Urgent reduction in greenhouse gas emissions is needed to avoid irreversible tipping points: time is running out

ABSTRACT This essay addresses climate change and its main causes over the last three decades. Between 1992–2021, global emissions of greenhouse gases (GHGs) have risen continually. Specifically, the major socioeconomic sectors – including (1) energy, (2) industry, (3) land-use/land-use change/agriculture, (4) transportation, (5) building/construction, and (6) waste treatment/disposal – have emitted enormous amounts of GHGs. Between 1992–2019, the combined annual GHG emissions have risen by 53% – from 32.6 to 49.8 Gt CO2 equivalent (CO2e). The combined GHG concentration has increased by 33% – from 382 ppm CO2e in 1992 to 508 ppm CO2e in 2021. The combined radiative forcing has surged by 45% – from 2.226 W m−2 in 1992 to 3.222 W m−2 in 2021. At the current emission rate, the entire GHG credit for limiting global warming to 1.5°C or 2.0°C – according to the Shared Socio-Economic Pathway (SSP) 1–1.9 or SSP1–2.6, respectively – in 2100 compared to preindustrial levels may be fully exploited by~2030. Limiting global warming to 1.5°C or 2.0°C will require total GHG emissions to peak before 2025 at the latest, and be reduced by 43% or 25%, respectively, in 2030 relative to 2019, followed by zero net emissions in the early 2050s or 2070s, respectively.


Introduction
Over recent decades, global climate change has become evident, manifested by rising temperatures, increasing severity and duration of droughts over extensive parts of the world (Naumann et al., 2018), and higher frequency and magnitude of intense rainstorms causing devastating floods (Liao et al., 2019).The establishment of the United Nations Framework Convention on Climate Change (UNFCCC) in 1992 and the concurrent UN Earth Summit in Rio de Janeiro primarily aimed at formulating the intergovernmental basis for collaborative efforts in climate change mitigation and adaptation (Seo, 2017).The subsequent establishment of the Conference of Parties (COP) mechanism was particularly aimed at implementing the UNFCCC policy, while serving as the worldwide executive authority for climate-related decision making (Wamsler et al., 2020).Between 1995-2021, world leaders met at numerous COP annual meetings to coordinate the intergovernmental climate changerelated actions (Stavi, 2022).Following COP21, held in Paris in 2015, the Paris Agreement was published, highlighting the need to limit global warming compared to preindustrial levels (the 1850-1900 baseline).Specifically, to negate irreversible climate tipping points that may trigger a cascading series of events with severe consequences and sometimes unexpected climatic feedbacks (Wunderling et al., 2021), the maximum temperature increase by 2100 was set to 1.5°C or 2.0°C at the most (IPCC, 2019).Despite these intergovernmental efforts, over the past three decades , global temperature anomaly relative both the 1901-2000 and 1951-1980 baselines has increased substantially (Figure 1(a)).
Over the past three decades, the radiative forcing (the atmospheric heating effect) of CO 2 , CH 4 , N 2 O, and halogenated compounds has increased by 62% (from 1.325 to 2.140 W m −2 ), 13% (from 0.467 to 0.526 W m −2 ), 58% (from 0.133 to 0.210 W m −2 ), and 16% (from 0.300 to 0.348 W m −2 ), respectively, yielding a combined increase (by all GHGs together) of 45% (from 2.226 to 3.222 W m −2 ) (Figure 3(a)).The Annual Greenhouse Gas Index (AGGI) -formulated by the National Oceanic and Atmospheric Administration (NOAA) -was introduced in 2006 and since then is updated annually.It calculates the ratio between total radiative forcing due to these gases in a given year and the total radiative forcing in 1990.Between 1992-2021, the AGGI rose by 45% (from 1.028 to 1.488) relative to the 1990 baseline (https://gml.noaa.gov/aggi/aggi.html:Figure 3(b)).
The introduced Shared Socio-Economic Pathway (SSP) (also known as the Representative Concentration Pathway (RCP)) scheme defines the boundaries for radiative forcing in order to negate climate tipping points (Shukla et al., 2019).The main global tipping points that have been identified are polar ice sheet collapse, sea level rise, permafrost thawing, disruption of major weather systems, forest die-off, and ocean acidification (WMO, 2022).According to this scheme, limiting global temperature increase to 1.5°C or 2.0°C would necessitate restricting the end-of-century radiative forcing to 1.9 W m −2 (SSP1-1.9 or RCP1.9: 'Taking-the-green-road-1 st ') or 2.6 W m −2 (SSP1-2.6 or RCP2.6: 'Taking-the-greenroad-2 nd '), respectively (Shukla et al., 2019).However, as shown in this essay, current emission rates show little if any chance of achieving this ambitious goal.In the following section, trends of emissions by the major socioeconomic sectors are concisely discussed.The

The energy sector
Over the last three decades, global GHG emissions from fossil fuel use in the energy sector -including electricity and heat production, as well as other related processes -have consistently risen, despite a slight decrease associated with COVID-19 lockdowns, which abruptly slowed economic activities (BP, 2021, https:// ourworldindata.org/emissions-by-fuel).In 1992, global CO 2 emissions across this sector encompassed 8.4 Gt from coal, 9.2 Gt from oil, and 4.0 Gt from gas, yielding a total of 21.6 Gt.Peaking in 2019, emissions included 14.7 Gt from coal, 12.2 Gt from oil, and 7.4 Gt from gas, and yielded a total of 34.3 Gt (https://ourworldindata. org/emissions-by-fuel).Despite the 5.1% decrease in CO 2 emissions from fossil fuels across this sector in 2020, a sharp recovery of global economy in 2021 has led to a 6.0% rise in global emissions, spiking to the highest ever level of 36.3Gt (IEA, 2022), equating a 68% increase since 1992.Despite the constant growth of the renewable energy sub-sector over time, global electricity demand is currently met by 63.3% fossil fuels, 10.4% nuclear, and only 26.3% renewable sources.Among the fossil fuels, the current share of GHG sources is 36.7%coal, 23.5% gas, and 3.1% oil (% of the entire energy sector).Among the renewables, sources are 15.8% hydropower, 5.3% wind, 2.7% solar, and 2.5% other (% of the entire energy sector) (https://ourworldindata.org/electricitymix). Including all sub-sectors, the energy sector is responsible for 29-35% of global GHGs (https://www.epa.gov/ghgemissions/global-greenhouse-gasemissions-data; https://www.eea.europa.eu/data-andmaps/daviz/change-of-co2-eq-emissions-2#tabdashboard-01).

The land-use & land-use change (LULUC) and agricultural sector
Within this sector, the conversion of natural lands to croplands and grazing lands encompasses a major source of GHGs.Often, particularly in the tropics, this includes deforestation and burning of vegetation cover, an enormous source of CO 2 emissions (FAO, 2020).Further, land-use changes lead to the decomposition of large amounts of safely-stored soil organic carbon, which oxidises and turns into atmospheric CO 2 (Bennetzen et al., 2016b).Prescribed fires, a regular management practice in extensive grasslands and savannahs, also emit considerable amounts of CO 2 (FAO, 2020).In croplands, another major source of GHGs is attributed to nitrogen fertilisers, which emit huge amounts of N 2 O. Agricultural CH 4 is mostly attributed to livestock systems, and specifically to ruminant animals, as well as to paddy cropping systems, and particularly to rice cultivation (Bennetzen et al., 2016a;Islam et al., 2020).Despite a decreasing trend in global GHG emissions from some of these subsectors during the last decades, other sub-sectors have faced an increasing trend.For example, between 2000-2018, along with declining deforestation rates, land-use change-imposed emissions decreased by 22% -from 5.0 to 3.9 Gt CO 2 e. Simultaneously, emissions from crop and livestock systems increased by 15% -from 4.6 to 5.3 Gt CO 2 e -with livestock systems contributing two thirds of this measure.Combining these divergent trends results in a 3.0% net decrease in GHG emissions across the entire LULUC/agricultural sector over this period (FAO, 2020).This trend consists with other studies, which showed a net decrease in global GHG emissions across this sector since the early 1990s, and attributed this trend to improved efficiency in agricultural production systems (Gaihre et al., 2015;Bennetzen et al., 2016a;2016b).Overall, emissions from this sector encompass 17-24% of global GHGs (FAO, 2020; https://www.epa.gov/ghgemissions/global-greenhouse-gas-emissions-data; https://ourworl dindata.org/emissions-by-sector).

The building/construction sector
Emissions from this sector are distributed among the sub-sectors of construction and related industries (43%), residential (direct and indirect: 20%), non-residential (direct and indirect: 17%), and other (20%).Until 2018, global emissions by this sector continuously increased.The COVID-19 economic slowdown led to a two-year decrease, which has since returned to an increasing trend.Yet, over time, improved efficiency across all subsectors, alongside the introduction of advanced building methodologies and the expansion of innovative construction materials, have somewhat mitigated the rising emissions (UNEP, 2021).Among the construction materials, cement is the dominant GHG emitter, followed by steel, aluminium, glass, wood, and copper (Zhong et al., 2021).The production of cement is highly carbon-intensive, as enormous amounts of fossil fuels are needed to heat a mixture of limestone and clay to over 1,400 °C.The following chemical reactions between the two substances release an additional 600 kg CO 2 for each tonne of cement produced (Nature Editors, 2021).Emission of GHGs have tripled in the last three decades and doubled in the last two decades.Global emissions from this industry encompassed 915, 1,305, 2,339, and 2,897 Mt CO 2 in 1992, 2001, 2011, and 2021, respectively (https://apnews.com/article/climatesc i e n c e -c h i n a -p o l l u t i o n -3 d 9 7 6 4 2 a c b b 0 7 f ca7540edca38448266).In total, this sector accounts for 6-11% of global GHGs (https://www.epa.gov/ghgemissions/global-greenhouse-gas-emissions-data; https:// ourworldindata.org/emissions-by-sector).

The waste treatment/disposal sector
Wastes from several sources, including food and other biogenic materials, municipal solids, industrial solids, sewage sludge, and more (Crippa et al., 2021), emit considerable amounts of GHGs.Within this sector, landfills and wastewater are the main source of GHG emissions, specifically CH 4 .In landfills, CH 4 encompasses~50% of all GHG emissions.In sewage sludge facilities, the CH 4 share of total GHGs varies greatly, depending on the properties of the processed material, processing procedures, moisture content, and ambient temperature, which determine the prevailing aerobic vs. anaerobic conditions and the efficiency of methanogenesis (IPCC, 2006a).Overall, waste treatment/disposal accounts for 20-30% of global anthropogenic CH 4 emissions (Crippa et al., 2021; https://climatechampions.unfccc.int/a-clarion-call-toreduce-and-phase-out-of-open-waste-burning/).In addition to CH 4 , solid wastes also emit smaller amounts of CO 2 and N 2 O (IPCC, 2006a).Yet, when solid wastes are intentionally (or spontaneously) burnt in open dumps or residential sites -a common practice in many developing countries (Gautam & Agrawal, 2021) -CO 2 emissions, and to some extent N 2 O emissions, become much greater (IPCC, 2006b).Further, this mismanagement practice accounts for 11% of global emissions of the short-lived GHG black carbon (https://climatechampions.unfccc.int/a-clarioncall-to-reduce-and-phase-out-of-open-waste-burning/).Overall, emissions from this sector are responsible for 5-12% of global GHGs (Gautam & Agrawal, 2021; https://climatechampions.unfccc.int/a-clarion-call-toreduce-and-phase-out-of-open-waste-burning/).

Time to act
On the bright side, GHG reductions in the LULUC/agriculture sector over the last decades (Bennetzen et al., 2016a), and the projected continued decreasing emissions in the coming decades (Bennetzen et al., 2016b) are encouraging.Such sectorial trends are rather remarkable, especially due to the global increase of 47% in human population over the last three decades -from 5.3 billion in 1990 to 7.8 billion in 2020 (https://ourworldindata.org/ world-population-growth), and the projected increase to 9.8 billion in 2050 (https://www.un.org/en/desa/worldpopulation-projected-reach- 98-billion-2050-and-112billion-2100).At the same time, incessantly increasing emissions in sub-sectors such as electricity production, heat generation, metal manufacturing, chemical production, road and aviation transport, and cement production, and moreover in entire sectors such as energy, industry, transportation, waste treatment/disposal, and building/ construction, demonstrate the dark side (Minx et al., 2021).Specifically, under the SSP1-2.6/RCP2.6 scenario, global GHGs by building and construction materials alone are forecasted to increase from 3.5 GT CO 2 e in 2020 to 4.6 CO 2 e in 2060 (Zhong et al., 2021).
Climate tipping points occur when global warming pushes temperatures beyond a critical threshold, generating accelerated and irreversible impacts, with complex chains of interactions among the cryosphere, ocean/ atmosphere, and biosphere (McKay et al., 2022).For example, the melting ice sheets in Greenland release fresh water into the ocean, and decelerate the Atlantic Meridional Overturning Circulation (AMOC).The latter, partly driven by dense, salty water descending towards the ocean floor, becomes weaker, decreasing heat transport from the tropics to the North Pole.In turn, this warms the seawater in the Southern Ocean, increasing ice sheet melting in Antarctica, causing sea level to rise, and accelerating the melting of additional ice sheets.The weakened Gulf Stream -which encompasses an important part in the AMOC and has substantially warmed over the last decades -has become less effective in regulating the mild climatic conditions over Western Europe, disrupting precipitation patterns across the region.Further, the interrupted Atlantic currents adversely affect climatic conditions across the Amazon, where reduced precipitations have led to desiccation and mass mortality of trees (Wunderling et al., 2021).The dead trees halt to assimilate carbon, and moreover, become available to fuel highseverity forest wildfires (both in the Amazon Basin and elsewhere), releasing enormous amounts of CO 2 into the atmosphere, thus generating a climate positive feedback loop (Choat et al., 2018).The simultaneous thawing of extensive carbon-rich permafrost systems, induced by global warming, emits huge amounts of CO 2 and CH 4 , generating an additional climate positive feedback loop (Natali et al., 2021).
Therefore, in terms of climate change tipping points, time is running out.In 2021, global emissions of CO 2 alone led to an 8.7% reduction in the remaining credit for limiting global warming to 1.5°C.If current emission rates continue, the entire credit may be fully exploited within approximately one decade (Z.Liu et al., 2022).Despite some uncertainties in terms of climatic processes and feedbacks, as well as in the relative share of emissions by each socioeconomic sector, there is no doubt that the next few years are critical.Limiting global warming to 1.5°C or 2.0°C by 2100 will require total GHG to peak before 2025 at the latest, and be reduced by 43% or 25%, respectively, in 2030 relative to 2019, followed by zero net emissions in the early 2050s or 2070s, respectively (IPCC, 2022).However, according to a recent study, the current global warming of~1.1°Ccompared to preindustrial times already lies within the lower end of five climate tipping points' uncertainty ranges.Also, according to the very same study, global warming ranging between 1.5-2.0°Cwould likely trigger several tipping points, such as ice sheet collapse in Greenland and west Antarctica, extensive permafrost thawing in northern latitudes, and die-off of low-latitudinal coral reefs, stressing the need to limit global warming to 1.5°C (McKay et al., 2022).
One way or another, in order to successfully mitigate climate change, conclusive intergovernmental policies, coupled with effective decision making at all levels, should call for immediate, substantial and binding emission reductions across all socioeconomic sectors, as the time to act is now, before the window of opportunity closes.Policymaking and legislation at all levels should generate the transformation to circular economy and promote complementary environmental strategies.In the energy sector, there is a need in accelerating the development of renewable energy technologies, increase their energy efficiency, and invest in energy storage means.In the industry sector, it is necessary to foster cleantech and environmentallyfriendly industries, and convert to renewable raw materials.In the LULUC/agricultural sector, supporting afforestation, reforestation, agroforestry, and silvopasture, promoting conservation agricultural practices, such as reduced tillage systems, water-saving means, and precision nutrient management, ensuring the restoration of degraded lands, and investing in foodtech innovations, are urgently needed.In the transportation sector, encouraging the use of public transportation means and investing in innovative/clean transportation means are necessary.In the building/ construction sector, progressing green building methods, renewable construction materials, and the use of energy-saving architectural methods, should be promoted.In the waste treatment/disposal sector, maximising the reuse, recycle, and recovery of materials, while minimising waste disposal should be sped up.Concordantly with the reduction of GHGs, such steps are also expected to decrease the emission of contaminants, thus sustaining environmental quality and human health (Stavi, 2022).