- Technical Contributors:
- Mary E. Black, University of Arizona
- Shallin Busch, National Oceanic and Atmospheric Administration
- Brandon Goshi, Metropolitan Water District of Southern California
- USGCRP Coordinators:
- Fredric Lipschultz, Senior Scientist and Regional Coordinator
- Christopher W. Avery, Senior Manager
<b>Gonzalez</b>, P., G.M. Garfin, D.D. Breshears, K.M. Brooks, H.E. Brown, E.H. Elias, A. Gunasekara, N. Huntly, J.K. Maldonado, N.J. Mantua, H.G. Margolis, S. McAfee, B.R. Middleton, and B.H. Udall, 2018: Southwest. In <i>Impacts, Risks, and Adaptation in the United States: Fourth National Climate Assessment, Volume II</i> [Reidmiller, D.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, K.L.M. Lewis, T.K. Maycock, and B.C. Stewart (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 1101–1184. doi: 10.7930/NCA4.2018.CH25
The Southwest region encompasses diverse ecosystems, cultures, and economies, reflecting a broad range of climate conditions, including the hottest and driest climate in the United States. Arizona, California, Colorado, New Mexico, Nevada, and Utah occupy one-fifth of U.S. land area, extending across globally unique ecosystems from the Sonoran Desert to the Sierra Nevada to the Pacific Coast. The region is home to 60 million people, with 9 out of 10 living in urban areas and the total population growing 30% faster than the national average.1 The Nation depends on the region for more than half of its specialty crops such as fruits, nuts, and vegetables.2 The Southwest also drives the U.S. technology sector, with more than 80% of the country’s technology capitalization located in California.3
Ecosystems in the Southwest gradually transform from deserts and grasslands in hotter and lower elevations in the south to forests and alpine meadows in cooler, higher elevations in the north. Natural and human-caused wildfire shapes the forests and shrublands that cover one-quarter and one-half of the region, respectively.4 To conserve habitat for plants and wildlife and supply clean water, timber, recreation, and other services for people, the U.S. Government manages national parks and other public lands covering half of the Southwest region.5 Climate change is altering ecosystems and their services through major vegetation shifts213 and increases in the area burned by wildfire.7
The California coast extends 3,400 miles (5,500 km),8 with 200,000 people living 3 feet (0.9 m) or less above sea level.9 The seaports of Long Beach and Oakland, several international airports, many homes, and high-value infrastructure lie along the coast. In addition, much of the Sacramento–San Joaquin River Delta is near sea level. California has the most valuable ocean-based economy in the country, employing over half a million people and generating $20 billion in wages and $42 billion in economic production in 2014.10 Coastal wetlands buffer against storms, protect water quality, provide habitat for plants and wildlife, and supply nutrients to fisheries. Sea level rise, storm surges, ocean warming, and ocean acidification are altering the coastal shoreline and ecosystems.
Water resources can be scarce because of the arid conditions of much of the Southwest and the large water demands of agriculture, energy, and cities. Winter snowpack in the Rocky Mountains, Sierra Nevada, and other mountain ranges provides a major portion of the surface water on which the region depends. Spring snowmelt flows into the Colorado, Rio Grande, Sacramento, and other major rivers, where dams capture the flow in reservoirs and canals and pipelines transport the water long distances. Complex water laws govern allocation among states, tribes, cities, ecosystems, energy generators, farms, and fisheries, and between the United States and Mexico. Water supplies change with year-to-year variability in precipitation and water use, but increased evapotranspiration due to higher temperatures reduces the effectiveness of precipitation in replenishing soil moisture and surface water.11,12,13,14
Agricultural irrigation accounts for nearly three-quarters of water use in the Southwest region,15,16 which grows half of the fruits, vegetables, and nuts2 and most of the wine grapes, strawberries, and lettuce17 for the United States. Consequently, drought and competing water demands in this region pose a major risk for agriculture and food security in the country. Through production and trade networks, impacts to regional crop production can propagate nationally and internationally (see Ch. 16: International, KM 1)18
Parts of the Southwest reach the hottest temperatures on Earth, with the world record high of 134°F (57°C) recorded in Death Valley National Park California,19 and daily maximum temperatures across much of the region regularly exceeding 98°F (35°C) during summer.20 Greenhouse gases emitted from human activities have increased global average temperature since 188021 and caused detectable warming in the western United States since 1901.22 The average annual temperature of the Southwest increased 1.6°F (0.9ºC) between 1901 and 2016 (Figure 25.1).23 Moreover, the region recorded more warm nights and fewer cold nights between 1990 and 2016),24 including an increase of 4.1°F (2.3°C) for the coldest day of the year. Parts of the Southwest recorded the highest temperatures since 1895, in 2012,25 2014,26 2015,27 2016,28 and 2017.29
Figure 25.1: Temperature Has Increased Across the Southwest
Extreme heat episodes in much of the region disproportionately threaten the health and well-being of individuals and populations who are especially vulnerable (Ch. 14: Human Health, KM 1).30 Vulnerability arises from numerous factors individually or in combination, including physical susceptibility (for example, young children and older adults), excessive exposure to heat (such as during heat waves), and socioeconomic factors that influence susceptibility and exposure (for example, hot and poorly ventilated homes or lack of access to public emergency cooling centers).31,32,33 Communicable diseases, ground-level ozone air pollution, dust storms, and allergens can combine with temperature and precipitation extremes to generate multiple disease burdens (an indicator of the impact of a health problem).
Native Americans are among the most at risk from climate change, often experiencing the worst effects because of higher exposure, higher sensitivity, and lower adaptive capacity for historical, socioeconomic, and ecological reasons. With one and a half million Native Americans,35 182 federally recognized tribes,36 and many state-recognized and other non-federally recognized tribes, the Southwest has the largest population of Indigenous peoples in the country. Over the last five centuries, many Indigenous peoples in the Southwest have either been forcibly restricted to lands with limited water and resources37,38,39 or struggled to get their federally reserved water rights recognized by other users.40 Climate change exacerbates this historical legacy because the sovereign lands on which many Indigenous peoples live are becoming increasingly dry.
Further, climate change affects traditional plant and animal species, sacred places, traditional building materials, and other material cultural heritage. The physical, mental, emotional, and spiritual health and overall well-being of Indigenous peoples rely on these vulnerable species and materials for their livelihoods, subsistence, cultural practices, ceremonies, and traditions.41,42,43,44
In parts of the region, hotter temperatures have already contributed to reductions of seasonal maximum snowpack and its water content over the past 30–65 years,45,46,47,48,49 partially attributed to human-caused climate change.45,46,48,49 Increased temperatures most strongly affect snowpack water content, snowmelt timing, and the fraction of precipitation falling as snow.48,50,51,52,53,54
The increase in heat and reduction of snow under climate change have amplified recent hydrological droughts (severe shortages of water) in California,14,55,56,57,58 the Colorado River Basin,12,13,59and the Rio Grande.45,60 Snow droughts can arise from a lack of precipitation (dry snow drought), temperatures that are too warm for snow (warm snow drought), or a combination of the two.48,51
Periods of low precipitation from natural variations in the climate system are the primary cause of major hydrological droughts in the Southwest region,61,62,63,64,65,66,67,68 with increasing temperatures from climate change amplifying recent hydrological droughts, particularly in California and the upper Colorado River Basin.12,13,14,56,57,59
Under the higher scenario (RCP8.5), climate models project an 8.6°F (4.8°C) increase in Southwest regional annual average temperature by 2100.23 Southern parts of the region could get up to 45 more days each year with maximum temperatures of 90°F (32°C) or higher.23 Projected hotter temperatures increase probabilities of decadal to multi-decadal megadroughts,61,62,69,70 which are persistent droughts lasting longer than a decade,69 even when precipitation increases. Under the higher scenario (RCP8.5), much of the mountain area in California with winters currently dominated by snow would begin to receive more precipitation as rain and then only rain by 2050.71 Colder and higher areas in the intermountain West would also receive more rain in the fall and spring but continue to receive snow in the winter at the highest elevations.71
Increases in temperature would also contribute to aridification (a potentially permanent change to a drier environment) in much of the Southwest, through increased evapotranspiration,69,70,72,73 lower soil moisture,74 reduced snow cover,71,75,76,77 earlier and slower snowmelt,75 and changes in the timing and efficiency of snowmelt and runoff.50,54,75,76,78,79 Some research indicates increasing frequency of dry high-pressure weather systems associated with changes in Northern Hemisphere atmospheric circulation.80,81 These changes would tend to increase the duration and severity of droughts67,74 and generate an overall drier regional climate.69,70,72
Climate models project an increase in the frequency of heavy downpours, especially through atmospheric rivers,74,82 which are narrow bands of highly concentrated storms that move in from the Pacific Ocean. A series of strong atmospheric rivers caused extreme flooding in California in 2016 and 2017. Under the higher scenario (RCP8.5), models project increases in the frequency and intensity of atmospheric rivers.83,84,85,86 Climate models also project an increase in daily extreme summer precipitation in the Southwest region, based on projected increases in water vapor resulting from higher temperatures.20,87,88 Projections of summer total precipitation are uncertain, with average projected totals not differing substantially from what would be expected due to natural variations in climate.88
The Southwest generates one-eighth of U.S. energy, with hydropower, solar, wind, and other renewable sources supplying one-fifth of regional energy generation.89 By installing so much renewable energy, the Southwest has lowered its per capita and per dollar greenhouse gas emissions below the U.S. average.90 Climate change can, however, decrease hydropower and fossil fuel energy generation.91 California has enacted mandatory greenhouse gas emissions reductions,92 and Arizona, California, Colorado, Nevada, and New Mexico have passed renewable portfolio standards to reduce fossil fuel dependence and greenhouse gas emissions.93
Figure 25.2: Actions Responding to Climate Change Impacts and Vulnerabilities
What Is New in the Fourth National Climate Assessment
This chapter builds on assessments of climate change in the Southwest region from the three previous U.S. National Climate Assessments.94,95,96 Each assessment has consistently identified drought, water shortages, and loss of ecosystem integrity as major challenges that the Southwest confronts under climate change. This chapter further examines interconnections among water, ecosystems, the coast, food, and human health and adds new Key Messages concerning energy and Indigenous peoples.
Since the last assessment, published field research has provided even stronger detection of hydrological drought, tree death, wildfire increases, sea level rise, and warming, oxygen loss, and acidification of the ocean that have been statistically different from natural variation, with much of the attribution pointing to human-caused climate change. In addition, new research has provided published information on future vulnerabilities and risks from climate change, including floods, food insecurity, effects on the natural and cultural resources that sustain Indigenous peoples, illnesses due to the combination of heat with air pollution, harm to mental health, post-wildfire effects on ecosystems and infrastructure, and reductions of hydropower and fossil fuel electricity generation.
This chapter highlights many of the increasing number of actions that local governments and organizations have been taking in response to historical impacts of climate change and to reduce future risks (Figure 25.2). Some examples include voluntary water conservation and management in California and the Colorado River Basin, restoring cultural fire management in California, and rooftop solar policies in California, Colorado, and Nevada. Many state and local governments have issued climate change assessments and action plans.
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