using more energy around the planet. Today, roughly 80% of energy used still comes from fossil fuels including coal oil and natural gas. are more than just a collection of coordinated technologies, they enshrine social practices and values. There is no one global energy system; energy is not distributed, delivered and used equally around the world.
As a privileged citizen of an industrialized country, my experience of energy is vastly different from most of the world's population. When I wake up, shut off the alarm clock, turn on the light, start the hot shower and the coffee maker, and get cold milk from the refrigerator, energy use is almost invisible. When I flip on the light switch, I expect light.
This is the privilege of the energy rich, the roughly 2.2 billion people of the 7.7 billion on the planet today who have the luxury of not having to think about energy. For these people, there is more than enough energy for basic comforts, health, food, transportation and wellbeing. There is enough for them to travel by airplane and car, to use cell phones and have Jacuzzis, extra freezers and nose-hair trimmers. This is not to say that energy use is uniform even within energy rich countries; energy disparities do exist, and some citizens in these countries still experience energy poverty.
Abundant energy has enabled rapid economic growth of industrialized societies, and this development has been responsible for the bulk of historical greenhouse gas emissions.
But today, energy demand in many rich countries is flat to declining. In 2018, the twentyeight European Union countries had flat or negative energy consumption growth due to policies supporting increased system efficiencies, investments in renewables and a mild winter. While reducing greenhouse gas emissions has become a modern rallying cry for the energy rich, others often have different priorities.
At the other end of the spectrum are the energy poor, the 1.1 billion people who live without access to electricity; and an additional 2.5 billion without access to modern cooking fuels. In these societies, lighting is often provided by candles or kerosene lanterns, while wood, dung, or charcoal provide energy for cooking and heating. Cooking over a three-stone fire requires significant amounts of both wood and time. Fire is dangerous, and it also creates smoky particulate matter which causes respiratory and eyesight problems, mostly in women and children who do the bulk of global wood gathering and cooking.
For these people, whose meagre energy use has not contributed to global climate change, affordability and access to energy is paramount. Our future depends on how we will collectively make and use energy. This will be shaped by how we design, travel and live in our cities, communities and homes. Today, a zero-carbon energy system pushes the limits of technology and faces immense political and economic barriers. With the EU Green Deal goal of a carbon neutral Europe by 2050, and at least €100 billion to support it, this is a critical first step. 9 Whether we like it or not, our energy systems are changing. It remains to be seen how they can be adapted to support our collective futures on Earth.
As we look forward, humility should accompany our energy system transitions. The ancient Greeks believed that hubris led to punishment and suffering. This is the tale of Odysseus, who was punished for his arrogance, and of Prometheus, who stole fire from the Gods as a gift to humanity. Praying to the weather gods will not save us from the next hurricane, fire, or the impacts of a changing climate. We should not lose faith in science or political systems, but we might want an extra set of oars at the ready. They may come in handy when that diesel motor sputters out. It is easy to cast blame on lower-and middle-income countries where rapid deforestation is currently underway. But we must remember that inhabitants of the industrialized world live in previously forested cities and towns, and farm previously forested fields. Nearly half of global forests were cleared by humans across history, not just in recent decades.

Endnotes
We also consume agricultural products that come from deforested tropical areas, fueling economic drivers of deforestation. And in higher-income countries, some natural forests are still being converted to plantations of non-native species such as eucalypts or pines.
Such introduced species significantly alter local ecology, and can bring other risks as well. have turned British Columbia's vast forests from a net carbon sink to a net carbon source, contributing to rather than mitigating greenhouse gas emissions.
The world's forests are also critical reservoirs of biodiversity, containing 80% of all terrestrial species globally and about three quarters of birds, with most of these found in the tropics. In North America, for example, the total number of birds has dropped by 29% since 1970, a reduction of about three billion individuals. 6 Climate change is only one of the human factors destroying the library of life; others include overharvesting, 7 pollution, 8 and loss of habitat. A recent UN report concluded that up to one million species are at risk of extinction. 9 To slow or avert this biodiversity crisis, forests must be restored or maintained to provide habitat for the many species they house. about 500 billion trees. 10 At first glance, this appears to be a win-win solution, restoring native forests, generating forest-based goods and services for local communities, enhancing greenhouse gas sinks and creating habitat for biodiversity. But this view from space misses many details that will determine the feasibility of this solution on the ground. 11 Afforesting grasslands won't actually increase carbon sequestration, as we now know that grassland soils contain as much carbon as forests, and they regenerate soil carbon faster than forests. We must also consider the individuals and communities that will benefit or be harmed as a result of reforestation. Per-capita greenhouse gas emissions are highest in high-income countries, while deforestation is greatest in those with lower incomes. Some people living in poverty who are already being disproportionately impacted by climate change may also suffer losses in livelihoods from widespread reforestation through loss of grazing or agricultural land. If local communities are not involved in designing and benefiting from reforestation, tree planting programs are destined to fail.

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Earth 2020 In some places, adapting landscapes to climate change may require planting fewer, not more, trees. While planting more trees per hectare might result in more carbon sequestration, forest managers are shifting to lower-density forests in drought-prone areas to provide trees with sufficient water and mitigate wildfire risks. Some forests will likely shift to grassland ecosystems with further warming. We need to better understand where and when these ecological shifts will occur, and how they will impact carbon storage.
In places where tree planting offers more benefits than risks, we need to consider climate change when selecting trees to plant. Changes in climate over the lifetime of a typical tree will impact forest health and reduce carbon sequestration. Populations of trees of a given species vary genetically, depending on the climates they have evolved in. Scots pine from Finland and Spain, for example, differ considerably in their growth timing, cold hardiness and drought tolerance. Tree species and populations used today for reforestation will need to be carefully chosen to increase the likelihood that they will be healthy and productive throughout their lifetimes as climates change. We are dealing with a moving climate target, and without a crystal ball to pick the best trees for the uncertain climate 50 or 100 years from now, we should hedge our bets by planting a variety of species, each with high genetic diversity. We may also need to assist the migration of genetic populations and even species into new habitats as they become climatically favourable. Fast-growing plantations of non-native species might grow and fix carbon more rapidly than natural forests, and provide some economic and social benefits, but they will not provide critical habitat for rapidly declining biodiversity.
One thing is for certain: we need to dramatically reduce deforestation globally. Higherincome countries with high carbon footprints should continue to encourage and financially support efforts to slow and reverse tropical deforestation. All nations should support their community-driven efforts to restore degraded forest ecosystems and marginal agriculture lands with a diversity of tree species that provide a variety of resources. If deforestation is to be slowed and reforestation is to succeed, trees must be worth more alive than logged to local people.
Sustainable forest management must also be routinely practiced everywhere.
Harvesting crop trees after longer rotations will increase rates of carbon sequestration per year. Thinning stands and using the harvested wood in long-lived products will reduce mortality and improve carbon balance. For example, the use of wood to construct buildings with long lifespans helps store carbon, and provides a substitute for building materials like cement with larger carbon footprints. Partial harvesting and rapid reforestation after harvest will accelerate carbon sequestration, and some stands should be left untouched in the hopes they will become the old growth trees of the future. And we should conserve ancient forests that provide habitat for biodiversity and may be irreplaceable in new climates.
We also need to increase tree cover in urban areas, adapting urban infrastructure and environments to climate change and mitigating some greenhouse gas emissions. Urban forests help cool cities, improve air quality and quality of life, and have positive effects on both mental and physical health. Many cities have lost large numbers of urban trees due to introduced invasive insects and diseases, including the emerald ash borer and Dutch elm disease in North America, and, in Europe, ash dieback due to a fungal pathogen.
Adapting urban forests to climate change is best done by planting a diversity of species and cultivars.
Trees have long been a symbol of environmental movements, and tree planting is an important tool in the fight against climate change. But planting trees is not enough. Tree planting will not replace the systematic societal changes to energy, transportation and food production systems needed to slow the pace of climate change and other human impacts on forests. 12 We have many opportunities to help the survival of forests and the species they house, while also mitigating climate change. If we reduce greenhouse gas emissions, reverse deforestation and manage forests sustainably, trees will continue storing carbon cheaply and efficiently, providing habitat for biodiversity and a multitude of products that support human well-being across the globe. 12. IPCC, 'Summary for policymakers', in reality is not that simple. With many victims downstream, large numbers could 'free-ride' the system, claiming that they don't care about pollution and thus declining to contribute toward the costs of pollution reduction. This outcome is a classic market failure, which can be fixed by government intervention. under some circumstances, but history has shown the limits of this approach. Economists had already explained why private markets fail to provide 'public goods' such as roads, law and order, or military defense. A lighthouse is the quintessential example, with two key attributes. First, once the lighthouse is built, its light can provide navigational benefits to many boats in the area who use the resource without ever depleting it -the light is available to additional boats at the same time and at no additional cost. Second, no business could recover the cost of building the lighthouse, because boaters would realize they can see the light whether they pay or not. These free riders cause the private market to fail, even though the social benefits may greatly exceed the costs of building the lighthouse.
In 1968, Hardin didn't use economic terminology, but his reasoning was impeccable: a clean environment represents a public good that provides health and aesthetic benefits to millions of people simultaneously. Once provided, clean air is available to others to breathe at the same time and at no additional cost. Moreover, consumers will not buy clean air, because they can breathe whether they pay or not. With this free-riding behavior, no business would voluntarily pay the costs associated with cleaning up the air. Other firms who do not clean up will be able to charge a lower price for their goods or services, thus gaining a market advantage. Once again, we see the failure of a private market, even though the social benefits of clean air greatly exceed the costs.
In the absence of viable private markets, government can increase social welfare by providing a clean environment; it can regulate firms, require scrubbers, tax pollution and prohibit improper disposal of waste. These clean-up activities certainly have costs, especially for generation of electricity or transportation of goods, and industries may have to cover their costs by increasing product prices. But, if environmental protection is done wisely, then collective health benefits can greatly exceed the additional costs to businesses and consumers.
As a thought experiment, consider a particular environmental protection proposal where total health and aesthetic benefits exceed total costs. Suppose also that the benefits and costs are distributed equally across all voters. In this scenario, the proposal would provide a net benefit to everyone, and support for the proposal should be unanimous. Most often, however, the benefits and costs of environmental protection are not shared equally. And therein lies one of the major economic problems of enacting environmental protection.
Even for policies with positive net benefits overall, some segments of society receive disproportionate benefits, while others bear disproportionate costs. Critically, economic analysis can be used to measure the distribution of these gains and losses resulting from any proposed policy. It can also help design a policy package that simultaneously achieves pollution reduction and desired objectives regarding the distribution of gains and losses.

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Earth 2020 When it was first established in 1970, the US Environmental Protection Agency (EPA) focused on technological and legal frameworks to control pollution, with little consideration of economic implications. Engineers were employed to determine the 'best' ways to cut pollution, and lawyers wrote regulations requiring the adoption of those recommended technologies. Under this approach, as it developed in the immediate aftermath of the first Earth Day, environmental protection was viewed as a moral imperative, and costs were not taken into account. In contrast, the early pioneers in environmental economics devoted significant attention to analyzing both the costs and the benefits of different environmental protection schemes. They often found that costs of actual legislative and regulatory changes were more than three times as high as those for alternative policies that would achieve the same degree of environmental protection. In other words, more economically efficient approaches could lead to greater environmental protection for the same level of financial investment.
Enter the ideas of John Harkness Dales. In 1968, Dales published a brilliant idea for minimizing the cost of achieving any given degree of environmental protection. 4 Government could limit the total amount of pollution at an appropriate low level, print a fixed number of permits or licenses, and let polluters bid for the permits or trade with each other. The key innovation of Dales' idea was to recognize that a particular required mitigation technology cannot logically be 'best' in all different circumstances. Policymakers in the nation's capital cannot possibly know as much about production technologies as the engineers inside each firm, especially when those technologies vary across firms. The same pollution reduction could be achieved by letting each firm determine their own 'best' method.
As an example, regulators might require the most advanced (and likely most expensive) flue-gas scrubber to remove sulfur dioxide from emissions of coal-fired electricity generating plants, but cost-minimizing engineers within the firm might be able to cut pollution the same amount at lower cost. They could switch from high-sulfur coal to low-sulfur coal, or from coal to natural gas, or change the dispatch order between coal plants and gas plants, or use renewable power like wind and solar. If the goal is a target pollution reduction, then the method of reduction should not matter. Moreover, not all of those strategies need to Environmental Economics 81 be available to every firm. With permit trading, a firm with limited options can essentially pay a different firm to do their required pollution reduction, through a so-called 'cap-andtrade' approach. Imagine ten firms that each hold 1,000 one-metric-ton permits for sulfur dioxide emissions. Together, these firms are collectively limited to 10,000 metric tons of emissions, but they do not all have to cut by the same amount. A firm with only fossil-fuelfired power plants could switch some output from coal to gas plants, while also buying additional permits from some other firm in sunny Arizona with abundant solar power.
With a single market price, say $100 per metric ton of sulfur dioxide emissions, a fixes the price of pollution, but it does not necessarily limit the total quantity of emissions.
Firms facing a fixed price will decide their quantity of pollution and thus the total amount they are willing to pay for it. If policymakers knew the total quantity of pollution that 82 Earth 2020 would result under a given taxation scheme, they could fix that quantity of pollution by printing a fixed number of tradable permits. Under this permit system, the market would be expected to produce an equivalent pollution price for the same quantity of emissions.
But the critical difference, pointed out by Weitzman, relates to future uncertainty as market conditions evolve. Limiting the quantity of pollution through a permitting system is great for ensuring a clean environment, but firms cannot be sure what price they will have to pay in the future. That uncertainty can inhibit investment and reduce growth, raising costs.
On the other hand, setting the price of pollution through taxation is great for ensuring a known cost of production (and thus certainty for investors), but this approach creates uncertainty about the total amount of resulting pollution.
Which policy, taxation or permitting, better maximizes total social welfare -accounting for all economic and environmental costs and benefits? The answer depends on the relative impacts of uncertain economic costs as compared to uncertain environmental costs. We face many environmental problems ranging from contaminated water, climate change, offers the opportunity to ponder this remarkable atmospheric eggshell around us. Only a few minutes after takeoff, we reach cruising altitude around 10,000 m above sea level, with two-thirds of the atmosphere below us. This lowest portion of the atmosphere -the troposphere -contains gases that support life on our planet, as well as all the pollutants that damage our lungs. The troposphere is also home to the greenhouse gases that warm us and the clouds that cool us. If our plane were to fly another 10 or 20 km higher, it would pass through the ozone layer in the stratosphere, which filters out biologically damaging ultraviolet radiation from the Sun. As we look back to the first Earth Day in 1970, we can ask ourselves how Earth's atmosphere has changed, and what the future may yet hold.
For much of human history, our Earth-bound species has largely taken the air around us for granted, the atmosphere conceived of as an invisible and infinite conduit to the heavens above. This notion was radically challenged with the rise of industrialization, when coal-darkened skies became common in cities across North America and Europe. The 1952 acidic fog episode in London provides a famous example. At that time, England was burning poor quality bituminous coal, creating high levels of carcinogenic soot particles in the air, as well as sulfur dioxide, which is harmful to our respiratory system. In early December of 1952, low winds created a stagnant pool of air that trapped the coal fumes were initially viewed as a shining example of human industrial ingenuity -non-toxic, non-flammable substances with many uses in cleaning, refrigeration and aerosol sprays.
Yet, in the decades that followed, the true environmental impact of these compounds would come to capture global attention.
The first measurements of global CFC abundance were reported in the 1970s by James Lovelock, who would later propose the Gaia hypothesis of Earth as a self-regulating environment sustaining life. Even though the CFC sources were largely in the northern hemisphere, where populations and industry are dominant, the abundance of these compounds was just as high south of the equator. This was one of the first indications that anthropogenic pollutants experience widespread global transport across geopolitical boundaries. Attempting to explain the behavior of CFCs in the atmosphere, researchers discovered that they decompose in the stratosphere, releasing chlorine that catalyzes ozone destruction. 2 At the time, CFCs were being used around the world in a wide range Air 87 of applications, and their atmospheric concentrations were rising dramatically. Once released, there was no easy way to remove these molecules from the atmosphere.
The problem of CFC-driven ozone destruction captured public attention in the mid-1980s, with the discovery of a massive ozone hole over the Antarctic continent. This phenomenon, seen as a large and recurrent loss of ozone in the region each spring, is driven by the chlorine released from CFCs. Its location over the southern pole is attributable to unique meteorological factors that isolate cold air masses over Antarctica and make them particularly susceptible to chlorine-mediated ozone loss. Ground-based observations of the ozone layer over the Antarctic continent, which started in the late 1950s as part of the International Geophysical Year, were the first to detect the ozone hole in the mid-1980s. 3 Subsequently, satellite-derived images of the widespread Antarctic ozone hole became emblematic of human impacts on the environment -a dystopian view of human technology gone horribly wrong.
As terrifying as detection of the ozone hole was, global action was swift and extremely effective. The Montreal Protocol, first signed in 1987 and amended a number of times thereafter, led to the banning of CFC production globally. 4 Other ozone-depleting substances, such as methyl bromide, once used to fumigate strawberry fields, have also since been banned. Ozone-friendly CFC replacement compounds are now widely used, with society barely noticing the transition. Yet, the lifetime of CFCs is so long -fifty to one hundred years -that significant ozone depletion continues over the Antarctic and more slowly at mid-latitudes. 5 Once the CFCs have been naturally cleansed from the atmosphere, ozone levels will hopefully return to those present on the first Earth Day. The enactment of the Montreal Protocol and the saving of the ozone layer has undoubtedly adverted millions of cases of skin cancer. An environmental success story indeed! T hroughout the 1970s, as stratospheric ozone loss became a cause for significant concern, ozone began to accumulate in the lowest layers of the atmosphere. Increasing ground-level ozone concentrations, first identified in Los Angeles and subsequently in other large cities around the world, resulted from chemical reactions between organic molecules (including gasoline fumes) and nitrogen oxides (emitted by car engines) in sunlight. The

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Earth 2020 resulting ozone caused significant damage to a variety of organic materials, from rubber tires and windshield wipers to people's breathing passages. Los Angeles, home to plenty of sunlight and automobiles, became the posterchild of photochemical air pollution, or 'smog', as it came to be known.
In response to the smog crisis of the 1970s and 1980s, California developed air pollution control strategies that are now widely adopted across the globe. Catalytic converters were added to automobile exhaust systems to remove organic and nitrogen oxide vapours, and internal combustion engines were designed to use gasoline that combusts much more efficiently, with computer-controlled tuning of air-to-fuel ratios. These measures have had a dramatic effect on air quality in Los Angeles and other major cities. Whereas the Los Angeles' automobile population has grown enormously from the 1970s, when the smog pollution was at its worst, ground-level ozone concentrations have dropped by roughly two thirds over the last forty years. In the 1970s, visitors to Los Angeles were surprised to find that the city is ringed by a range of mountains, which were infrequently invisible through the haze. Today, visibility is much improved. Additional progress will be made as gasolinepowered engines give way to electric and hydrogen-based vehicle propulsion systems.
Even so, ozone production will continue from organic precursor molecules derived from a variety of consumer products, such as paints, solvents, personal care products and indoor cleaning agents. 6 These sources, long overlooked, are currently unregulated, posing an on-going challenge for long-term air quality improvement. But it seems only a matter of time before these chemicals, like automobile exhaust and CFCs, will also be subject to strict environmental regulation.
The factors that led to urban smog in cities around the world also created additional atmospheric pollution problems. In the 1970s, forests and lakes were dying in northeastern North American and northern Europe as a result of acid rain produced from sulfur and nitrogen oxides released from coal burning and vehicle exhaust. In extreme cases, the acidity of rainwater approached that of vinegar, and this low pH precipitation was deposited onto land and water surfaces with devastating effects. The strong acidity had direct biological impacts on marine and terrestrial ecosystems, and a variety of indirect effects, including the leaching of toxic metals from soils. There was also another, very visible, manifestation Similarly effective action was taken to combat atmospheric lead pollution. The environmental toxicity of lead, from cookware to paint, has impacted human societies for millennia, and lead poisoning was famously suggested as a cause for the decline of the Roman Empire. But it was not until the twentieth century, when lead became widely used as a gasoline additive that the concentrations of this metal began to increase on a global scale through long-range atmospheric transport. As with CFCs and acid rain, the solution to atmospheric lead contamination was clear and remarkably effective. In a 1996 amendment to the US Clean Air Act, lead was banned from all gasoline products, and over the next two decades, human blood levels of lead dropped by more than 80%. U nlike CFCs and lead, air-borne particulate matter continues to be an important component of air pollution. 7 These small solid and liquid particles, much smaller than the width of a human hair, have serious health consequences when present in high abundance. This is particularly true for the smallest size class of particles, which are readily inhaled into the lungs. The landmark 'Harvard Six Cities' study, initiated in the 1970s, has continually monitored the mortality of people living in six American cities. 8 After correcting for occupational hazards and smoking rates, the study has shown a strong correlation between rates of excess mortality and high amounts of air-borne particulate matter. This Over the past five decades, human-derived sources of atmospheric particulates have increased dramatically. These anthropogenic particles are derived from both fossil fuel and vegetation burning, as well as specific industrial activities such as metal smelting.
The world's population is more urbanized than it has ever been, with over half of us now living in cities. The growth of megacities with populations of more than ten million people has been remarkable, with most of these cities in industrially developing countries.
These urban centers have extremely high air particle levels, resulting from the burning of dirty coal and agricultural wastes, widespread street cooking and the unregulated use of many commercial products, including small motorcycles without air pollution controls.
Moreover, indoor air quality remains a serious problem in millions of homes around the world in which cooking is still performed over inefficient stoves using wood, coal or dung fuels. As reported by the recent Global Burden of Disease study, the air quality in or near these homes is one of the leading causes of pollution-related death globally, particularly for women and children. 9 The implementation of better ventilation, more efficient cook stoves, and cleaner fuels are needed to address this global health problem. While ozone depletion, acid rain and urban air pollution are being addressed, enhanced global warming associated with atmospheric release of greenhouse gases -most notably carbon dioxide, methane and nitrous oxide -remains a larger, daunting challenge. Ice core records show that the concentrations of these gases in the atmosphere are significantly higher than at any time over the last 800,000 years, with a rate of increase that may be unprecedented in Earth's history. 10 Unlike CFCs or lead, whose industrial sources could be traced to specific sources (spray cans and leaded gasoline, for example), CO 2 emissions result from the combustion of all fossil fuels, from coal and oil to wood and natural gas.
For this reason, a reduction in CO 2 emission requires nothing less than a whole-scale transformation of global energy production systems.
The global warming challenge mirrors previous global air pollution threats. In the case of ozone depletion, lead, acid rain and smog, society recognized the central role played by key compounds -CFCs, sulfur dioxide, nitrogen oxides and particulate matter -and policies were put in place to successfully control these emissions. We can only hope that these previously successful approaches can provide a template for tackling global warming and transforming our energy supply network, with sound science and technological innovation tied to effective public policy. Though it has been suggested that carbon capture may be necessary to limit global warming to 1.5°C, we hopefully will not need to rely on other geoengineering schemes -such as injection of aerosol particles into the stratosphere to block incoming sunlight -to avert the most dire warming scenarios. Indeed, it is heartening to see the price for wind and solar energy rapidly dropping, to the point that these non-carbon energy sources are now economically competitive with fossil fuel energy in many places. If we apply the same focus and energy used to address air pollution issues over the past half-century, we can remain optimistic that the one-hundredth anniversary of Earth Day may see the atmosphere returning towards its pre-industrial character.
Earth 2020 Endnotes W hen people think about responding to climate change, they typically think about reducing emissions of carbon dioxide (CO 2 ) and other heat-trapping greenhouse gases. Had we started on a path to reducing these emissions at the time of the first Earth Day -when the science was already indicating that our emissions would cause global warming -then climate change might be behind us today. Instead, fifty years later, our collective emissions are higher than they have ever been. Cutting emissions is absolutely essential, but it is no longer sufficient. 1 We must transform our entire global energy infrastructure, not just to reduce our emissions of greenhouse gases, but to eliminate them altogether. That won't happen overnight, and even if we succeed in that challenge over the next few decades (which we must), there will still be substantial global warming. This is our new reality in 2020.
Because CO 2 remains in the atmosphere for a long time, reaching zero emissions won't eliminate climate change, it will just stop making the problem worse. 2 Like a driver careening towards the car in front of us, the first thing we must do is to take our foot off the gas pedal. But that alone won't necessarily prevent the damage. The next step is to apply the brakes -and quickly -to lessen the impending impact. And even then, we might need airbags to avoid the worst possible consequences.
Over the past several decades, as our failure to limit greenhouse gas emissions has become ever more apparent, there has been increasing interest in applying the brakes on 94 Earth 2020 global warming by removing CO 2 from the atmosphere after it has been emitted. This set of ideas, known as carbon dioxide removal (CDR) or negative emissions technologies, 3 includes 'natural' methods such as planting trees, or changing agricultural practices to store more carbon in the soil; artificially fertilizing the oceans to encourage phytoplankton blooms that consume CO 2 and sequester some of it in the deep ocean; chemically capturing CO 2 from the air through reaction with various minerals; or enhancing the rate of weathering of rocks, the natural process that will ultimately remove atmospheric CO 2 over the coming millennia. 4 The challenge today is that while many CDR approaches have promise, none of them currently satisfies three essential criteria.
First, carbon removal needs to be scalable. Each tree planted, for example, will ultimately absorb something in the order of 1 ton of CO 2 over the next forty years. By comparison, we are currently emitting nearly 1300 metric tons of CO 2 per second. There is roughly a trillion more tons of CO 2 in the atmosphere than there was at the dawn of the industrial revolution, and, if we ramp down to zero emissions over the next twenty-five years, we will have emitted half that amount again. There simply isn't enough available land for tree planting alone to solve the problem we've created. 5 There are similar scaling limitations on other carbon removal approaches as well, in particular those that most closely mimic natural ecological processes.
Second, carbon removal needs to be reasonably economical. While planting trees might be relatively cheap, the current projected costs for more globally scalable Clearly, such unintended consequences must be factored into any future considerations.
With further research and development, there are carbon removal approaches that, when implemented together, might avoid all three of the challenges above. But at the same time, it would be foolhardy to assume that these approaches to CO 2 reduction can be relied on with certainty to avoid future climate change. And it would be even more unwise to continue to emit CO 2 today on the assumption that our children and grandchildren can figure out how to remove it.
We are thus left with no certain pathway to avoid serious climate change impacts. So, we cannot achieve this additional reflection by doing things like painting roofs white; there just aren't enough roofs. There are, however, at least two proposed approaches that could plausibly reflect enough sunlight to significantly influence global climate.
One such approach would mimic the cooling effect that occurs after large volcanic It is, in principle, possible to deliberately mimic this process of solar reflectance (without all of the ash and other negative impacts of a volcanic eruption). The stratospheric-aerosol approach would cool the planet, and would thus counteract many -but not all -of the impacts of climate change. We don't currently have aircraft that fly high enough with the capacity to deliver a useful payload, but these engineering challenges appear surmountable.
In fact, one of the concerns with this idea is that the direct costs might be low enough to make the idea more enticing than it should be!
Another solar geoengineering idea is to enhance the formation of reflective low clouds over the ocean. Satellite imagery reveals that ships in some parts of the ocean leave behind 'cloud tracks' that can persist for up to a week. This phenomenon occurs when aerosol pollution from the ship enhances the formation of cloud droplets, either creating a cloud where none previously existed, or making more, smaller droplets that make existing clouds 'brighter'. In either case, the result is the same; more sunlight is reflected back to space.
Achieving this effect does not necessarily require adding pollution; spraying salt water into the right type of clouds might be sufficient.
Spraying salt water into clouds may be more benign than adding sulfate to the stratosphere, but we don't understand the physics of cloud-aerosol interactions well enough to know how well this approach might work. Cloud brightening also comes with its own set of issues. While stratospheric aerosols spread roughly uniformly across the globe, marine clouds that can be brightened might only exist over about 10% of the Earth's surface.
Achieving the same global cooling effect through cloud enhancement would require much larger changes over smaller areas, resulting in potentially significant impacts on regional weather patterns.
Beyond any technical challenges of solar geoengineering, there are other significant questions to be addressed, from the details of its physical impacts, to broader societal issues such as public acceptability, ethics and international relations. For example, both cloud-brightening and the introduction of stratospheric aerosols have the potential to change precipitation patterns. Climate models suggest that these precipitation changes will typically be smaller than those we would experience if we allowed climate change to grow without geoengineering. But that might not be true everywhere, and there is still considerable uncertainty in model predictions. In addition, stratospheric aerosols could delay the recovery of the ozone layer through their interactions with the long-lived chlorine compounds (CFCs) that were phased out by the 1987 Montreal Protocol. 9 And what goes up 98 Earth 2020 must come down -so there may be ecological impacts as sulfate aerosols are eventually returned to Earth's surface in the form of acid rain (though the amount of acid rain would likely be a small increment over today's background levels). We simply don't know enough today to adequately inform future decisions. More research might uncover reasons why geoengineering would always be a bad idea, or might conclude that the consequences of not deploying these approaches outweigh these concerns.
More challenging still are the societal and governance questions. 10  Land-based glaciers and ice sheets develop when winter snowfall persists through successive summers, building up and compacting under its own weight into frozen layers that may be hundreds and sometimes thousands of meters thick. Inputs of snow on the ice-sheet surface are balanced by losses in the form of basal melting and iceberg production at ice-sheet marine margins and, in some milder areas, by surface melting and water runoff. Because of their origins from snow, glaciers and ice sheets contain fresh water. In total, about 70% of the planet's fresh water is presently locked away in these glaciers and ice sheets.

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Earth 2020 Today, the great ice sheets of Antarctica and Greenland cover areas of 13 Compared to land-based glaciers and ice sheets, sea ice is much more variable in distribution and thickness over annual cycles. In the polar oceans, the sea-surface freezes each winter to produce ice that extends over about 15 and 19 million km 2 of the Arctic and Southern oceans, respectively. Unlike glaciers and ice sheets, much of this sea ice is shortlived, with a large portion of it melting each summer to give a minimum extent of about 4 to 5 million km 2 in the Arctic and approximately 3 million km 2 in the Antarctic. The edge of the sea ice retreats poleward as the summer proceeds, with protected fjords and inlets often being the last to become clear of ice. As a result of the seasonal cycle of ice growth and melting, sea ice is usually only a few meters thick at most, as compared to hundreds or thousands of meters for glaciers and ice sheets.
A third type of ice is permafrost, which occurs in polar and high-mountain areas where the ground is permanently frozen to depths of ten to hundreds of meters. In summer, ice in the upper meter or so of the soil matrix melts to produce a soft 'active layer', which refreezes again each winter. Permanently frozen ground occupies vast areas of the Arctic beyond the margins of modern glaciers and ice sheets, including much of northern Earth's climate has varied during the Quaternary with a periodicity of about 100,000 years. Each 100,000 cycle can be broken down into colder (glacial) and warmer (interglacial) intervals, and the last few of these cycles are recorded in the ice itself, most notably in an Antarctic ice core over 3 km long. The sequential depth-layers of this ice core contain a frozen archive that preserves the recent climate history of Earth, including temperature and greenhouse-gas concentrations going back about 800,000 years. 3 From this record, we know that the warm period in each glacial-interglacial cycle is typically much shorter than the cold phase, making up no more than 10-20% of the cycle. Today, we are in the most recent interglacial period, which started when the Earth began warming after the water that absorbs much greater amounts of solar radiation. Computer models suggest that this 'ice-reflectance effect' (sometimes known as ice-albedo effect) will continue over the coming decades, amplifying ongoing climate warming. The exact trajectory of that warming, estimated at between less than 2° and about 5°C by 2100, will depend on the future evolution of our economic, industrial and agricultural activities, and their impact on atmospheric greenhouse gases. 8 Over the past four decades, the availability of comprehensive satellite-based measurements has radically changed our understanding of global ice distributions. Today, we know that many glaciers around the world, and parts of the massive Greenland and Antarctic ice sheets, are already thinning and retreating as a result of atmospheric and ocean warming. Since the first Earth Day, half a century ago, glaciers in many Arctic and mountain areas have thinned by tens of meters and undergone kilometers of retreat. 9 Furthermore, the Greenland Ice Sheet has shown a clear trend towards increased melting and mass loss since the turn of the twenty-first century, with almost the entire ice-sheet surface subjected to melting in some recent summers. 10 Summer melting is now also commonplace at lower elevations in parts of the western Antarctic Peninsula, and even the 2 million km 2 West Antarctic Ice Sheet is affected, with thinning and retreat detected using very accurate satellite radar and laser altimeters. 11 Land-based glaciers and ice sheets hold huge volumes of water, which can be released into the ocean. The loss of mass from glaciers and ice sheets thus has enormous implications for global sea level change, which affects low-lying communities world-wide. 12  scenarios, respectively. 13 In either case, many millions of people will be displaced globally.
A second area of major concern is the decline in Arctic summer sea-ice extent, which has been monitored systematically by satellites over the past forty years. 14 September

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Earth 2020 sea-ice minima have declined from around 7-8 million km 2 to values often less than 5 million km 2 over this period. For perspective, this reduction in sea-ice surface area is roughly equivalent to the land mass of India. This decline is set to continue due to icereflectance feedback, and computer models predict that the Arctic Ocean will be largely devoid of summer sea ice within a few decades. 15 The loss of Arctic sea ice will exert a major influence on Arctic marine ecology and the humans that depend on it, with significant geopolitical implications linked to new shipping routes and resource exploration potential.
There is another possible, and somewhat paradoxical, consequence of sea-ice decline in a warming Arctic, which is related to the effects of sea-ice formation on ocean-circulation patterns. 16 When sea ice forms, salts from seawater are rejected from the forming crystals and released into the underlying surface waters, producing very cold and salty water masses that sink to the ocean depths. In the Labrador Sea off the coast of Greenland, this deepwater formation forms one branch of a large 'ocean conveyer belt' that transports heat and nutrients southward at great depth in the North Atlantic. The upper portion of this circulation is the northward return flow of warm Gulf Stream water in the top 1,000 m or so of the North Atlantic. If the formation of deep water slows or even stops (as appears to have happened more than once in Earth's geological history), the Gulf Stream and its northward transfer of heat will also slow. This, in turn, would lead to the somewhat counter-intuitive cooling of North West Europe, or at least to a reduced warming trend. 17 The huge areas of permafrost covering much of the Canadian and Eurasian Arctic are also vulnerable to warming. Although more difficult to measure from satellites than changing glacier extent and thickness, it appears that permafrost is responding to the enhanced Arctic warming of recent decades. 18 In most permafrost areas, ground temperatures and the rate of degradation in permafrost thickness and extent have increased over the past twenty to thirty years. This summertime melting has created challenging conditions for travel on the unstable ground, and has also begun to destabilize some built structures, such as houses and pipelines. Of potentially wider significance, a deepening of the biologically active upper layer of permafrost will increase the rate of organic matter decomposition in the soil, releasing methane to the atmosphere. 19 Methane is about thirty times more potent than carbon dioxide as a heat-trapping or greenhouse gas, 20   In our new and powerful role over the planet, we have now become capable of engineering our own demise. We must come to fully appreciate the long-term consequences of our collective and individual actions. My hope is that these pictures will stimulate a process of thinking about something essential to our survival, something we often take for granted -until it's gone.    of ice that sits below the ocean surface was already committed to collapse over the next couple centuries. Today, collapse is clearly underway in multiple sectors of the ice sheet. 2 Fortunately, the planet's two other major ice sheets are so far proving more stable.
The East Antarctic holds 53 m sea level equivalent of ice; of this, 19 m sea level equivalent sit with their base below the ocean surface and are potentially vulnerable to the same instabilities playing out in West Antarctic. But so far this ice sheet has only shrunk by a few centimeters and does not seem in imminent danger of collapse. The Greenland Ice Sheet contains enough water to raise sea level by 7.4 m. While geological records from past warm periods suggest we may already have warmed the planet enough to lose a substantial chunk of this ice sheet, it currently appears that its loss will take many millennia. 3 While the ice sheets are the major (and most visible) driver of sea level change today, they aren't the only important factor. Indeed, as of the fiftieth Earth Day in 2020, they weren't even the dominant one. From 1993 -when the first satellite providing global sea level observations was launched -to 2020, global average sea level rose by about 8 cm. Of that 8 cm, about 40% was due to the thermal expansion of ocean water as it warmed, and another quarter to melting mountain glaciers. 4 The remaining 35% was due to accelerating ice losses from both Greenland and West Antarctica. 5 That was the global story. Around the world, however, for a variety of reasons, different places experienced different rates of sea level change. For one thing, surface winds and ocean currents are important drivers of local sea level changes -indeed, the dominant driver on a year-to-year basis. Other factors also come into play. Over the twentieth century, many inhabited river deltas, such as the Mississippi Delta in Louisiana, experienced sea level rise several times greater than the global average. In these areas, which rest upon loosely consolidated sediments, the weight of the sediments can lead to a sinking of the land surface, and thus a relative sea level rise -a process accelerated when humans pump water, oil or gas out from between the sands. Conversely, other areas -such as parts of  And we may see even more dramatic changes over the longer term. Computer models and geological records of past warm periods suggest that 3°C of global warming would lead to about 10 m of total rise over the next two millennia if the planet were left to its own devices. 11 Such a drastic increase in global sea level would flood 2.6 million km 2 of habitable land surface -an area currently home to over 10% of the global population.
But it looks increasingly unlikely that the planet will be left to its own devices. While mid-century proposals from India and the Alliance of Small Island States to engineer the planet's climate with stratospheric aerosol pollution seem to have been quieted by threats from China's risk-averse leadership, efforts to artificially remove carbon dioxide from the atmosphere have seen rapid growth in the last decade. 12 It's quite possible that, by Earth Day 2120, the rate of deliberate removal of atmospheric carbon dioxide will match the mid-century rate of human emissions. If such efforts can be sustained, Earth's temperature may cool back to near its pre-industrial levels by the first half of the twenty-third century, with global sea level rise slowing to a more measured pace by the twenty-fourth century. It can also include softer infrastructure, such as periodically replenished beach dunes.
Oyster reefs and salt marshes also provide substantial protection against waves, although they are generally less effective in protecting against longer-lasting storm surges and tides. A key to the success of the CAP process is that it is not imposed by the federal or state government; rather, these higher levels of government participate in a supporting role, providing funding for the process and incentives for participation, as well as identifying 148 Earth 2020 (and occasionally removing) barriers that might prevent options from playing out. Often, public universities -generally much more highly trusted than federal or state government, especially early on in the CARCAA era -play critical roles both as conveners and as sources of expert knowledge. The CAPs demonstrated that community voices could have real impact. They showed that communities, universities and higher levels of government working together could limit some of the damage of climate change's increasingly severe effects, and even create beauty in new public works.
Even so, efforts to cope with intensifying coastal flooding around the world have been  A rereading of the Brundtland Report, more than three decades after its publication, is a rather poignant reminder of the pioneering work, and even optimism, of this Commission as it grappled with the major social, economic, environmental and geo-political issues of the day, while envisioning a different future. Yet the authors also presented a clear warning that 'the time has come to take the decisions needed to secure the resources to sustain this and coming generations'. The first IPCC report appeared in 1990, just a few years after the publication of the Brundtland Report. Together, these two documents were a significant driving force for the 1992 United Nations Conference on Environment and Development (the Rio Conference). It was here that the United Nations Framework Convention on Climate Change (UNFCCC) was born, as part of a package of measures for the twenty-first century, including the Rio Declaration on Environment and Development, 8 Agenda 21,9 the Convention on Biological Diversity (CBD), 10 and the Forest Principles. 11 The Preamble to the UNFCCC 12 contains the following principles, which resonate with the underlying norms of International Law as well as sustainable development: that the Earth's climate and adverse effects are a common concern of humankind; that the greenhouse effect will warm Earth's surface and atmosphere and adversely affect natural ecosystems and humankind; that there is a need for an appropriate international response in accordance with common but differentiated responsibilities; that developed countries have a 154 Earth 2020 historical but also current responsibility for their emissions, while emissions originating in developing countries will need to grow in future; that developed countries should take immediate action to develop comprehensive strategies; and that responses to climate change should be coordinated with social and economic development. Pertinently, the Parties acknowledged that low-lying small island developing states, and other developing countries prone to floods, drought and desertification, are particularly vulnerable to the adverse effects of climate change. The ultimate objective of the UNFCCC was to achieve stabilization of greenhouse gas (GHG) concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system. Such a level was to be achieved within a timeframe (not clearly articulated in 1992) sufficient to allow ecosystems to adapt naturally to climate change and to ensure sustainable food production and economic development. The first step towards legally-binding GHG emissions targets was the 1997 Kyoto Protocol, whereby developed countries agreed that overall emissions would be capped at 5% below 1990 levels by the end of 2012. 14 Developing countries were not required to meet any targets, and this was seen by some nations as a major point of contention.
With the early focus on reducing GHG emissions in the mid-1990s, there was a view that identifying climate change adaptation options would be tantamount to accepting the reality of climate change -at a time when the science was more tenuous than it is now.
Developed countries were also concerned that accepting the need for adaptation amounted to an implicit assumption of responsibility, with the associated duty to compensate. At the same time, many developing countries were reluctant to discuss adaptation lest it derail developed country commitments to mitigation. 15 But as the science became clearer, and the failure of global efforts to reduce GHG emissions increasingly apparent, more attention shifted towards adaptation. At the Cancun negotiations in December 2010, the Parties to the UNFCCC established the Cancun Adaptation Framework, 16 in which Parties were requested to start making assessments of their vulnerability to climate change, plan adaptation actions, strengthen institutional capacities, build resilience and enhance their climate-related disaster risk reduction strategies.
By 2013, following the IPCC's Special Report Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation (SREX), 17 it had become apparent that many extreme weather and slow onset events were linked to a warming climate. Based on the best available science, the Parties established the Warsaw International Mechanism for Loss and Damage associated with Climate Change Impacts (the Mechanism), 18 under the Adaptation Framework. The Mechanism acknowledged that the loss and damage associated with climate change impacts cannot all be reduced by adaptation. 19 The Mechanism called on countries, amongst other things, to: undertake impact, vulnerability and adaptation assessments; 20 engage in climate resilient development, 21 enhance climate change disaster risk reduction; 22 and understand and cooperate on Climate Displaced Persons, migration and planned relocation at the national, regional and international levels. 23 E ven with the growing discussion around climate adaptation strategies over the past decade, there has been continued, if faltering, discussion of mitigation through control of greenhouse gas emissions. The most recent instalment, drafted in 2015 and signed in 2016, is the Paris Agreement, 24 which committed Parties to limit the increase in global average temperature to well below 2°C above pre-industrial levels, and pursue efforts to 156 Earth 2020 limit the temperature increase to 1.5°C. 25 For the first time, both developed and developing country Parties must prepare, communicate and implement successive voluntary nationally determined contributions (NDCs) that will be implemented through domestic mitigation measures. New NDCs must be communicated every five years and be informed by a Global Stocktake of emissions, starting in 2023. 26 Each successive NDC must represent a stronger target than the previous one, and developed countries are still expected to take the lead by undertaking economy-wide absolute emission reduction targets.

One of the important accountability mechanisms for Parties is the Enhanced
Transparency Framework, which requires developed and developing countries to report every two years on progress towards meeting their emissions reduction targets. The information provided will be subject to a technical expert review, which will identify potential compliance issues and areas for improvement. A disappointing feature of the Paris Agreement is that it does not provide a basis for any liability or compensation for the impacts of climate change. However, a Task Force on Displacement was established to deal with the millions of people who will ultimately be displaced as a result of climate change.
Some believe that the Paris Agreement may be our final curtain call. Indeed, the   The development of early warning systems for extreme weather events has become possible in recent years because of improved weather forecasting capabilities. Today, in 2020, national weather services can forecast temperatures 5-6 days in advance more accurately than their predecessors in the early 1970s could forecast a single day in advance.
These improvements have come from increased computing power, which has allowed ever more complex and realistic mathematical simulation of the atmosphere, and the increased availability of satellite observations to drive and refine model predictions.
Thanks to these greatly improved weather forecasts, bureaucrats, politicians, the media, medical and emergency services and the public now have several days to implement strategies to minimize deaths from high temperatures and other extreme weather events.
As global warming increases the frequency and intensity of heat waves, these alert systems can reduce some of the likely human cost of climate change.
Increased monitoring and forecasting skills allow us to observe the occurrence of a wide variety of weather extremes in addition to heat waves, and to predict their future trajectories. Extreme cold events also cause many deaths and illness in many parts of the world. Cold events have been decreasing in frequency and intensity in countries around

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Earth 2020 the world, as another consequence of global warming. Nonetheless, cold events still occur, but these events can now be predicted days in advance, allowing us to reduce their health impacts. The combined effect of global warming, which reduces the frequency of cold events, and the improved forecasting of these events, will continue to reduce their deleterious effect on the human population.
Although the changing frequency of hot and cold extremes is very clear over the past fifty years, patterns in other extreme weather events are more difficult to identify.
In some cases, this may reflect the absence of any real change in frequency of these events. But in other cases, it may be that changes in the way we observe particular weather Droughts are also changing as a result of global warming. Global warming has meant that droughts today are accompanied by higher temperatures than they would have been fifty years ago, and this trend towards warmer droughts will continue into the future. In some areas, there is also evidence that droughts have become more frequent (although this is not universal). Whether droughts in the future will be drier or longer-lasting will depend on the regional impacts of global warming on atmospheric circulation, which we cannot currently predict with confidence. It is even harder to predict how floods will change in the future, because of the complex factors at play. Over the past several decades, there has been an increase in intense precipitation events associated with global warming, and we can expect such changes to continue into the future. But the extent to which this increased rainfall will lead to flooding will depend, among other things, on alterations in the land surface (such as increasing road surfaces) and changes in riverbanks and drainage systems. Once again, our improved ability to monitor heavy rainfall events, using radar and satellites, has improved our ability to provide more timely forecasts of What about the smaller scale weather extremes, such as hailstorms and tornadoes?
These short-term events are notoriously difficult to monitor, with historical records of such extremes relying heavily on subjective reports from observers. Such reports are, in turn, dependent on population density, amongst other factors. Thus, an increasing population in an area, for example, might lead to increased reports of hailstorms, even if the actual frequency is not changing. Disentangling such reporting biases from any real climate-forced change in these small-scale extremes is beyond our capabilities at present.
We are thus left with little confidence in any apparent trends in these extremes, even over recent decades. Nevertheless, improved systems for detecting these small-scale extreme weather events, and our increased ability to issue and distribute short-term forecasts of their movement, have begun to allow populations to avoid some of the associated damages.
Continued improvements in these monitoring and forecasting systems should help to further reduce the associated damage of short-lived extreme weather events, even if global warming increases their frequency or severity.
T he past half-century has seen substantial changes in the frequency and intensity of some extreme weather events. But these fifty years have also seen advances in our ability to monitor and predict these extreme weather events (and others), thereby reducing the associated human impacts. In particular, meteorologists have vastly improved their ability to predict extreme hot and cold days, storms, bushfire weather and cyclones several days in advance. These improvements have led to the development of alert systems that have reduced the loss of life previously caused by such extremes. More can be done to improve these forecasts, and their public dissemination and use in communities. At the same time, seasonal climate forecasting has developed from a pie in the sky idea into a well-developed science, at least in some parts of the world and for some seasons. With this new tool, we can now predict some droughts and seasonal tropical cyclone activity, well in advance, providing opportunities for longer-term planning and disaster-reduction strategies.
The 2019-2020 Australian bushfire season provides a case in point. with the challenge of assuming greater responsibility for an Earth whose complex dynamics elude full understanding, and whose very capacity to sustain human life is seen by many as gravely threatened.

Earth 2020
In the rapidly industrializing nineteenth century, the urge to protect nature arose in an acutely emotional register; a sense of irreparable loss as nature's tranquil beauty was ravaged by the smoke, filth and noise of the machine age. Only gradually did people learn that producing goods on mass scales not only did violence to pristine landscapes, but also harmed the health and wellbeing of all living things and the ecological and If it takes a village to ensure the well-being of young children, then it is hardly surprising that it took a massive, collective effort to establish the scientific facts of climate change. Since

1988, that work has been led by the Intergovernmental Panel on Climate Change (IPCC), a body created by the UN Environment Program and the World Meteorological Organization
to assess the mountains of data on Earth's changing biophysical systems, and to clarify the nature and severity of those changes in relation to human well-being. Divided into three working groups -on science, impacts and policy -the IPCC has always insisted on its political neutrality. Its work, the IPCC repeatedly asserts, is policy-relevant but not policy- prescriptive. Yet, it soon became apparent that policy neutrality could hardly remain a realistic option if the IPCC's claims were to be taken at face value. Since its first Assessment Report (AR) in 1992, the IPCC has issued a total of five ARs (a sixth is in the offing), and many additional special reports on specific effects and assessment methods. The basic conclusion If genuine reductions in GHG emissions are to be achieved, humanity will need to harness not only science and technology, but also its collective moral will. The astronomer and gifted science popularizer Carl Sagan offered a foretaste of that thought in his 1994 book, The Pale Blue Dot. 9 The floating images of Planet Earth brought home to Sagan the smallness and isolation of human existence. There is no sign in the vastness of space that humanity's salvation will come from anywhere else other than Earth and its human in the next half-century will have to build on the understanding that science and planetary stewardship are co-produced. Inevitably, the politics of the Anthropocene will also have to be a politics of precaution. In our early fishing days, energy was provided by the muscles of fishers, and later by the wind. Even with sophisticated sailing vessels, the energy density that could be used for fishing was limited, and ultimately dependent on sunlight, which grew the food that fishers ate and fueled the winds that powered sailing vessels. This changed radically in the 1880s,  It wouldn't be long, however, before signs of trouble began to appear.
The first collapses occurred in the California sardine fishery (of 'Cannery Row' fame) in the early 1950s, and the Peruvian anchoveta fishery in 1972, then the largest fishery in in the world. Both these collapses could be attributed to natural oceanographic processes, including, strong El Niño conditions in the case of Peru, which warmed surface waters and reduced the supply of nutrients for plankton. In contrast, the collapse of the Norwegian spring spawning herring in the 1960s was the first to be linked directly to overfishing.
The most blatant case of overfishing, combined with the active suppression of the voices that warned about an impending collapse, would come about twenty years later, with the demise of the Northern cod fishery of Canada. Within just twenty to thirty years, giant trawler vessels reduced the cod population to less than one percent of its historical abundances, which had consistently yielded annual catches from small vessels and traps ranging between 100,000 and 200,000 tonnes over the previous five centuries. 6 And yet, when the fishery was closed in 1992 resulting in the loss of 40,000 jobs, Canada's fishery managers blamed the seal predators and a cold winter.
Other collapses, large and small, began to accumulate in various parts of the world, and, in 1996, the global catch trend began to decline for the first time since the post-war years.
In other words, 1996 was the year of peak global catch, and catches have been dropping ever since, despite increasing fishing effort. Other impacts of fishing on fish populations also became apparent. Most notably, fishers began to see a change in the kinds of fish being caught, as large predatory fish on top of marine food web (e.g., cod, swordfish and tuna) became increasingly scarce, while smaller forage fish, such as herrings, sardines and anchovies, began to make up an increasing proportion of total catches. This 'fishing down the food web' 7 had a significant impact on ecosystem functioning, and also on ocean sea floor habitats, which became increasingly affected by the use of trawls and other bottomimpacting gear. This led to a decrease in the resilience of both fish stocks and marine ecosystems.
The 'fishing down' concept and the development of ecosystem-wide thinking began to broaden fisheries management to include the ecosystem effects of fishing in general.

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This drew attention not just to the quantity of fish taken by fisheries, but the total removal of marine living organisms from ecosystems, and the impacts of various fishing gear on their habitats and biodiversity. And as fisheries and marine science began moving away from a narrow focus on the maximum catch that could be taken from marine ecosystems, fisheries economics, management, governance and policy were also having their moments During the 1970s, TACs became increasingly adopted around the world, but it did not take long before it became clear that this approach, on its own, was insufficient to protect fish stocks. For one thing, appropriate TACs, based on sound ecological understanding of fisheries populations, were difficult to set, and even more difficult to enforce. Second, even if  Although things look dire, there is a path forward to a better future. The best fisheries science available shows that the decades-long global decline in fish catches could be reversed if the world's maritime countries reduced the fishing effort in their exclusive economic zones. This, together with the removal of harmful subsidies, 11 the elimination of illegal fishing, a greater emphasis on future benefits and the closure of the high seas to fishing, would allow the fish to rebuild their abundance, and allow for higher catches than at present. The problem is that either the politicians do not accept the results of fisheries science (as is sometimes also the case for climate science), and/or are unable to stand up to the industrial fisheries lobby, to which they have largely ceded the exploitation and quasi-ownership of public marine resources. Unless things change, largely uncontrolled industrial fishing is likely to continue until the bitter end -whatever that may be. Add marine pollution to this (including plastic) and the multiple ocean stressors generated by climate change (sea surface temperature rise, ocean acidification and deoxygenation), 12 and the future might look rather bleak. We desperately hope that the world takes suitable action on multiple fronts to chart a different path forward. for a single purpose (think a desk, or book), are being replaced by 'smart materials' with multi-functionality. This revolution has led to spectacular combinations of the elements from across the periodic table into modern materials and gadgets, with capabilities well beyond what was imaginable even a generation ago. As a result, we are now faced with tens of thousands of chemicals used in our homes and in our everyday products. For the vast majority of these chemicals, we have only a limited understanding of how they will behave once released into the environment.
A major problem is that many synthetic chemicals are persistent in nature, meaning they do not readily break down following release into air, water or soils. This persistence allows the chemicals to accumulate in the environment, and to be transported long distances from their original sources if they are released to air or water. Heavy metals occur naturally on Earth, but their abundance in environments where they are likely to accumulate in organisms has increased dramatically due to human activities. For example, cumulative anthropogenic releases of Hg over the past 500 years have been fifteen times higher The deadliest example of such impact was the gas leak and explosion that occurred in December 1984 at the Union Carbide pesticide plant in Bhopal, India. Over 500,000 people were exposed to methyl isocyanate gas, and more than 2,000 eventually died from acute symptoms. The ground hugging deadly fog was a powerful image of the toxic effect of pesticides, but even more dire was the contamination of soils and groundwater, which led to an estimated 15,000-20,000 premature deaths in the subsequent two decades, painting a dark picture of the long-term effects of chemical exposures. Over the past several decades, we have learned that exposures to a wide range of anthropogenic chemicals are associated with diverse deleterious health outcomes. There are critical windows of vulnerability to chemical exposure -such as the developing fetus during the third trimester of pregnancy, when the brain is developing most rapidly, and during the first several years of life when the body's immune programming is taking place.

In their landmark 1996 book, Our Stolen Future, Theo Colburn, Dianne Dumanoski and John
Peterson Myers brought together large amounts of scientific data linking declining human fertility with a rise in exposure levels to estrogen-like structures present in many common synthetic chemicals. 9 They put forward the so-called 'endocrine disruptor' hypothesis, arguing that hormone-like synthetic compounds were taking a heavy toll on humans and wildlife, interfering with the organism's natural chemical signaling pathways. We now also know that exposure to various forms of arsenic (As), PFAS and polychlorinated biphenyls Nixon. Yet, despite its mission to 'protect human health and the environment', 12  Unfortunately, however, PFAS have become the latest example of chemical whack-a-mole; one compound is phased out, only to be quickly replaced by another whose environmental properties and health consequences are largely unknown. The same game of chemical whack-a-mole has been played for different brominated flame retardants and plasticizers such as bisphenol-A in water bottles and other products. Each banned chemical is replaced by new compound that is initially assumed to be safe, but later discovered to be a regrettable substitution and problematic in its own right. has developed a computational toxicology screening tool known as 'ToxCast' that uses high throughput screening assays to understand potentially adverse impacts of exposure for living organisms. 15 Use of this and other emerging screening tools would be simple and inexpensive prior to widespread use of chemicals in commerce.
The path toward sustainability in chemicals management is achievable. Tom Graedel from Yale's School of Forestry has shown that tracking the use of chemicals from manufacturing to disposal can improve conservation and optimize material flows. 16 Similarly, the movement towards a 'circular economy' has demonstrated to manufacturers that eliminating chemical releases through reuse rather than disposal can be profitable,   land surface is used for growing crops or grazing animals. 2 By comparison, urban areas make up less than 5% of Earth's surface, yet this small area is home to over half the human population. 3 Human influence is also evident in much of the remaining lands that have not been cleared, with estimates suggesting that more than three quarters of the world's lands bear the footprint of our species. 4 Earth 2020 To fully appreciate human impacts on Earth's surface, we can look to past drivers of global land use change, as we work to shift our practices towards a more sustainable future. 5 During the first stage of human history (the Paleolithic age, between about two million and ten thousand years ago), the use of stone tools and the control of fire enabled humans to migrate from their origins in East Africa to Eurasia, Australia and the Americas. The use of fire by Palaeolithic hunters changed landscapes and was also partly responsible for the extinction of megafauna. Observations of human-induced landscape burning can be traced to antiquity, as in a Carthaginian reference to western Africa around 500 years BC: '…during the day, we saw nothing but forests, but by night many burning fires … we saw the land at night covered with flames. And in the midst there was one lofty fire, greater than the rest, which seemed to touch the stars'. 6 The The Green Revolution has been deemed a massive success by some scholars and policymakers, considering the overall increase in total global crop production. 14 Indeed, total calories produced increased more than 30% from 1961 to 2013, reaching an average of 2884 daily kcal per person, which is more than enough to meet the average minimum daily requirements of every person on Earth. 15   All in all, more than two billion people across the world remain malnourished, even as the expansion of global agriculture is arguably the single most important driver of global environmental degradation. One of the key United Nations Sustainable Development Goals is to 'End hunger, achieve food security and improved nutrition and promote sustainable agriculture'. 18 How do we reduce food insecurity, feed the additional two to three billion people of the future, lower the environmental footprint of agriculture and make it more resilient to climate change -all at the same time?
This is, no doubt, a daunting challenge, but a number of wide-ranging solutions are now on the table. Many scholars recognize the need for continued increases in crop yields, particularly in areas characterized by high poverty rates with limited agricultural infrastructure such as irrigation, roads and markets. They call for sustainable intensification to produce more food at lower environmental costs, and argue that new agri-food technologies, such as precision agriculture and genetically modified (GM) foods are important components of this pathway. But others challenge this 'productivist paradigm', pointing out that increased food production is the wrong objective given the existing market failures that result in the poor global distribution of calories, and the fact that lower yields often reflect a lack of resources rather than technology. These scholars argue for a focus on food sovereignty, which advocates for growers and eaters to work together, along with scientists and the public sector, to develop regionally-adapted solutions for more equitable and ecological farming systems. 19 These systems employ agro-ecological methods like crop diversification and integration with animal agriculture to address soil nutrient deficiencies, while also contributing to dietary diversity and improved food security. of that year, I was invited to participate in 'Survival Day' -our neighborhood equivalent of Earth 2020 the second Earth Day. My brother, Tom, was a high school teacher, and he helped organize this first-ever, local environmental event. Speakers included distinguished representatives from advocacy organizations, Health Department officials, university professors, as well

as industry representatives from Allied Chemical Company, Bethlehem Steel and Niagara
Mohawk -the regional energy provider. As a college student, without any fancy title or Other CO 2 sampling sites were soon established at strategic locations worldwide, along with the creation of an international network of ocean weather ships, which, at its peak, included twenty-two Atlantic and twenty-four Pacific Ocean stations collecting oceanographic and meteorological observations. In the decades that followed, these long-term data sets would prove to be critical for detecting anomalous ocean conditions and for establishing baselines against which future ocean states could be compared.
In the decade that followed Keeling's early CO 2 measurements, scientific progress  The early ship-based oceanographic surveys and time-series stations were critical for providing important baseline observations. But given the vastness of the planet's oceans, these measurements could not even hope to cover all of Earth's marine waters.
Fortunately, just as ship-based oceanographic programs were ramping up, ocean science entered into the satellite age. In 1978, the coastal zone color scanner (CZCS) was launched on the Nimbus 7 satellite, providing the first dedicated imagery of ocean color, which was used to measure the concentration of photosynthetic plankton in marine surface waters.
Initially designed as a one-year proof of concept, the CZCS mission ran until 1986, and yielded unprecedented information on the spatial and temporal patterns of biological productivity across the oceans. Since that time, improved satellite remote-sensing of ocean color, temperature, salinity, wind, sea level, sea ice and other key environmental variables has revolutionized our understanding of oceanographic processes on regional-to-global scales. Beyond its effect on global temperature, increasing atmospheric CO 2 concentrations are altering the chemistry of the surface ocean in a way that is negatively impacting many marine organisms. To date, approximately 25% of the CO 2 that has been emitted by human activities has been absorbed by the surface ocean. 9 On the face of it, this CO 2 enrichment might be expected to benefit ocean life by stimulating marine photosynthesis.
The reality, however, is more complex. Perhaps most importantly, there is the problem of ocean acidification, which is a direct result of increasing ocean CO 2 levels, since dissolved CO 2 reacts with seawater to produce carbonic acid. A s we reflect on the past fifty years of ocean change, we must also look to the future. In the face of significant challenges, we can take solace, and perhaps even inspiration, from the diverse marine microbial assemblages that have thrived on our planet for billions of years. These microorganisms possess enormous genomic potential and metabolic flexibility, and this has provided them with resilience in the face of environmental change.
In the end, marine microbes will survive and adapt to climate change, although it is unclear how humankind will fare. Despite an ever-growing knowledge base concerning 210 Earth 2020 the sea around us built on observations, measurements and computer models, the ocean is still grossly under-sampled. Consequently, major uncertainties still exist regarding climate change impacts on the ocean and its inhabitants. Human influence on climate is indisputable and accelerating, and now, more than ever, we need to embrace a holistic view of the coupled Earth systems. Basic science is critical, but so too is fact-based education, aggressive advocacy for our planet and effective action.

Endnotes
Earth 2020 depletion that did not materialize. However, rather than generating any singular predictions, the computer simulations were conceived as a means of exploring many plausible if-then scenarios. Critics also commonly overlooked the fact that many model runs predicted overshoot and decline even when resources were assumed to be limitless. In these model runs, it was Earth's capacity to assimilate human wastes and emissions (represented as pollution in the model), rather than the supply of raw materials and fuel, that became the critical environmental constraint on the continued growth of the economy.
Today, nearly half a century after the MIT study, there is growing consensus that environmental pollution caused by wastes and emissions is of far greater concern than depletion of non-renewable resources. Not all extracted fossil fuels are burned, though. Today, 14% of oil production and 8% of natural gas extraction is used to make petrochemicals, such as plastics, fertilizers and a multitude of other chemicals. Petrochemicals production as a whole experienced enormous growth since the end of the Second World War, and the rise of plastics, in particular, is its most visible manifestation. As a mass-produced material, plastics are barely seventy years old. 5 In 1970, the year of the first Earth Day, global annual production of plastic polymer resins, fibers and additives was 37 million metric tons, or megatons (Mt).
In 2017, global annual plastics production had reached an astounding 438 Mt, an elevenfold increase in less than fifty years. By the end of 2017, humankind had produced a total of 9.2 Gt of plastic. That is the equivalent mass of 900,000 Eiffel Towers, or 88 million blue whales, or 1.2 billion elephants. If spread out ankle deep as low density plastic waste, it would cover an area the size of Argentina, the eighth largest country in the world. The growth of global annual plastic production has been so large and sustained that half of all plastic ever made by humankind was produced in just the last thirteen years. In other words, in just a little more than the past decade alone, we have doubled the total amount of plastic ever made.
While some might regard the global rise of plastic as a fantastic economic success story, others see an environmental tragedy. Many plastic products are short lived -plastic toys, household items, or fast fashion made from synthetics, for example. But it is packaging that has the shortest lifetime of all plastic products. Packaging accounts for around 36% of plastic production, most of which is used once and then disposed of. As a result, much plastic becomes waste soon after it was produced, and plastic waste generation can thus be expected to closely track plastic production. Unfortunately, solid waste generation data are much harder to come by than material production data -clear evidence that we consider the generation of solid waste an inconvenience, and treat it as an afterthought.
We love buying new things, in alluring and convenient packaging, but we also seem to expect that the old stuff will just disappear once we throw it into our garbage bins. This may have been true at some point in the past, when the majority of our trash would rot or corrode away. It is certainly not true for the plastics we have made so far, since they do not biodegrade on any reasonable timescale. In fact, all of the plastic we have made and did 216 Earth 2020 not burn, or otherwise destruct thermally, is still present on this planet. This is estimated to be 86% of the plastic waste humankind has thus far generated. An estimated 6 Gt of plastic waste is therefore present somewhere on this planet: in landfills, or open dumps, or in the natural environment. Another estimated 3 Gt is currently in use and will become waste as soon as we're done with it, which won't be long.
While we can estimate how much plastic currently resides on the planet, we do not know where exactly it is. Conventional plastic polymers don't biodegrade, but become brittle and disintegrate into smaller and smaller pieces, which then disperse in the environment as so-called micro-plastics. Wherever we look for plastic we find it. Plastic has been found in ocean creatures of all sizes and trophic levels, from plankton and seabirds to fish and whales. It's on the ocean surface, in the water column, and on the world's beaches, river beds and ocean floor, including its deepest point, the Mariana Trench, more than 11 km below sea level. Plastic has been found in arctic sea ice, in snow, rain, tap water, bottled water and beer. In the year 2010 alone, 5 to 13 Mt of plastic entered the world's oceans from land due to littering or mismanagement of plastic waste. 6 Terrestrial plastic pollution has so far received less attention than plastic marine debris, but we know that plastic is also everywhere in the soil. In fact, due to its ubiquity, plastic has recently been proposed as a geological indicator of the proposed Anthropocene, the period in which many geological The virgin plastics industry frequently states that using plastic is actually environmentally beneficial, since it replaces heavier and more impactful materials, including metal and glass.
This argument not only implies that the environmental impacts of plastic production, use and disposal are lower than those of alternative materials, but also that plastic is being used instead of these other materials. Unfortunately, global production of all humansynthesized materials has been increasing, so we're actually using plastic in addition to everything else, not instead of it. In the fifty years since the first Earth Day, global annual production of hydraulic cement increased seven-fold, while primary aluminum and crude steel production grew by factors of five and three, respectively. Along with our increasing use of various materials, solid waste generation in general is also increasing year over year. Producing and using more materials each year does not just mean that there will be more waste when these materials reach the end of their useful lives. Materials cause environmental impacts throughout their life cycles, such as ecosystem disturbance during 218 Earth 2020 extraction, and wastes and emissions all along their supply chains. What we throw into our garbage bins is just the tip of an ever-growing 'wasteberg'.
Material recycling, which has recently been repackaged as part of the 'circular economy', is unlikely to be a panacea. 8 Collection and reprocessing of solid waste into secondary material has its own environmental impacts. These impacts are typically much lower than those of making the same material from primary resources, like ores, but they are still significant. Recycling therefore only decreases environmental damage if it significantly reduces primary material production. So far this has not happened. The currently empty promise of recycling is perfectly illustrated by a petrochemical engineer, who stated that 'we passionately believe in recycling' while overseeing the construction of a brand-new, 'as big as you get' virgin plastic plant that will be fueled by abundant Marcellus Shale gas. 9 Another proposed solution that is not going to work on a global scale is the so-called 'bioeconomy', in which fuels and materials are made from biomass rather than non-renewable resources. Utilizing waste from agriculture and forestry for fuel and material production is certainly attractive, but there is nowhere near enough biowaste for a large-scale replacement of non-renewable fuels and materials. Instead, this would require vast amounts of dedicated crop production and thus agricultural land. It is now clear that the climate change and land use impacts of global food production are imposing significant stress on Earth's terrestrial ecosystems. 10 Imagine how much those impacts would increase if we produce biomass not just for our food and feed, but also to supply all our fuels and materials. To make things worse, some bioplastics and biofuels, such as polyhydroxyalkanoates (PHAs) and corn-based ethanol, have been shown to have greenhouse gas emissions that are similar or even higher than those of their fossil-based competitors. A global bio-economy would also massively increase other environmental impacts, such as eutrophication from fertilizer runoff, where excess of nutrients leads to harmful algal blooms and oxygen depleted 'dead zones' in water bodies.
A t this point, the only meaningful path forward will have to include substantial reductions in the amount of materials we produce and use, unless we are willing to see further increases of CO 2 in the atmosphere, plastic in the oceans, nitrogen in our estuaries and coastal waters and so on. It is telling when the CEO of Recology, a major resource recovery company, publishes a newspaper op-ed entitled 'It is time to cut the use of plastics'. 11 I have no doubt that a large reduction in our material footprint is compatible with a good life.
If anything, maintaining the latter will require the former, since the relentless growth of the global economy seems to be finally hitting the environmental limits of this big, but finite, planet. This brings me back to The Limits to Growth study, conducted almost fifty years ago. One final error of its critics is the belief that history proved it wrong a long time ago. But the standard scenario in the report made projections well beyond the year 2050, finding that global population would peak around that time and decline sharply thereafter.
It remains to be seen whether history will falsify this grim prediction. Perhaps we think of a gentle spring rain that nourishes wildflowers and newly planted crops. We may envision a clear lake with fish schooling just under the surface, or a rushing stream with migrating salmon leaping from the water. In wealthier countries or regions, we may think of filling a glass of water from the tap, or of our morning shower, often without realizing what a privilege it is to have safe water delivered reliably to our homes. We may also recall water being used to fight fires or for irrigation, transportation and hydropower.
Negative images may also come to mind, such as the environmental devastation associated with water pollution or its diversion for agriculture or hydropower. Or we might think of the torrential downpours that cause flooding and the loss of human life and property.
These examples illustrate that people across the globe can experience fresh water in vastly different ways.
Throughout history, water has always been of central importance for human welfare.
Each person needs about 80 L of water per day for drinking, cooking and hygiene (30,000 L per year) and about a factor of forty more (1. were also a concern in Europe. In 1986, a fire at a chemical storage facility in Basel near The first Earth Day did not need to focus on access to safe drinking water and adequate sanitation, since these problems had already been solved in industrialized countries. The to 45% for sanitation, and from 61% to 71% for drinking water. Even in the Least Developed Countries, access to safely managed drinking water increased from 25% to 35%. 6 The Achieving the SDGs will require that we find balances between our direct uses of One example that is both motivating and cautionary is the past success of industrialized countries in providing safe water supply and sanitation -the central objective of SDG 6. This success is the basis for today's conventional paradigm for water and wastewater management, which reflects the climate, geography and historical development of Western Europe and North America. In this paradigm, water is used to transport waste, and the rapid conveyance of storm water away from urban areas is prioritized to prevent flooding. Systems are highly centralized, and water supply and wastewater are strictly separated. In water-scarce regions of LMICs, however, using water for waste conveyance is not a practical use of a scarce resource, and the required investment in sewers would be prohibitively expensive. An attractive alternative is to reclaim water, energy and nutrients from wastewater (or from fecal sludge). This approach, which relaxes the strict distinction between water and wastewater, is also being taken up by industrialized countries as they move toward circular economies and replace aging infrastructure. Looking to the future, it seems that the fully centralized model of industrialized countries may not be the dominant, much less the only, way forward.
Infrastructure investment provides another example where caution should be exercised in emulating the past successes of industrialized countries. Past infrastructure investments for water conveyance and storage increased water security and, through hydropower production, energy security. Today, LMICs face massive infrastructure deficits, which call for trillion-dollar investments. 9 Thousands of major hydropower dams, with capacities of at least 1 MW, are in the planning or construction phase with most sited in LMICs. 10  agricultural fertilizers into coastal waters has resulted in the formation of oxygen-deficient dead zones extending over more than 10,000 km 2 .
In some LMICs, point-source pollution is accompanying rapid industrial development.
This is a particular concern with the expansion of chemical and pharmaceutical production in the Asia Pacific. 15 In rapidly growing economies, regulatory controls on the discharge of industrial effluents are often inadequately enforced. Import of single-use and poorly The diverse challenges related to water use across the globe call for locally-appropriate, sustainable balances between meeting direct human needs for water and preserving the capacity of the water environment to provide ecosystem services. Location is critical because water issues arise from local conditions (such as climate and past infrastructure investment) and viable solutions to water issues can only be implemented by people and institutions with local authority and responsibility. 16 At the same time, demands and pressures on water resources in a specific location often reflect consumption patterns in distant countries importing embedded (or virtual) water, especially in the form of agricultural products. In Switzerland, for example, over 80% of the national water footprint derives from imported goods and services. 17 The inherent linkages between water, food and energy pose challenges to conventional, sector-based approaches to resource management, but also offer opportunities to leverage synergies. Water use in agriculture, for example, could be reduced by application of sensors and precision technologies to avoid excessive irrigation, re-use of treated wastewater and adoption of diets that include less meat.
Progressing toward a circular economy will be a key element in decoupling human well-being from resource exploitation, including unsustainable demands on fresh water. 18 Recovery of the nutrients nitrogen and phosphorus from human excreta could provide renewable and less energy-intensive fertilizers for agriculture. Separated plumbing systems for greywater (i.e., wastewater not contaminated with fecal waste) could allow in-home re-use, substantially decreasing the demand on the water supply to the household. Decentralized systems for water and wastewater could reduce the need for water distribution systems and sewers with their associated costs and often-substantial loss of water through leaks.
There will not be a 'one-size-fits-all' solution to meet the water-related challenges of the SDGs. We will need to work cooperatively to develop a shared portfolio of approaches that can be adapted for local conditions. 19 Cooperation must extend across sectors and include the co-production of knowledge by actors with different expertise, backgrounds, experience and responsibility. Advances in technology must be effectively To realize the SDGs, we can take the attributes of water as a guide. Water flows around obstacles -SDG implementation must also be adaptive and appropriate to local contexts.
Over time, water can wear away the hardest stone -we must also be persistent. Water has great power that can be destructive but can also be harnessed productively -as individuals and collectively, we too have the power to transform our societies. Investing an issue like climate change with meaning, ethics and morality has generally been the work of social groups -conversely, this implies that one's social group can also undermine confidence in scientific facts and the immediacy of concerns about climate change. 8 Scholars who study media, science and social movements have been arguing for some time that diverse communities play a huge role in the circulation of facts, but the exact and varied nature of this influence is only just beginning to be understood. This is a departure from the view that audiences demand and require textbook-like scientific facts.
Instead, the work of journalism might better be considered as helping audiences develop a relationship with evolving climate facts that have been occasionally revised, but more often elaborated and affirmed.
When social movements are active online -providing source materials, perspectives and reporting -it becomes even more essential that journalists understand the varied contexts and audiences for their reporting, whilst also introducing new ideas about accountability into the discussion. The Columbia Journalism Review recently argued that climate change connections should be articulated when reporting on all environmental concerns. 9 While it makes good 'news' sense to link high profile disasters like hurricanes and fires, it is equally important for business and policy decisions to be linked to climate change. Holding corporations and governments accountable to ever clearer metrics for climate mitigation and adaptation may not be an easy news story to pitch or tell, but, in the longer term, it is an arguably more imperative one.
Perhaps an even greater challenge to address in media coverage is the representation of diversity in human experience and relations, and the importance of acknowledging the long histories of these relations. Erasing ongoing and historic relations between humans and non-humans, or even recognizing the inter-connectedness of our social structures and societies doesn't just wave away the challenges of contending with them, even in the name of shared concerns. Its successful injection into orbit was a moment of enormous consequence. It transformed Earth orbit into a buffer zone between humans and the wider solar system. In the years that followed, the formerly featureless 'orbital space' rapidly accumulated a population of robotic satellites and the junk they generated in their decay.
In the orbital space surrounding Earth, objects are in continual movement, and places are defined by velocity and height above the planet's surface. This is no longer a geography, which maps places on Earth, but an orbitography. Over the past six decades, human objects have colonized this orbital space, dividing it into zones and regions with distinct characteristics.
Low Earth orbit (LEO) ranges from around 200 km to 2,000 km above Earth's surface. Today, in 2020, we are facing a transformation of Earth's orbital landscape with the launch of proposed mega-constellations of internet telecommunications satellites. The first of these have already been launched, even though the effects of injecting tens of thousands of new objects into an already congested region of space are not fully understood.
Notwithstanding the optimistic assurances of commercial operators that the satellites will quickly re-enter Earth's atmosphere, it is clear that that predictions of the onset of the Kessler syndrome will have to be revised.
No longer will people on Earth have to scan the skies systematically to pick out a lone silver sphere, as they did in 1957. Satellites sightings will become the norm, rather than the exception; they will be our constant companions whenever we look heavenwards. The burning shards of re-entering spacecraft will cease to cause fear and astonishment. And, in a few decades, the people who remember the sky before Sputnik 1 shattered its peace will be gone. Soon, the whirling graveyard of space junk punctuated by living robots will be all we have ever known. In my childhood and teens, I served as a deckhand under my grandfather (an experienced commercial fisherman) and my mother (who was also raised fishing on the water), soaking up their knowledge and experience, accumulated over many generations.
When I was twenty, I got my first chance to captain my own boat on the River, accompanied by a deckhand and my thirteen-year-old sister, Charlee. We were fishing along a bend of the river, in a place with notoriously swift currents and many rocks.
At one point, the currents became too strong, tearing our net and pushing it into our engine, where it soon became hopelessly tangled. I screamed to cut the engine, which we did, but it was too late. The propellers were wrapped up in the fragments of the net and the engine was useless; we were sitting ducks. The current immediately began to pull us dangerously close to the rocks, which threatened to puncture our fiberglass boat and sink us. I knew that we were in mortal danger, and that the lives of the boat's occupants were in my hands.

Earth 2020
In the physical scramble to save the net and the boat, I was burned and cut by rope, and (to make matters worse) also stung by a wasp. My hand immediately started swelling and

Daniel Pauly is a Professor of Fisheries at the University of British Columbia, Vancouver
Canada, where he directs the Sea Around Us project, which is devoted to studying, documenting and mitigating the impact of fisheries on the world's marine ecosystems.
The concepts, methods and software his team has developed have been used in over 1000 widely-cited publications, and have led to his receiving numerous scientific awards.

Navin Ramankutty is a Professor and Canada Research Chair in Global Environmental
Change and Food Security at the University of British Columbia. His research uses global data and models to explore solutions to improve global food system sustainability. He is Her research focuses on energy system transitions, including interactions between technologies, decision making, policies and institutions. She works with practitioners in government, civil society and the private sector to shape the next generation of energy system transitions. She was awarded a Carnegie Fellowship and was selected as a Leopold Leadership Fellow. She is also a board member at the Vermont Energy Investment Corporation.

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Earth 2020
Fi� y years has passed since the fi rst Earth Day, on April 22nd, 1970. This accessible, incisive and � mely collec� on of essays brings together a diverse set of expert voices to examine how the Earth's environment has changed over these past fi � y years, and to consider what lies in store for our planet over the coming fi � y years.
Earth 2020: An Insider's Guide to a Rapidly Changing Planet responds to a public increasingly concerned about the deteriora� on of Earth's natural systems, off ering readers a wealth of perspec� ves on our shared ecological past, and on the future trajectory of planet Earth.
Wri� en by world-leading thinkers on the front-lines of global change research and policy, this mul� -disciplinary collec� on maintains a dual focus: some essays inves� gate specifi c facets of the physical Earth system, while others explore the social, legal and poli� cal dimensions shaping the human environmental footprint. In doing so, the essays collec� vely highlight the urgent need for collabora� on and diverse exper� se in addressing one of the most signifi cant environmental challenges facing us today.
Earth 2020 is essen� al reading for everyone seeking a deeper understanding of the past, present and future of our planet, and the role that humanity plays within this trajectory.
As with all Open Book publica� ons, this en� re book is available to read for free on the publisher's website. Printed and digital edi� ons, together with supplementary digital material, can also be found at www.openbookpublishers.com