The Fourth National Climate Assessment, Chapter 25: Southwest

Chapter 25 of the Fourth National Climate Assessment (NCA4) is an assessment of climate change and variability, climaterelated risks, impacts and adaptation in the U.S. Southwest. The chapter builds on assessments of climate change in the Southwest from the three previous U.S. National Climate Assessments. Each assessment has consistently identified drought, water resource reliability, and loss of ecosystem integrity as climate change challenges for the Southwest region. Chapter 25 further examines interconnections among water, ecosystems, coastal and marine systems, food, and human health and adds new key messages concerning energy and Indigenous peoples. The Southwest chapter is one of 29 chapters in Volume II of the Fourth National Climate Assessment Impacts, Risks, and Adaptation in the United States. The National Climate Assessment fulfills the mandate of the Global Change Research Act (GCRA) of 1990 to provide the nation with a timely assessment and analysis of scientific findings of the effects of global change on multiple economic and natural resource sectors in the United States, and an analysis of observed and projected trends in global change. Chapter 25, Southwest, was written by a team of scientists and practitioners with expertise spanning areas specified in the GCRA, after extensive stakeholder engagement that involved the collection of input on key climate-related challenges, impacts, and opportunities in the Southwest region. The chapter went through multiple rounds of public and governmental review, during 2017 and 2018. This poster will focus on the findings from Chapter 25.

Water Resources. Water for people and nature in the Southwest has declined during droughts, due in part to human-caused climate change. Intensifying droughts and occasional large floods, combined with critical water demands from a growing population, deteriorating infrastructure, and groundwater depletion, suggest the need for flexible water management techniques that address changing risks over time, balancing declining supplies with greater demands.
Ecosystems and Ecosystem Services. The integrity of Southwest forests and other ecosystems and their ability to provide natural habitat, clean water, and economic livelihoods have declined as a result of recent droughts and wildfire due in part to humancaused climate change. Greenhouse gas emissions reductions, fire management, and other actions can help reduce future vulnerabilities of ecosystems and human well-being.
The Coast. Many coastal resources in the Southwest have been affected by sea level rise, ocean warming, and reduced ocean oxygen-all impacts of human-caused climate changeand ocean acidification resulting from human emissions of carbon dioxide. Homes and other coastal infrastructure, marine flora and fauna, and people who depend on coastal resources face increased risks under continued climate change.
Indigenous Peoples. Traditional foods, natural resource-based livelihoods, cultural resources, and spiritual well-being of Indigenous peoples in the Southwest are increasingly affected by drought, wildfire, and changing ocean conditions. Because future changes would further disrupt the ecosystems on which Indigenous peoples depend, tribes are implementing adaptation measures and emissions reduction actions.
Energy. The ability of hydropower and fossil fuel electricity generation to meet growing energy use in the Southwest is decreasing as a result of drought and rising temperatures. Many renewable energy sources offer increased electricity reliability, lower water intensity of energy generation, reduced greenhouse gas emissions, and new economic opportunities.
Food. Food production in the Southwest is vulnerable to water shortages. Increased drought, heat waves, and reduction of winter chill hours can harm crops and livestock; exacerbate competition for water among agriculture, energy generation, and municipal uses; and increase future food insecurity.
Human Health. Heat-associated deaths and illnesses, vulnerabilities to chronic disease, and other health risks to people in the Southwest result from increases in extreme heat, poor air quality, and conditions that foster pathogen growth and spread. Improving public health systems, community infrastructure, and personal health can reduce serious health risks under future climate change.
The authors examined the scientific literature in their areas of expertise. The team placed the highest weight on scientific articles published in refereed peer-reviewed journals. Other sources included published books, government technical reports, and, for data, government websites. The U.S. Global Change Research Program issued a public call for technical input and provided the authors with the submissions. The University of Arizona Center for Climate Adaptation Science and Solutions organized the Southwest Regional Stakeholder Engagement Workshop on January 28, 2017, with over 70 participants at the main location in Tucson, AZ, and dozens of participants in Albuquerque, NM, Boulder, CO, Davis, CA, Los Angeles, CA, Reno, NV, and Salt Lake City, UT, all connected by video. Participants included scientists and managers. The author team met the following day for their only meeting in person. Subsequently, authors held discussions in regular teleconferences. Since 2000, drought that was intensified by long-term trends of higher temperatures due to climate change has reduced the flow in the Colorado River (top left), which in turn has reduced the combined contents of Lakes Powell and Mead to the lowest level since both lakes were first filled (top right). In the Upper Colorado River Basin that feeds the reservoirs, temperatures have increased (bottom left), which increases plant water use and evaporation, reducing lake inflows and contents. Although annual precipitation (bottom right) has been variable without a long-term trend, there has been a recent decline in precipitation that exacerbates the drought. Combined with increased Lower Basin water consumption that began in the 1990s, these trends explain the recently reduced reservoir contents. Straight lines indicate trends for temperature, precipitation, and river flow. The trends for temperature and river flow are statistically significant. Adapted from Water Resources Research 53: 2404-2418.   In response to sea level rise and storm surge in San Francisco Bay, federal, state, and local agencies, supported by voter-approved funds, are restoring coastal habitats and levees to protect cities from flooding. Crop water savings: The risk of reduced food production increases as climate change intensifies drought. In the Gila River Basin, local government agencies have lined 15 miles (24 km) of irrigation canals to reduce seepage from the canals, saving enough water to irrigate approximately 8,500 acres (3,400 hectares) of alfalfa and other crops each year. Cultural fire restoration: Reintroduction of cultural burning by the Yurok Tribe in northern California reduces wildfire risks and protects public and tribal trust resources. Desert soil restoration: In Utah, transplanting native and drought-resistant microbial communities improves soil fertility and guards against erosion. Health protection: To reduce heat-associated injury and deaths on Arizona trails, the City of Phoenix and Arizona tourism organizations developed a campaign "Take a Hike. Do it Right." Signs at trailheads and on websites remind hikers to bring water, stay hydrated, and stay aware of environmental conditions. Ranching and habitat: The Malpai Borderlands Group in Arizona and New Mexico integrates native plant and wildlife conservation into private ranching. Rooftop solar: The state governments of California, Colorado, and Nevada have enacted policies that support rooftop solar on homes, which reduces greenhouse gas emissions, improves reliability of the electricity generation system, and creates local small businesses and new jobs. Water conservation: Drought in the Colorado River Basin has reduced the volume of water in both Lake Mead and Lake Powell by over half. The United States, Mexico, and state governments have mobilized users to conserve water, keeping the lake above a critical level. Wildfire fuel reduction: In response to severe wildfires, the City of Flagstaff, Arizona, enacted a bond to fund reduction of fire fuels in forests around the town.