The Southwest encompasses diverse natural ecosystems, vibrant cultures, and productive economies. This vast region spans nearly 700,000 square miles, or 18% of the US land area.1 The Southwest is home to more than 60 million people and is among the fastest growing and most economically productive areas of the country. Southwest ecosystems provide society with food, energy, and water; regulate climate; protect against disasters and disturbances; and offer the settings and inspiration for meaningful social, cultural, recreational, and spiritual experiences (Figure 8.17).
White, D.D., E.H. Elias, K.A. Thomas, C.E. Bradatan, M.W. Brunson, A.M. Chischilly, C.A.F. Enquist, L.R. Fisher, H.E. Froehlich, E.A. Koebele, M. Méndez, S.M. Ostoja, C. Steele, and J.K. Vanos, 2023: Ch. 28. Southwest. In: Fifth National Climate Assessment. Crimmins, A.R., C.W. Avery, D.R. Easterling, K.E. Kunkel, B.C. Stewart, and T.K. Maycock, Eds. U.S. Global Change Research Program, Washington, DC, USA. https://doi.org/10.7930/NCA5.2023.CH28
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Climate change is negatively impacting human health and well-being (KM 15.1), cultural heritage, property, built infrastructure, economic prosperity, natural capital, and ecosystem services across the Southwest (Figure 28.1). Impacts include rising air temperatures2 and sea surface temperatures, both attributable in part to human activities;3 changes to the timing, form, and amount of precipitation;4,5,6 sea level rise and associated flooding events;7 increases in extreme heat events;8 summertime heat stress9,10 and heat-related mortality;11 surface and groundwater reductions;12,13,14,15,16 increased wildfire risks;17,18,19,20,21 and changes to ocean chemistry. These impacts pose heightened risks to overburdened and frontline communities and to Indigenous Peoples (KMs 4.2, 15.2, 16.1).
Southwest ecosystems transition from deserts and grasslands in hotter and lower elevations to forests and alpine meadows in cooler, higher elevations. The region supports important terrestrial and marine biodiversity and ecosystems, including the Sonoran Desert, the Sierra Nevada, and the Pacific Coast. The southern deserts commonly see temperatures between 105° and 115°F, and Phoenix has the hottest climate of all major US cities. The California coast stretches 3,400 miles (5,500 km), and its coastal wetlands provide critical habitat for fish and wildlife, protect water quality, and buffer against storms and floods.
The region is heavily urbanized, with 9 out of 10 people living in cities such as Albuquerque, Denver, Las Vegas, Los Angeles, Phoenix, Salt Lake City, and San Francisco. The region is also a major hub for software innovation, information technology, and semiconductor manufacturing. California’s economy alone contributed more than $3.21 trillion (in 2022 dollars) to the US GDP in 2021, about 12% of the total US economy.22 The region also encompasses expansive rural areas with livelihoods centered on ranching, mining, agriculture, and tourism.
Indigenous Peoples and Tribal lands are essential to the social, cultural, and geographic identity of the region. The Southwest is home to 182 Federally Recognized Tribes,23 as well as numerous state-recognized Tribes and Tribes seeking state or federal recognition. California has the largest number of Federally Recognized Tribes (109) and the largest Indigenous population of any state.23 Arizona, New Mexico, Colorado, and Utah are home to seven of the most populous Tribes, ranging from 10,000 to more than 300,000 members. Nine Tribes in the Southwest are considered “large land-holding Tribes,” five of which are among the ten largest reservations in the US, ranging in size from 600,000 to 16 million acres. The largest US federal Indian reservation—the 16-million-acre Navajo Nation Reservation—occupies portions of Arizona, New Mexico, and Utah.
The Federal Government manages nearly half of the total land area of the region through national parks, forests, fish and wildlife reserves, military installations, and public lands.24 In Nevada, the Federal Government is responsible for managing more than 80% of the total acreage of the state.24 Thus, the Federal Government is central to adaptation and mitigation in the Southwest.
Over the past five years, climate change impacts in the Southwest have become increasingly apparent and widespread.25 At the same time, understanding and modeling of how these impacts affect specific sectors and processes have improved. For instance, advances have been made in understanding and modeling of water,26,27,28 food and agriculture,29 wildfire,19 invasive species, biodiversity loss (KM 8.2), ecosystem transformations, human health,30 and human migration across the Southwest.31,32 Furthermore, research has advanced understanding and modeling of interdependencies, feedbacks, and cascading risks for interconnected systems (KM 18.1) such as the food–energy–water nexus (KM 18.3).33,34
To address these climate change impacts, governments, nongovernmental organizations, and private enterprises are increasingly responding with planning and actions to reduce current and future risks and increase adaptive capacity. Adaptation efforts that are effective, feasible, and just—including nature-based solutions such as green infrastructure for flood mitigation—have been shown to reduce climate risk, increase resilience, and provide co-benefits to related societal goals (KM 8.3).35 There is an awareness of new approaches to equity and environmental justice for frontline communities, as well as Indigenous Peoples (KM 16.2) across the Southwest. These approaches recognize, protect, and apply diverse knowledge systems, including Indigenous Knowledges (KM 16.3). Social science has also improved our understanding of inclusive, participatory, and collaborative decision-making to solve problems in this region and beyond.36,37,38
While this chapter focuses on climate impacts, risks, and adaptation actions in the Southwest, it also recognizes efforts underway to mitigate greenhouse gas emissions (Figure 32.20) throughout the region at multiple scales. California, Colorado, and New Mexico are members of the US Climate Alliance, committed to reducing net greenhouse gas (GHG) emissions in line with the Paris Agreement (KM 32.5). California has committed to carbon neutrality by 204539,40 and released a detailed plan with targets to achieve this goal,41 as well as augmenting funding across sectors.42 Both Colorado and New Mexico have statewide greenhouse gas reduction goals.43,44 At the local level, dozens of cities in all Southwest states are committed to emissions reductions in line with the Paris Agreement through the bipartisan Climate Mayors network (KM 32.5). For example, the Phoenix Climate Action Plan states that the city is on track to meet its goal of 50% reduction in GHG emissions (below its 2018 baseline) by 2030 and is committed to carbon neutrality by 2050.
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Climate change has reduced surface water and groundwater availability for people and nature in the Southwest (very high confidence), and there are inequities in how these impacts are experienced (high confidence). Higher temperatures have intensified drought and will lead to a more arid future (very likely, high confidence); without adaptation, these changes will exacerbate existing water supply–demand imbalances (likely, high confidence). At the same time, the region is experiencing more intense precipitation events, including atmospheric rivers, which contribute to increased flooding (high confidence). Flexible and adaptive approaches to water management have the potential to mitigate the impacts of these changes on people, the environment, and the economy (medium confidence).
The Southwest region is historically arid and marked by episodes of intense drought and precipitation (KM 4.1).45,46 Climate change is exacerbating these conditions, as increasing temperatures are leading to hotter extreme heat events, drier soils, greater atmospheric evaporative demand, and reduced flows in major river basins such as the Colorado and Rio Grande.14,47,48,49,50 For example, between 1913 and 2017, annual average discharge from the Colorado River decreased by 9.3% for each degree Celsius of warming (Box 28.1).49 Additionally, since 2000 the Southwest has experienced an exceptional “megadrought”—defined as an episode of intense aridity that persists for multiple decades—that is recognized as the driest 22-year period in 1,200 years.51
Mountain snowpack is one of the most important sources of water in the Southwest, serving as a natural reservoir to supply water to drier, lower elevations for irrigated agricultural, municipal and industrial uses, and ecosystems (KM 4.1). Observed declines in western snowpack over the last century have been predominantly driven by warming trends,4 leading to smaller snowpack volumes, higher-elevation snow lines, and earlier snowmelt (KM 3.4).6,52 These processes are exacerbated by the deposition of dust and other light-absorbing particles on snowpack, which accelerates snowmelt.53 The resulting decrease in snow cover also reduces the albedo, or reflectivity, of the land surface, resulting in a positive feedback cycle that further increases solar radiation absorption, warming, and snowmelt.49,54,55 These changing snowpack dynamics are expected to have different influences on the timing and volume of snowmelt-driven streamflow in different basins,56 potentially disrupting the ability of existing water infrastructure, including hydropower, to meet the region’s needs5,49 and altering ecosystem dynamics. Persistent low-snow years are projected to occur in the next half century if climate change continues unabated (Figure 28.2).5
In addition to extended periods of record-low precipitation, higher temperatures driven by climate change have increased evapotranspiration and reduced soil moisture, which can reduce the volume of runoff produced from a given amount of precipitation.16,47,50,59 These trends have negatively impacted natural resource management and agricultural production (KM 11.1) by increasing stress on vegetation.60 Coupled with increases in demand and subsequent water withdrawals, reduced streamflow has caused many of the region’s lakes and reservoirs, such as the Great Salt Lake, to reach historically low water levels.61 Furthermore, greater variability in streamflow threatens the region’s ability to consistently produce and use hydropower, impacting a typically reliable and low-carbon source of energy.62
Katelyn GarciaThe Dust We Will Breathe(2022, inkjet print)Artist’s statement: The Dust We Will Breathe is a photograph of the drying lakebed of the Great Salt Lake, a graveyard of once underwater mounds made of microbial organisms. Human consumption is mostly to blame for the lake reaching historic lows, which is compounded by climate change and the west’s current megadrought. If no drastic changes in consumption are made, the lake will be gone in 5 years. Every day that more lakebed is exposed, we will breathe in more of its toxic dust.View the full Art × Climate gallery.Artworks and artists’ statements are not official Assessment products.
Climate warming will also reduce groundwater recharge from rainfall, snowmelt, and runoff in some areas, thereby reducing groundwater storage.63,64 These effects are exacerbated by groundwater pumping to satisfy the needs of agricultural irrigation,65,66 which is the biggest consumer of fresh water in the region. The Central Valley aquifer of California is one of the most stressed aquifers in the world; during the 2012–2016 drought, about two-thirds of the valley’s surface water deficit was due to groundwater pumping, which caused land subsidence (the gradual sinking of land) in some areas67 and declines in groundwater quality.
Despite the region’s increasing aridity, flooding from extreme precipitation events (KM 3.5) and snowmelt conditions (KM 4.1) also poses a threat to life and property, as well as to freshwater ecosystems.68,69 Due to climate change, snowmelt-driven flooding is expected to occur earlier in the year due to earlier runoff.70 Moreover, atmospheric rivers, which have driven much of historical flooding in the region, are expected to intensify under a warming climate.71,72 Flooding from sea level rise may also threaten water infrastructure and supplies in areas such as the Sacramento–San Joaquin Bay Delta region.73,74
Critically, the impacts of these climate-driven changes are experienced disproportionately by certain communities in the region, including Indigenous communities (KM 16.1). A lack of clean water and sanitation services in Indigenous communities came to national light in 2020 due to COVID-19, which spread 3.5 times faster among Indigenous than non-Indigenous communities in the initial stage of the pandemic,75 due in part to the lack of access to potable water in some Indigenous communities. A major impediment to water access is the cost of water infrastructure, which averages $600 per acre-foot of water for non-Indigenous families with piped delivery, compared to $43,000 per acre-foot of water for Navajo families relying on hauled water (no dollar year available).76,77 Furthermore, many Tribes in the region continue to lack access to water because their water rights have not been adjudicated through settlements or other processes, which could further exacerbate water shortages for other users.78
Other examples of overburdened communities experiencing disproportionate water-related impacts of climate change include certain Black communities, which face disproportionately higher flood risk in Los Angeles,79 and Hispanic and low-wealth communities, which receive lower-quality drinking water80 and may be systematically excluded from water management processes (KM 4.3).81
In response to these interrelated climate challenges, people across the Southwest have implemented adaptive water governance and management approaches. Examples include California’s Sustainable Groundwater Management Act82 and various conservation and drought response measures in the Colorado River basin,37,38,83 which incentivize collaboration among diverse participants to develop innovative solutions (KM 12.4). Transitions toward more sustainable water management under climate change also include innovative infrastructure (e.g., enhanced aquifer storage, recharge, and recovery) and institutional practices (e.g., integrative land and water management practices, changes in rate structures, water sharing agreements, and reservoir operations).84,85,86,87 Social science studies in Southwest cities such as Denver, Phoenix, and Las Vegas indicate widespread support for innovative management strategies for urban water sustainability88 and opportunities for targeted educational interventions for demand management strategies based on residents’ attitudes toward climate change.89 The extent to which these adaptation actions mitigate changes in water availability depends on interacting climate and social dynamics (KM 3.4).
Large-scale marine heatwaves and harmful algal blooms have caused profound and cascading impacts on marine coastal ecosystems and economies (high confidence). Without implementation of adaptation or emissions-reductions measures, human-caused warming will drive more frequent and longer marine heatwaves (very likely, very high confidence), amplifying negative coastal effects (medium confidence). Sea level rise, along with associated impacts such as flooding and saltwater intrusion, will have severe and disproportionate effects on infrastructure, communities, and natural resources (likely, very high confidence). The California State Government has applied climate science to planning and decision-making for sea level rise, and multiple regions are moving toward climate-informed and adaptive strategies for fisheries (high confidence). However, climate planning and adaptation solutions for aquaculture are less clear (high confidence).
The coastal region of the Southwest encompasses approximately 3,400 miles of coastline and nearly 70% of the state’s 39.4 million people. California’s 19 coastal counties employ more than 12 million people92 and in 2012 accounted for 80% of the state’s GDP ($57.25 billion in 2022 dollars).93 Furthermore, California is showing leadership through adaptation actions nationally.
California coastal sea surface temperature has increased an average of 0.4°–0.6°F per decade since the 1970s94 and is projected to increase into the future under climate change (Ch. 2).95,96 Human-caused warming also contributes to marine heatwaves (Figure 28.4), or incidences of exceptionally warm ocean temperatures, which have already had significant impacts on human and natural systems (Box 10.1).97,98,99,100 The change in average cumulative intensification of MHWs for the entire US coast is presented in Figure A4.11. As the ocean warms, including in California coastal waters, MHWs increasingly exceed thermal limits of ecosystems, amplifying impacts99 including shifts in marine species composition,101 lower abundance and nutritional quality of important small prey fishes,102,103 and a potential influence on mass seabird mortalities and reproduction.104,105 Similarly, Tribal/Indigenous Traditional Knowledge demonstrates significant declines in five coastal species of cultural significance for the Tolowa Dee-ni’ Nation, the Cher-Ae Heights Indian Community of the Trinidad Rancheria, the Wiyot Tribe, and the InterTribal Sinkyone Wilderness Council, a Tribal consortium of ten Tribal Nations.106 Such ecological changes disproportionately impact coastal communities and economies (KM 9.3),107,108,109 including cultural resources for Indigenous Peoples (KM 15.2).106,110 The 2014–2016 Northeast Pacific marine heatwave was followed by others in 2018111and 2019–2020.112 These MHWs can coincide with and contribute to other climate-related extremes such as drought100 and harmful algal blooms (HABs).113
Commercial and recreational wild fisheries, as well as aquaculture (aquatic farming), will continue to be negatively affected by MHWs and HABs,107 resulting in severe economic ramifications.113,114,115,116 Extreme ocean warming events also have compound effects: an MHW contributed to the loss of more than 90% of Northern California’s bull kelp, a foundational species for the ocean ecosystem, resulting in large economic losses in fisheries (Focus on Blue Carbon),117,118 including the red abalone, a species now listed as critically endangered by the International Union for Conservation of Nature. Further, extreme event–related delays and closures disproportionately impacted smaller-scale fishing operations.119
The widespread impacts of MHWs and HABs underscore the need for effective adaptive approaches to fisheries management. While fishers in California are coping with MHWs by fishing in different areas or for different species,108 it will be challenging to manage fisheries in the long term under extreme warming events.115 Marine protected areas (MPAs), which are considered a management strategy for climate-driven ocean changes, may not buffer widespread effects of MHWs on species in southern California kelp forests.120 Adopting an ecosystem approach that considers multiple management options instead of one species in isolation121,122 appears to improve management under climate change.123 Applying a more coordinated disaster risk management approach to MHWs and extreme HAB events appears to correspond to better adaptive fisheries management, emphasizing the need for improving coordination and consistency across governing bodies, communities, and fishers on the frontlines (KM 10.3).124,125
Human-caused ocean warming coincides with increasing ocean acidification (OA) and declining oxygen levels (hypoxia) of the deeper, more nutrient-rich upwelled coastal waters. Under a very high scenario (RCP8.5), sardines, a commercially and ecologically important species, are predicted to move poleward, resulting in substantial shifts in catch.109,126 Under the same scenario, increased acidity due to the ocean’s chemical response to absorption of carbon dioxide is projected to increase the mortality of calcifying invertebrates (such as oysters and other bivalves), which are important to aquaculture and the food web, and result in a loss of food sources for some fishes and invertebrates.127
Potential adaptation solutions include an ecosystem management approach to marine habitats and fisheries, as well as enforcing water and land-use regulations, which are expected to buffer some climate impacts.128 Protection and restoration of foundational eelgrass and kelp forests in California waters provides essential habitat, and these ecosystems can also improve local pH and oxygen conditions.129,130 The Fishery Ecosystem Plan adopted by Pacific Fishery Management Council in 2013 includes guidance on OA and hypoxia,128 but additional strategies—such as flexible permitting, better coordination with fishing communities, and adaptable control rules—may be needed to improve outcomes.131 Nature-based aquaculture solutions, such as conservation and restorative aquaculture, also have potential to mitigate local OA impacts132,133,134,135 but are just emerging in California.136
Sea level rise (SLR) poses risks to the California coast through an increase in flooding, impacts from storm surges, and loss of coastal habitats and beaches (Figure 28.5; KM 9.1). Seas are projected to rise, on average, 0.79–1.25 feet for the California coastline by 2050, 3.10–6.63 feet by 2100, and 6.11–11.90 feet by 2150 (Intermediate to High scenario).7 California has more people living below 3.3 feet (1 m) of elevation than any other state except Louisiana;137 the population living in the mapped 100- and 500-year coastal floodplains increased approximately 10% from 2010 to 2020.137 SLR is also expected to exacerbate inequities in communities and result in compounding impacts, such as saltwater intrusion polluting groundwater.7,138,139,140,141,142,143 Furthermore, coastal Tribes in California are observing rising sea levels, which, when combined with the loss of kelp forests, are increasing the risk of coastal erosion, destruction of cultural artifacts, and limited access to traditional shoreline sites.110
By 2050, for all emissions pathways, SLR effects on tide and storm surge are expected to cause more frequent moderate to major high tide flood events, and coastal communities are already experiencing minor to moderate high tide flooding (KM 9.1).7
Coastal energy and transportation infrastructure is expected to be negatively impacted by flooding from SLR. The projected inundation of energy substations in low-lying areas during storm events and from extreme SLR under a very high scenario (RCP8.5) is expected to cause electricity service interruptions to thousands of customers and increase maintenance and repair costs.139 Analysis of California’s transportation fuel network found that docks, terminals, and refineries are most exposed to coastal flooding.143 The California Department of Transportation has begun adaptation planning efforts that consider a variety of strategies beyond hardening infrastructure, including nature-based strategies to limit the impacts of flooding (KM 8.3), as well as planning to avoid loss of coastal resources and access.144
Sea level rise and increased coastal flooding will disproportionately impact frontline communities (KM 9.2).145 The Toxic Tides project found that under a very high scenario (RCP8.5), SLR in California146,147 would result in increased flooding to over 400 industrial facilities and contaminated sites, including power plants, refineries, and hazardous waste sites, with 440 projected to be at risk of at least one flood event per year by 2100.148 Any flooding of hazardous sites would increase risks of contamination in surrounding frontline communities.149
Residents of affordable housing, typically low-income communities, are especially vulnerable to SLR, with a greater percentage of affordable housing exposed to SLR than the general housing stock in some coastal states.140 California is in the top four states nationwide with the most units of affordable housing exposed at least four times per year to coastal flooding based on projected sea levels for the year 2050 under a very high scenario (RCP8.5).140 By 2050, California is also projected to see a 40% increase in the number of units at risk of flooding, compared to 2000.140 For affordable housing residents, flood risk is compounded by the threat of displacement due to rising property values and rents. Strategic city-level adaptation and resilience efforts, combined with community and infrastructure improvements, could protect these residents from potential displacement.140,150
Higher seas are raising the coastal groundwater table, exposing communities to flooding from water that emerges from underground (KMs 9.1, 9.2).138,141 Communities in low-lying areas such as San Francisco Bay are most at risk, and areas with shallow coastal water tables are projected to see widespread flooding from groundwater emergence.138,141 Subsidence exacerbates this threat; coastal residents residing in subsiding locations experience an average relative sea level rise of up to four times faster than the global rate.142,151 These risks have not been well addressed in adaptation planning. Furthermore, the impacts and adaptation needs are expected to be higher than reported if only overland flooding due to SLR—which does not include flooding from subsidence or groundwater intrusion—is considered in community and infrastructure planning.142
Adaptation planning for SLR as a field has advanced,152 as coastal managers have reported an increased concern regarding the threat of SLR and local, regional, and state governments in California apply climate science to decision-making (KM 9.3).153 California has instituted policies requiring consideration of climate change in state and local government decision-making and infrastructure planning.154,155,156,157 Specifically for the coast, there is guidance on how to apply SLR risk assessment and projections into planning, including specific guidance for critical infrastructure.158,159 This landscape of statewide policy and guidance is directly informing local coastal adaptation planning. Of 19 coastal counties, 18 have completed a vulnerability assessment, developed an adaptation policy, and/or updated the state-mandated safety elements of their general plans to include climate adaptation.160
While adaptation planning along the California coast has advanced significantly, many of these efforts have not yet been implemented.160 This is partly because of financing and implementation challenges, especially for local governments that lack resources and must overcome institutional and governance issues (KM 31.5).152,161 Despite these challenges, California is ahead of many other parts of the US coast in employing adaptation strategies and appears to be well positioned for increased adaptation.152
Continuing drought and water scarcity will make it more difficult to raise food and fiber in the Southwest without major shifts to new strategies and technologies (high confidence). Extreme heat events will increase animal stress and reduce crop quality and yield, thereby resulting in widespread economic impacts (likely, high confidence). Because people in the Southwest have adapted to drought impacts for millennia, incorporating Indigenous Knowledge with technological innovation can offer solutions to protect food security and sovereignty (medium confidence).
Across the Southwest, annual average minimum air temperatures, growing degree days, and average number of days above 86°F (the threshold used to define heat zones) are projected to increase due to climate change.162 By midcentury under intermediate (RCP4.5) and very high (RCP8.5) scenarios, projections show longer growing seasons, a northward shift in plant hardiness zones, and expanded areas of heat stress exposure to crops and livestock (KM 11.1).162 In California, increasing temperatures are expected to affect the timing of cool-season annual crops and the location of warm-season annual crops.163 Warmer winters would be detrimental to the chilling requirements for orchard crops.164 In California, fewer cold snaps are expected to reduce crop exposure to frost;165 however, “false springs” in the intermountain West are expected to increase vulnerability to late-season freeze events.166 During summer, a higher probability of heatwaves is expected (KM 2.2).167 The productivity of some economically important crops, such as upland cotton in Arizona, has already declined because of heat stress.168 While increased drought is the most prominent climate-driven risk to agriculture in the region, important farming areas such as California’s Central Valley also face damage from occasional large floods caused by atmospheric river events.169
Farmers and ranchers are particularly at risk from prolonged, severe drought (Figures 28.6, 8.6). Future temperature increases are expected to drive higher rates of evapotranspiration, increasing demand for fresh water for irrigation.168 The producers most vulnerable to local precipitation deficits are dryland farmers growing rain-fed crops and producers raising livestock on rangelands. Community-based snow-fed irrigation systems in high-elevation watersheds of New Mexico and Colorado, known as acequias, are particularly exposed to the shortfalls in annual snowpack.170 Under increasing aridity, agricultural practices such as fallowing and grazing on rangelands will need careful management to avoid increased wind erosion and dust production from exposed soils.171 Rising summer temperatures also degrade protective desert soil crusts formed by communities of algae, bacteria, lichens, fungi, or mosses, adding to airborne dust loads.172 The impacts of increasing aridity on agriculture are therefore twofold because dust deposits on mountain snowpack drive faster melting, depleting the snowpack173 and resulting in reduced surface water for irrigation. While just 22 of the 216 counties in the region are classified as “farming-dependent” by the USDA,174 agriculture is an important contributor to state and local economies and US food supply. California leads the Nation in agricultural cash receipts,175 primarily from fruits, nuts, and vegetables; direct farm sales to consumers; and farm expenditures.176 Climate change poses risks to both productivity and quality of fruit and vegetable products, requiring adaptations on farms and throughout the supply chain, including changes in crop calendars, nutrient and pest management strategies, post-harvest handling, and preservation methods.177,178
Reduced crop production due to climate change will carry major economic costs. Drought events have brought significant economic impacts on regional agriculture (KM 19.1); for example, the 2021 drought cost California farming sectors an estimated $1.28 billion (in 2022 dollars) and led to the loss of 8,745 full- or part-time jobs (KM 11.3).179 Modeling studies indicate that warming temperatures are expected to have a detrimental impact on the yields of almonds,164 wine grapes,180 and other high-value crops.169 Localized adaptation strategies include crop- and locality-specific combinations of irrigation, site management (e.g., use of cover crops and increased fallowing), and cultivar selection.181 Fallowing as a response to water shortages can bring its own challenges, such as increased dust and weed production, but it can also enhance ecosystem services such as groundwater recharge and improved ecosystem health.182
Climate warming is likely to lead to larger, more frequent, and more severe outbreaks of bark beetles, negatively affecting the quality and quantity of timber available to the region’s forestry and forest products industries.183 While wood products are minor economic contributors to the region’s inland states, costs could be considerable in California, where the industry has been estimated to contribute $44.8 billion (in 2022 dollars) and 177,000 jobs (KM 7.2).184
Over time, agricultural income in the region has become more dependent on crops than livestock.185 Because most Southwest croplands are irrigated, agriculture in the region had been thought to be less vulnerable to climate change than that in other parts of the country. However, future irrigation supply is uncertain as it depends on dwindling ground and surface water supplies (KMs 28.1, 4.1). For example, Arizona allows up to 73% of its water to be used for crop production,186 but the promise of continued irrigation water is less clear given the state’s rapidly growing population and decreased flows in the Colorado River.187 Crop irrigation, mainly of alfalfa, accounts for three-quarters of consumptive water use in the Great Salt Lake basin, where cuts to irrigation use are advocated as the state seeks to prevent total depletion of the lake and associated environmental and public health impacts.188 Strategies to reduce irrigation water use include switching from gravity flow and sprinklers to more efficient systems,186 but the costs of conversion can be difficult for farmers when climate change is already reducing yields.185 Federal insurance programs can assist farmers after climate-related crop or forage loss, providing short-term economic relief from effects of extreme events.189 However, some research suggests that federal insurance programs provide a disincentive for farmers to adapt to climate change impacts.189 Non-climate-related stressors can influence the capacity of agricultural communities to adapt to climate impacts.29 On the plains of Colorado and New Mexico, most rural counties are depopulating due to persistent out-migration of young adults, straining social services and reducing tax revenues.190 Yet the Southwest also has some of the fastest-growing areas in the US, including high-amenity rural areas and cities expanding into agricultural zones.191 Urban expansion can increase cropland loss while simultaneously increasing the number of small farms focusing on specialty crops rather than basic commodities,192,193 placing greater pressure on the region’s food supply as drought (KM 28.1)51 threatens agricultural production.194
Livestock production is the dominant use of agricultural land in large areas of the Southwest where crop production is unprofitable or infeasible. Animal agriculture accounts for about one-third of agricultural revenue, with about 70% from cattle.195 Climate change is expected to reduce the sustainability of cattle production that depends on rangeland ecosystems.195,196 Negative impacts are expected on the entire livestock food supply chain, affecting production and nutritional quality of forage, livestock health on rangelands and in transport due to heat stress and pest exposure, and shelf life of products during transport and storage.197,198 Forage from Bureau of Land Management rangelands is expected to decrease in Arizona and New Mexico, but it is less certain whether rangeland forage will hold steady in the central and northern portions of California, Colorado, Nevada, and Utah due to differences in moisture availability during the growing season.197,199
The cascading impacts of climate change in combination with urban population increases and other social and cultural factors pose an increasing threat to agriculture in the region.29 Urban growth in the Southwest has led to competition for water between farms and cities, mirroring global trends.200 Water transfers from rural to urban areas have been a feature of the Southwest for decades, often with negative consequences for rural and low-income communities.201,202 To meet water demands for a growing metropolitan region while preserving irrigated croplands, Colorado is experimenting with water policy innovations designed to encourage rural-to-urban transfers while minimizing impacts in rural areas, but adoption has been slow due to distrust on the part of agricultural communities and uncertainty about trade-offs.202 Market forces in California have encouraged growers to shift to crops with a high economic value but also a large water footprint, such as tree nuts203,204 and legal cannabis.205
Frontline communities including Hispanic populations, women farmers, migrant farmworkers, and Indigenous Peoples face challenges to water access in their homes as well as food security and health (KM 4.2).201,206,207 For example, the 2012–2016 drought in California’s San Joaquin Valley disrupted farmworkers’ employment and reduced food security, water security, and health.208 Mental health risks are also increasing as farmers and ranchers report moderate to severe levels of anxiety about climate change and the need to adapt.209 First-generation and women ranchers are disproportionately vulnerable to climate impacts because of limited experience with drought and weaker connections to rancher networks.210
Low-income urban communities are expected to be among the first to suffer food insecurity as climate change reduces the region’s food production. Strategies have been proposed to produce more food in urban settings, but these foods often do not reach low-income consumers, who have less access to food distribution systems and often cannot afford to pay the higher prices such foods often command.211 Indigenous Knowledge has been proposed as a significant resource for climate change adaptation (KM 16.3).212,213 Because people in the Southwest have adapted to drought impacts for millennia, employing Indigenous Knowledge can allow the region to serve as a “laboratory” for future climate-adapted food systems214 while enhancing food sovereignty.215
Adaptation solutions exist for ranching operations,196,216 but social and economic barriers, such as distrust of experts, the financial costs and time commitments of innovation, and adherence to tradition, have slowed information uptake.199 Climate change information is not routinely incorporated into ranchers’ risk management decisions217 and only recently has become a priority in federal agency rangeland management plans.197
People across the Southwest are exploring technological adaptations to climate impacts (KM 31.3). Adaptive conservation management approaches that focus on minimizing soil disturbance while maximizing soil cover, biodiversity, and the presence of living roots have been gaining traction with farmers through practices such as cover cropping and reduced-tillage and no-till farming (KM 11.1).218,219 Combined with reduced tillage, cover cropping improves soil structure, organic carbon content, and infiltration and water-holding capacity in irrigated cropland220 and positively impacts nutrient cycling, crop yield, and soil water conservation in limited-irrigation, semiarid cropping systems.221,222 However, some farmers and ranchers, such as those who operate on small acreages, often find it hard to access the resources to transition practices or may perceive the risks of change to be too great, including financial expense and the perceived need to learn new skills.223
Irrigation efficiency can reduce risks to farming and ranching operations due to increasing temperatures, unreliable precipitation, and reduced water resources. However, access to these solutions can be complicated due to farm or ranch location, access to surface water and groundwater, water rights, current irrigation methods, and crop types.224 In the Verde Valley of Arizona, limited access to materials, equipment, and financial resources, especially for small-scale producers, inhibits their ability to respond to water-related challenges.224 Indigenous Peoples face barriers in accessing support from the Natural Resources Conservation Service (NRCS) related to land tenure, financial assistance, institutional mismatches, and complexities in incorporating Indigenous agricultural methods in applications for NRCS programs.225,226
Increases in extreme heat, drought, flooding, and wildfire activity are negatively impacting the physical health of Southwest residents (high confidence). Climate change is also shaping the demographics of the region by spurring the migration of people from Central America to the Southwest (medium confidence). Individuals particularly vulnerable to increasing climate change impacts include older adults, outdoor workers, and people with low income (high confidence). Local, state, and federal adaptation initiatives are working to respond to these impacts (high confidence).
Since 2018, 31 large climate- and weather-related disasters have affected the Southwest, resulting in more than 700 fatalities and estimated damages totaling $67.3 billion (in 2022 dollars).2 Strong evidence indicates that extreme heat disproportionately impacts the health of frontline and overburdened communities in the region (KM 15.2), including the unhoused,227,228,229 outdoor workers, and migrant farmworkers (Figure 28.7; KM 11.2),230,231,232,233 as well as those with low income8,234 and older adults.235 Between 2016 and 2020, 7,687 hospitalizations in the Southwest were due to heat and heat-related illnesses, in comparison to 5,517 in the previous five years (2011 to 2015).236 Pre- and post-natal exposure to high heat and air pollution are shown to be particularly dangerous in the region.237,238,239,240
Extreme heat and high-ozone days in the region are expected to increase under climate change (KMs 2.3, 3.5).241 These changes are expected to increase heat and air-pollution exposure, illness, and premature death.242 Intensified aridity from higher temperatures and drought is expected to lead to more dust storms243 and more than double the number of deaths attributed to fine dust by 2080–2099 under a very high scenario (RCP8.5), relative to 1986–2005 (KM 6.1),244 with increasing exposures for outdoor workers during the warm season. The incidence of coccidioidomycosis (Valley fever) in the region has increased (Figure 15.2)245 and is associated with higher air temperatures and drier soils,246,247 with greater risk to those whose job requires dirt disruption. The annual average cost to the US economy of Valley fever for the 2000–2015 baseline was $4.8 billion per year (in 2022 dollars), which is projected to increase 390% by 2090 under a very high scenario (RCP8.5; Figure 15.2).248
Extreme heat exposure also affects the economy through decreased productivity and well-being in outdoor workers (Figure 28.7),249,250,251 especially among migrant agriculture workers in the region (KM 15.1).252,253 Impact estimates to productivity provided in Figure 28.7 are projected to result in a loss of 25% of the workday on all days in the third quarter (July–September) under a very high scenario (SSP5-8.5) by the end of century and cause important losses to the economy (KM 19.1). Dehydration due to working outdoors in extreme heat in California is linked to acute kidney illness even after a single day of exposure.254 Research into the mechanisms of chronic kidney disease related to climate change is ongoing, yet occupational heat exposure is a causal factor.255
While the annual average level of fine particulate matter (PM2.5) has seen decadal decline in the region due to strengthened air quality policies reducing emissions from controllable sources, disparities in PM2.5 exposure and related health concerns remain high in the region.260 Moreover, the frequency and severity of smoke events with PM2.5 exceedances of federal air quality standards have increased significantly due to wildfires (Figure 14.3). Since 2015 in Northern California, the annual average PM2.5 has increased because of wildfire events, which have taken over as the main source of PM2.5 exceedances.261 PM2.5 in wildfire smoke contributes to adverse health effects for firefighters262 and the public263,264 and can be more hazardous to health than similar levels of particulates from other sources.265 The costs of adverse respiratory and cardiovascular health outcomes can exceed the billions spent on wildfire suppression (KM 28.5).266 Chemicals in wildfire smoke also correlate with increased cancer risk.267 Direct exposure to the 2018 Camp Fire in California has been linked to mental health disorders such as depression and post-traumatic stress disorder.268 Wildfires can also cause other public health impacts, including water contamination when fires damage water distribution infrastructure,269 long-term loss of access to clean drinking water,270 and increased landslide risks (KM 28.5).
Increases in flooding in the region are projected with continued warming.271,272 These changes increase risks of water-borne diseases and exposure to toxic hazards and place stress on food, energy, and water supplies, as well as farmworkers’ health (including interconnected sectors) and their socioeconomic insecurity.39 Flooding exposures may increase as a greater proportion of the population across the region, on average, is living on 100-year floodplains (e.g., in California, between 1990 and 2020, 25,000 more people lived on 100-year floodplains).137 Flooding can also interrupt vector-control programs, such as for West Nile virus.273 The region is seeing challenges with West Nile virus, particularly in Arizona and California,274 with projected increases due to changes in the climate, human population, and mosquito distribution (KM 24.3).275,276
Limited occupational health and safety standards for farmworkers and other outdoor workers are of key concern, as intensifying wildfires and heat collide with harvest season each year, particularly for undocumented Latino/a and Indigenous migrants.231 The improvement of these standards at the state and national levels will be critical for health adaptation to climate impacts in the region. Moreover, the harm to farmworkers due to wildfire smoke is expected to be greater than previously thought, bolstering the argument for additional research and policies to help safeguard overburdened and stigmatized populations.277 Many Southwest cities experienced high rates of economic and population growth during the second half of the 20th century, particularly between 2015 and 2019.31 The region’s flourishing economy and proximity to the Mexican border result in a high influx of migrants.278,279 Migrants from Central America, spurred to migrate due to climate change in addition to poverty and violence (KM 17.2), are drawn by the Southwest’s strong economy and increase the number of vulnerable people and change the demographics in the region. Local, state, and federal efforts to both mitigate climate change and support essential human adaptation to increasing exposures will be vital in protecting the health of a growing and aging population and our most vulnerable communities (Figure 28.8; KM 15.1).280
The effects of climate change on other regions of the world—especially Central America—are changing the Southwest’s demographics. Decreasing agricultural productivity, increasing levels of food insecurity, and adverse climate effects are among the main reasons why people emigrate from the Northern Triangle (Guatemala, Honduras, and El Salvador) to the US (KM 17.2).32,279 In 2021, 42% of Central American immigrants to the US lived in the Southwest region,282 and about 43% of immigrants apprehended at the Southwest border in 2019 originated from the Northern Triangle.278 Many are poor, women, children, or Indigenous Peoples. Climate-related migration has been shown to affect people’s physical and mental health, resulting from exposure to weather extremes, disruption of social ties, and overcrowding of health systems in the host communities.283
Several programs have been developed to address the health impacts of climate change in the region, but financial constraints and political support affect their implementation.284 Since 2010, the CDC’s Building Resilience Against Climate Effects program in Arizona and California has developed and implemented strategies to protect communities from climate-sensitive hazards, including schools, healthcare facilities, and other at-risk populations.285 This program, currently in 10 cities across the country, has developed important resources and programs that can be scaled up for future climate resilience efforts. To protect the health and learning of school children, the Arizona Department of Health Services created new heat policy guidance, resulting in recommendations for school heat safety and adaptation strategies.286,287 In California, San Mateo County assessed asthma burden connected to local climate issues.288 Public health guidance in the region should focus on co-exposures to heat and wildfire smoke in adaptation efforts,289 particularly given the projected increase in childhood asthma due to wildfires.290 While data on private sector investment is limited, the private sector has historically lacked incentives to invest in adaptation (KM 31.6). Globally, in 2017–2018 only 1.6% of all adaptation financing came from the private sector.291 In the Southwest, however, certain sectors, such as insurance, came under pressure from the local authorities to get involved in tackling climate change by divesting their fossil fuel–based investments.292
In recent years, the Southwest has experienced unprecedented wildfire events, driven in part by climate change (high confidence). Fires in the region have become larger and more severe (high confidence). High-severity wildfires are expected to continue in coming years, placing the people, economies, ecosystems, and water resources of the region at considerable risk (very likely, high confidence). Opportunities for adaptation include pre- and postfire actions that reduce wildfire risk and facilitate ecosystem restoration and include traditional land stewardship practices (high confidence) and the application of Indigenous cultural fire (medium confidence).
Fire is a natural process in many ecosystems across the Southwest and is necessary for biodiversity, ecosystem services, and nature-based solutions (KM 8.2). Fire regimes associated with fire-dependent ecosystems are highly variable with elevation and across geographies.293 Long-standing policies and forest management—including fire suppression, widespread logging and livestock grazing, and elimination of Indigenous fire use—combined with the effects of a changing climate, have contributed to high tree densities, compromised ecosystem function, and the diversity, or heterogeneity, of forest attributes such as species, size classes, and geographic distributions.18,21,294,295,296 Consequently, many Southwest forests and fire-prone wildlands are susceptible to climate-mediated impacts including droughts, pests and disease (Box 7.1), and devastating wildfire.295,297 An abundance of scientific research strongly suggests that Southwest ecosystems, in the face of rapid climate change–induced transformation, will require active management interventions that increase forest heterogeneity and enhance ecosystem function and adaptive capacity (KM 7.3).298,299,300
Human-induced warming temperature trends, changes in precipitation patterns, and increases in vapor pressure deficit have driven the desiccation of fuels that influence wildfire patterns and behavior across the western US (KM 7.1).17,18,19,20,21,440 In the Southwest, fires have become larger, more frequent, and, in many areas, more severe (KM 7.1), with clear evidence of climate change as a major cause.301,302 Seven of the ten largest US wildfires in 2020–2021 occurred in the region. Of the 50 largest US wildfires in 2020, 22 occurred in California, and the 7 largest wildfires recorded in California have occurred since 2018.303,304,305 The three largest wildfires recorded in Colorado occurred in 2020; the largest fires in Nevada history burned in 2018;306 and the largest wildfires in Arizona, New Mexico, and Utah all occurred since 2007 (Figure 28.9; Focus on Western Wildfires). Large fires on non-forested western US rangelands also increased more than fivefold during the period 1984–2017.307 Much of this increase is driven by increases in invasive annual grass cover, caused partly by climate change and increased atmospheric carbon dioxide.308
Climate change causes cascading effects with other factors in Southwest ecosystems that are otherwise fire-adapted. For example, large areas of high-severity fire have driven ecosystem type conversions in many parts of the region.294,309 Semiarid to arid forest systems are particularly vulnerable to these effects and have experienced conversion to native grassland,310 shrubland, or non-native grassland (Figures 28.9, 8.6).294 The cumulative effects of fire-driven ecosystem changes continue to place ecosystems at high risk of vegetation type conversion (e.g., forests to shrublands), which can result in severe impacts on watersheds and aquatic resources.294 Effects include degradation of riparian systems; risks to riparian and riverine species, as well as to threatened and endangered species, from erosion caused by extreme precipitation events; and increased invasions by non-native species.311
Recent climate-induced aridification, including loss of snowpack, has also hindered postfire tree seedling and shrub establishment, limiting ecosystem recovery.312,313,441 This is particularly true of postfire conditions for water availability, quantity, and quality. Moreover, interactions between wildfire and natural drought variability are expected to increasingly exacerbate dry conditions that will further stress tree seedlings314 and drive potential future shifts in species composition or vegetation type.312,315 Conversions of coastal shrublands driven by human development also have interacted with climate-induced drying of vegetative fuels to generate atypical fire conditions.316
Projections of future climate change suggest that wildfire activity will continue to affect ecosystems and their services.298,317,318,319,320 Specific ecosystem responses to climatic changes will depend on interactions among vegetation type, moisture stress, disturbance regimes (e.g., pests, pathogens, and high-severity fire), and human land-use changes (KM 6.2).295,297,317 For example, climate change is predicted to lead to a loss of sagebrush ecosystems in the southern and eastern Great Basin because those ecosystems are less able to recover after fires in a warmer, drier climate.321
However, future wildfire trends are less certain in rangelands than in forests because fire size (measured by annual area burned) and severity (a shift from low-intensity fires to stand-replacing crown fires) depend on production of aboveground vegetation that varies annually with climatic conditions.322 Growth of the grasses that typically fuel wildfires is expected to decrease in Arizona and New Mexico,199,323 whereas elsewhere in the region, precipitation is projected to increase early in the growing season, which, when followed by hotter summers, will generate conditions ideal for fire ignitions.322
Climate-related increases in the frequency, severity, and extent of wildfires in the Southwest are endangering lives and property (Focus on Western Wildfires).17,18,19,20,21,324 Complete data across the region on wildfire-caused fatalities are sparse,325 but three of the five deadliest fires on record in California have occurred since 2017, costing 122 lives.303 Further, loss of life can be attributed to wildfire smoke, which has also been linked to increases in COVID-19 fatalities in Northern California326 and postfire debris flows that can leave slopes bare of vegetation and vulnerable to rapid erosion (Ch. 7; Focus on Western Wildfires).327 The risk of postfire debris flows in coastal communities is expected to increase due to an increase in heavy precipitation typically delivered by atmospheric river events (KMs 28.1, 8.1).328
Property losses due to wildfire are greatest in the Southwest compared to other regions. In 2021, 3,363 structures burned due to wildfires in California, the highest number lost in any state, while the December 2021 Marshall Fire in Colorado, a fast-burning grassland fire, burned more than 1,000 homes in just a few hours.305 The 2022 Calf Canyon/Hermit’s Peak Fire that burned 341,735 acres is New Mexico’s largest fire. Secondary impacts of wildfire, such as postfire debris flows (Figures 3.13, 6.5) on recently burned slopes, impose additional hazards to property.327 The estimated cost of fighting the 10 largest California wildfires in 2021 exceeded $2.25 billion (in 2022 dollars),305 with suppression costs representing only a fraction of a total economic impact that also includes loss of structures and infrastructure, increased medical costs, crop losses, water quality contamination, and other factors (KM 19.1).
Julia Lauercalifornia(2020, paper, ink)Artist’s statement: 'california' is a five-layer reductive woodcut that depicts an immense blaze trailing from a mountain range to a burning house. The piece was born in response to the California wildfires. The contrast between the bright flames and the dark sky is representative of the dramatic change between El Niño and La Niña years, which have a cyclical influence on fire in the region. The burning house speaks to the impact wildfires have on people, while the burning plains and mountains comment on the effect fire has on the planet.View the full Art × Climate gallery.Artworks and artists’ statements are not official Assessment products.
The increase in structures and infrastructure lost can be linked to population growth in the wildland–urban interface (Figure A4.14), where houses are built close to forests and other natural areas.329 The number of Americans living in the WUI doubled from 1990 to 2010, and the WUI population has risen fastest in areas such as the Southwest where wildfire risk is greatest.330,331 While migration to WUI counties shows modest reductions immediately after wildfire or extreme heat events,332 fires do not seem to drive current residents away; fewer than 6% of Sonoma County, California, residents who lost homes to wildfires in 2017 subsequently left the county.333
Impacts of fire on community livelihoods depend on exposure to wildfire risk and capacity to adapt (KM 7.3). Analysis of livelihood vulnerability in 14 fire-prone states found that New Mexico and Arizona residents were most vulnerable due to relatively high risk exposure and low to moderate access to resources needed to adapt to changing conditions.334 Low-income areas, communities of color, undocumented Indigenous migrants, sexual and gender minorities (KM 15.2),335 and unsheltered persons are most vulnerable to wildfire impacts,231,336 including water contamination from carcinogenic compounds.269 Populations with medical disabilities or limited mobility, older adults, and those who rely on medical equipment are also disproportionately at high risk during wildfires.337
Wildfires, moreover, often occur during farm harvest seasons, increasing health risks for workers.277 Southwest industries especially vulnerable to wildfires include wineries,338 tourism,339,340 forest products,183 and legal cannabis cultivation.341 For example, in the 2020 fires, the economic impact of smoke taint is estimated to have cost the California wine industry $4.2 billion (in 2022 dollars). Smoke taint occurs when smoke and ash permeate the skin of grapes and can affect the taste and smell of wine.342
Impacts of wildfire on natural environments can affect ecosystem services (KM 7.2) that people derive from those environments, including air quality (KM 28.4),266 water quality and availability,343 pollination,344 livestock forage and health,345 and outdoor recreation.346 Ecosystem service effects vary over time as short-term declines in services can be followed by improvement over a longer term as ecosystems recover.347 Wildfires in forested ecosystems chemically alter watersheds and can reduce drinking water quality,348 in some cases leading municipalities to issue drinking water advisories.349 Postfire hazards such as floods and debris flows further threaten water security (Focus on Western Wildfires).350 Smoke from the 2020 wildfires significantly reduced industrial solar energy production in Southern California.351
While high-severity wildfires (i.e., stand-replacing fire) generally have negative impacts, wildfires and prescribed fires that burn at low to moderate severities can have positive effects by reducing fuel loads, curtailing plant pests and diseases, and stimulating new vegetation growth.352 Prescribed burning, while an effective tool to reverse undesirable changes in forest vegetation structure due to fire suppression, can also reduce air quality in the short term.266
Forest resilience may be enhanced by thinning trees, leveraging low- and moderate-severity wildfires with traditional forest management treatments that adjust fuels, and better incorporating managed wildfire.299 Using prescribed fire in conjunction with mechanical forest treatments, such as thinning or pruning, also reduces tree densities and fuels.300 Prescribed fire can increase forest resilience during periods of climate-related stress, such as sustained drought,353 and can reduce the extent and intensity of the wildfire regime.354 Cultural fire use to meet Indigenous and Tribal objectives can be compatible with traditional fire application and help advance increased resilience to climate change.296,355
To decrease competition for water resources and increase forest resilience, reductions in tree density and fuels can lower the risk of high-severity wildfires and drought- and pest-induced mortality (KM 6.2).356,357 Recent evidence suggests that increasing these approaches can help adapt to the rapidly increasing risks.299,358 However, the implementation of prescribed fire may be curtailed due to public concerns about smoke and fires that escape management prescription.359
Natural lands, including forests and associated woodlands, play a central role in mitigating climate change (KM 32.1).360,361 However, climate variability, drought- and pest-driven tree mortality, wildfire, and other disturbances suggest that the Southwest will see a continued decline in terrestrial carbon storage.360,362 As a result, securing or even increasing ecosystem carbon storage is often an objective for forest management investment.363 Forest management treatments differ in their short-term carbon losses relative to the expected benefits of greater fire resistance, which leads to longer-term carbon stability (Box 7.2). In Southwest forests, reducing thinning area and increasing the area burned enhances the potential for a net carbon benefit when compared to no action.364 Reforesting areas where forest cover was lost due to mortality can help mitigate the effects of climate change,365 but planting additional trees that increase forest density would result in elevated fire risk and drought stress and therefore is not expected be an effective adaptation solution.300,366
Other adaptation solutions include power shutoff policies by utilities to reduce wildfire risk when extreme wind events are predicted to topple powerlines and telecommunications infrastructure.367,368,369 Power shutoffs are more likely in autumn due to a climate-related increase in the number of days with extreme fire weather.18 Power shutoffs may also increase homeowners’ intention to adopt solar power370 or fossil fuel–powered generators.371 Individuals who experienced shutoffs reported poorer physical and mental health immediately after the occurrence, yet they still supported the use of shutoffs as a wildfire risk-reduction strategy.372
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Following their selection in August 2021, the chapter lead author, federal coordinating lead author, and agency chapter lead author developed a comprehensive list of author candidates based on an analysis of prior National Climate Assessments, published scientific literature, science communication outlets, and policy-relevant reports. The final chapter authors were then selected based on their depth of knowledge, diversity of expertise, and experience in issues critical to the Southwest. Furthermore, authors were selected to represent diverse perspectives in terms of race, ethnicity, and gender identity. The Southwest chapter public engagement workshop was held virtually on February 4, 2022, with more than 90 participants. The workshop included an overview, a series of breakout rooms where the participants provided feedback on key topics, and a final roundup discussion among all participants. The author team held a debriefing session on February 7, 2022, to reflect on the input provided during the workshop. The author team met weekly during the development of the chapter.
Based on discussions among the author team, input from stakeholders, and consideration of the Fourth National Climate Assessment, the authors developed five Key Messages representing the valued assets and unique characteristics of the region. These include water resources, the coast, food and agriculture, demographics and human health, and wildfire. The chapter details the observed and anticipated effects of climate change on human and natural systems across those topics and outcomes to be avoided in the absence of adaptation and mitigation.
For the scientific assessment, the authors conducted a systematic evaluation of the body of scientific and technical knowledge to synthesize published studies, data, models, and assumptions while applying best professional judgment to assess uncertainties and conflicting findings. The chapter pays specific attention to factors that make specific systems and groups more vulnerable. The chapter identifies intersections between topics, cascading risks, and paths to resilience. The chapter also addresses cross-cutting themes including adaptation solutions and challenges, climate change equity and environmental justice, Indigenous Peoples and Knowledges, economics, infrastructure, and ecosystems and ecosystem services.
Read about Confidence and Likelihood
Instrumental data and paleoclimate data provide strong, abundant evidence that the early-21st-century drought in the Southwest is more severe than most, but not all, prior drought periods.28,45,46,373 Research has identified higher temperatures as being a major driver of drought severity through the mechanism of increased atmospheric demand.47 Recent research builds on an already-substantial evidence base demonstrating the decline of southwestern snowpacks over the last century.5,6,49,50 Research has shown how drought-induced shortages in surface water have increased groundwater pumping in California’s Central Valley,67 which provides one example of how drought may reduce future water access, especially if aquifer recharge is also reduced.63
Historical data supply insight into the potential impact of precipitation deficits on surface water, and various research studies demonstrate how precipitation deficit and higher rates of evapotranspiration feed into reduced soil moisture and infiltration, both in contemporary and future warmer climates. However, there is remaining uncertainty over the precise contribution of temperature to these declines in 20th-century Upper Colorado streamflow,373 the impact of changing snowpack dynamics on streamflow in and across different southwestern river basins,56 and the impacts of El Niño–Southern Oscillation on precipitation in the region.374,375
There are also research gaps on the impacts of climate change on the full hydrologic cycle of the Southwest. From the biophysical perspective, there’s a lack of information about aquifer recharge, specifically regarding temporal variability of recharge rates and locations.63 Another topic for further exploration is the relationship between drought, surface water supplies, and rates of groundwater pumping for agriculture in locations within the Southwest beyond California (e.g., Rio Grande and tributaries, Utah, Nevada). Further, there is limited research on climate change impacts on the dynamics of the North American Monsoon, including how to more effectively capture monsoon moisture to substitute for decreased winter precipitation, as well as on other extreme precipitation events such as atmospheric rivers.
Additionally, there are multiple paths for water adaptation in the Southwest for industry, agriculture, and communities to build resilience to a more water-scarce future. However, there are research gaps around the feasibility and long-term effects of these adaptation solutions.376,377 Moreover, the ability of different sectors and communities—including rural, low-income, Indigenous, and other frontline communities—to adapt to climate-induced water scarcity is highly variable across the Southwest.378 There is a gap in research on best practices to support these communities in adapting to current and future water scarcity.
Currently, Indigenous Peoples’ water rights are under-utilized, while water access remains a challenge for many Indigenous Peoples. As more Indigenous Peoples gain access to and use their water rights, there is limited research on how this might impact other water users and broader water management actions, especially under Colorado River drought policies.379
Measurements of snowpack, streamflow, and groundwater over the last century support the observation, with very high confidence, that climate change has reduced surface water and groundwater availability for people and nature in the Southwest.4,14,56 Moreover, evidence from the peer-reviewed literature supports the assertion of high confidence that certain Indigenous and frontline communities, including agriculturalists, will experience disproportionate impacts from reduced water availability and that long-standing institutional frameworks drive these inequities.187,380,381,382
The high confidence that higher temperatures have intensified drought and will very likely lead to a more arid future is based on recent evidence that shows that since the early 20th century higher temperatures have increased the proportion of precipitation being lost to evapotranspiration relative to its contribution to Colorado River flow—a trend that is expected to continue as the region warms.28,51
Without extensive adaptation, which is challenging because human systems have developed under the historical water cycle patterns (KM 4.2), there is high confidence and it is likely that these dynamics will further stress existing water supply–demand imbalances. Water supply imbalances primarily refer to the over-allocation of surface water supplies in the region’s major rivers, such as the Colorado and Rio Grande, due to committing more water to what are legally known as “beneficial uses” than is currently available. Concurrently, longitudinal studies and climate models suggest that there is high confidence that increased flooding will occur due to more intense precipitation events, such as those caused by atmospheric rivers,71,72 in the region’s future.
There is medium confidence that flexible and adaptive approaches to water management have the potential to address changing climate risks and mitigate the impacts on people, the environment, and the economy (KM 4.3). While there are abundant examples of such approaches from around the world and in multiple water-use sectors, assessments of the feasibility of these examples in the Southwest are lacking.
Recent research and observed events demonstrate that marine heatwave events will continue and increase in frequency alongside other stressors, impacting marine-resource dependent communities.35,95 Marine heatwaves and century-scale sea surface temperature warming trends near the coast of California have been attributed to human influence on climate.383 Adverse impacts to aquaculture and wild-capture fisheries have already been observed and are projected to worsen. Impacts are connected to the degradation of marine ecosystems from not only temperature effects but also ocean acidification, hypoxia, and harmful algal blooms. And while there is extensive literature on the California Current ecosystem and wild-capture fisheries, there are fewer studies on the long-term effects of extreme events and California aquaculture, and thus higher uncertainty.
Sea level rise impacts have been extensively modeled and compared with observed flooding. The most recent lines of evidence are synthesized with a large body of relevant literature included in the 2022 Interagency Task Force report on sea level rise7 and the Intergovernmental Panel on Climate Change Sixth Assessment Report from Working Group I.95
The authors focused on findings on the impacts from sea level rise, which centered around new understandings of sea level rise impacts to groundwater and infrastructure and communities. Energy and transportation infrastructure will continue to be impacted by increased flooding. Furthermore, recent studies have shown that climate impacts will disproportionately impact overburdened communities, but additional research specific to California is lacking.
Adaptation for infrastructure and communities has been accelerating in California through state and local governments, as seen through the release of laws, executive orders, and state guidance documents, as well as an increase in state, Tribal, local, and regional vulnerability assessments and adaptation plans. Government planning requirements and guidance for different sectors are increasing. The State of California also invests in downscaled climate science to inform state and local climate decision-making; the state’s climate change assessment has been codified in law. Less information on adaptive strategies for fisheries and aquaculture has been released.
Long-term, compound implications of greater extremes with other stressors and mitigation strategies are less certain. There is less research in California on combined, long-term impacts of marine heatwaves and multi-stressor effects on ecosystems, species, and aquaculture. However, multi-stressor research is advancing,384 including for the California Current,385 increasing our mechanistic understanding of climate change effects. Mitigation strategies of nature-based carbon dioxide removal are growing in interest and investment in the US (Ch. 32),386 including in California. While the emerging evidence suggests that ocean-based solutions, such as seaweed aquaculture carbon sequestration, are not a global “silver bullet” to mitigate carbon emissions, they may provide some local benefits.133 However, who benefits from such interventions and the impacts to surrounding ecosystems require further investigation to ensure positive outcomes for nature and people.387,388,389
There is a lack of research on the impacts of sea level rise on groundwater flooding, as well as the application of those findings to infrastructure and community analyses. These compounding threats have not been included in most adaptation planning to date, which could result in less adaptive solutions being implemented.
As governments and communities begin building adaptation projects, uncertainties about their efficacy remain. There is limited research tracking adaptation implementation, including research identifying potential ramifications, such as displacement of overburdened communities, economic injustices in terms of resources available for adaptation, and methods to ensure communities are part of decision-making for eventual retreat or relocation. Additional work could help identify how existing funding tools can support adaptation and resilience efforts and how to develop a more robust understanding of adaptation costs and benefits.
There is high confidence that climate change will increase marine heatwaves and harmful algal blooms (HABs), resulting in impacts to coastal ecosystems and economies. There is very high confidence and it is very likely that marine heatwave frequency, intensity, and extent will increase. Laufkötter et al. (2020)383 found that heatwaves have already increased twentyfold because of human-caused global warming and that the probability of occurrences increases in frequency under progressively warmer scenarios. Linkages between HABs and thermal conditions of the oceans have been demonstrated, but frequency and extent are linked to climate and non-climate (e.g., runoff) drivers.390 There is high agreement and evidence of negative impacts to marine ecosystems and resource-dependent communities in the literature, but under moderate to high climate mitigation scenarios, the severity of impacts will depend on adaptative interventions (e.g., nature-based solutions, adoption of ecosystem-based management).
Sea level rise will likely cause increased flooding on the Southwest coast, and there is very high confidence that those impacts will severely affect infrastructure and communities, with inequities in how these impacts are experienced. The latest climate models and sea level rise projections demonstrate increased confidence in relative sea level rise amounts by 2050. Sweet et al. (2022)7 state that relative sea level for the entire contiguous US coastline is expected to rise, on average, as much over the next 40 years (2020–2050) as it has over the last 100 years (1920–2020; 0.82–0.98 feet). Furthermore, by 2050 the expected relative sea level will impact tide and storm surge, leading to major and moderate high tide flood events occurring as frequently as minor high tide flood events occurred in 2022, which will impact infrastructure, communities, and ecosystems.7
Given the State of California’s progress and leadership on climate science and adaptation planning for sea level rise, there is high confidence that adaptation planning and implementation will continue. As evidence, the state’s fiscal year 2021–2022 budget included $4 billion (in 2022 dollars) for climate resilience programs, with other new climate resilience programs created in fiscal year 2022–2023. State agencies continue to release adaptation planning guidance (e.g., State Agency Sea Level Rise Action Plan 2022; Extreme Heat Action Plan 2022; State Adaptation Strategy 2021391,392,393) and new funding programs (e.g., Adaptation Planning Grant Program 2022; Transportation Climate Adaptation Planning Grants 2022; Local Transportation Infrastructure Climate Adaptation Project [LTCAP] Program 2022; Regional Resilience Planning and Implementation Grant Program 2022394,395,396,397). There is limited to no inclusion of aquaculture in California state climate planning (e.g., Lester et al. 2022398), and comparatively less coverage overall in the scientific literature compared to wild capture and agriculture,399 and thus there is high confidence that climate planning and adaptation solutions for aquaculture are less clear.
Temperature and precipitation data clearly show that this century has been warmer and drier than the last.50,379,400 This drying and warming trend is already resulting in measurable impacts on rangeland forage production,401 dryland agriculture,402 and irrigated crop yields. A substantial literature exists showing the impact of climate change on livestock production through changes in food quantity and quality, pests and disease, and animal health and well-being.403 Evidence shows that climate is already affecting the distribution of livestock agriculture in the east of region. The Southern Plains (in New Mexico) and Central Plains (in Colorado) saw large decreases in cow numbers in the 2011–2014 drought.404 Increasingly the Southwest’s agricultural economy relies on crop production, but research anticipates downward trends in the production of numerous high-value crops in California169 and reduced or less reliable supply of irrigation water that supports crop production throughout the region. Numerous examples of adaptation strategies exist, such as improved irrigation technologies, soil protection strategies, and shifts to Indigenous practices and crops, although there are barriers that inhibit widespread adoption of many of these practices.
Southwestern food and fiber production is diverse, spanning rangeland livestock systems, irrigated croplands and orchards, dryland, and Indigenous farming. Each of these systems is nested within local, regional, and international contexts that can exacerbate or alleviate vulnerability to climate change. This diversity, combined with factors other than climate change (e.g., global pandemics, inflation, supply chains, access to financial support) contribute to the complexity of assessing the impacts of climate change on southwestern agriculture. There are therefore research gaps and some uncertainty on how the adaptation options described in the agronomic and range management literature can be applied in the real world.
Research gaps remain in our understanding of not only the biophysical and socioeconomic capacity to support change in southwestern agrosystems but also the will of farmers and policymakers to change the type of crops that are grown. For livestock producers who rely on forage from public lands, there is uncertainty about how climate change will affect the number of animals the land can support and how possible reductions in stocking rate will affect the viability of ranches and ranching-dependent communities.
There is high confidence that continuing drought and water scarcity will make it more difficult to raise food and fiber in the Southwest without major shifts to new strategies and technologies. Climate models agree that the Southwest will continue to warm. This impacts the thermal tolerance and chilling requirements for economically important crops. There are multiple indicators that the Southwest is undergoing aridification; as a consequence, it is expected that there will be less available water for irrigated agriculture in the future.
Given substantive evidence in the literature, there is high confidence, and it is likely, that extreme heat events will continue to occur and are expected to worsen in intensity and increase in frequency in the 21st century.167 Extreme heat reduces crop quality and yield181,401 and affects livestock productivity (and in some cases survival), resulting in economic impacts.197,198,403 USDA Risk Management Agency indemnities data from 1989 to 2021 show that heat events are already driving crop production losses across the region.
There is a growing literature that advocates for greater inclusion of Indigenous Knowledge for informing adaptation solutions in Southwest agriculture. There is medium confidence that Indigenous Knowledge, along with technological innovation, has the potential to inform sustainable agricultural practice regionwide, as well as increasing Indigenous food sovereignty, because there are few studies to date that demonstrate how Indigenous Knowledge can be drawn on to adapt larger acreages or commercial operations to climate change. It is also evident in the literature that technical innovations (e.g., agrivoltaics, internet of things) have yet to become more widely adopted or applied to larger acreage operations.
Strong evidence and good agreement among multiple sources and lines of evidence show that extreme heat exposures already are leading to heat-associated deaths and illnesses across the region.228,233,405 Exposures to extreme heat for city dwellers are increasing, partly due to human-caused climate change and partly due to urban-induced warming.406,407 Regional-scale warming in the Southwest since 1901 exceeds what would be expected from natural variability and is partly attributable to human influence.3 Climate change has doubled the likelihood of an event capable of producing catastrophic flooding for California, and future increases to this risk are expected due to continued warming.271,272
Multiple lines of evidence indicate current and future increases in human exposures and adverse health outcomes to wildfire smoke in the region.277,408,409 Good agreement exists among models that the intensified arid conditions in the region will result in more dust storms243 and thus a higher incidence of respiratory ailments, including Valley fever.246,248
Evidence supports the need for increased investments in adaptative strategies that support social, physical, and health systems that enhance individual and community resilience to changes in climate, particularly extreme heat.288,410,411,412,413 Improving public health systems and community infrastructure in the region can reduce the health consequences of climate change.
Uncertainties exist in the attribution of changes in climate variables to specific health outcomes. The collection of and alignment of more environmental and health data will assist in understanding the long-term nature of how climate change affects specific health outcomes. Detecting direct relationships between climate change impacts and public health outcomes is also made more difficult due to confounding factors related to socioeconomics, vulnerability, exposure assessments, demographic shifts, migration, and community and individual characteristics. Detection and attribution studies are thus vital to the region for addressing multiple public health concerns (e.g., Ebi et al. 202030).
Related uncertainties in how regional changes in climate will affect public health exist due to variability in projections of extreme precipitation; uncertainties in the occurrence and intensity of climate-sensitive exposures that impact human health, including wildfire smoke exposures; and variability in local and regional ozone based on meteorological conditions and emissions-reduction targets achieved.
Uncertainties also exist in projecting climate-related changes to the abundance of vector-borne diseases and associated disruptions. While US rates of chikungunya and Zika dropped414 with widespread travel restrictions due to COVID-19, issues with West Nile virus (WNV) remain due to the endemic nature of the virus in the region. The most common WNV vector in the region (Culex quinquefasciatus) is abundant in urbans areas such as Los Angeles, Albuquerque, and Phoenix,415 and certain water management structures (e.g., catch basins, storm drains, and retention ponds) provide favorable environments for Cx. quinquefasciatus reproduction.416 There is also uncertainty around how heavy precipitation, as compared to drought conditions, impacts the abundance and distribution of vector-borne diseases connected to human activity and management of water. Transmission of WNV is also projected to shift northward,417 thus decreasing potential risks of transmission in parts of the Southwest.
Finally, considerable uncertainties exist in how individuals and communities will adapt to the effects of climate change in the region. Model projections of health impacts rarely account for adaptive capacity changes into the future. Improving adaptive capacity involves enhancing infrastructure, technologies, behavior, and the overall health of the population to cope with climate effects.
There is high confidence that increases in extreme heat, drought, flooding, and wildfire activity are negatively impacting the physical health of Southwest residents. Climate models agree that the Southwest is experiencing higher temperatures and more intense, longer, and more frequent heat events.3,8,373,418Extreme heat exposure causes mortality, morbidity, and lost productivity if dangerous exposures occur (KM 15.2).235,419 Drier conditions lead to a greater risk of wildfires and particulate matter, which adversely affect human respiratory and cardiovascular health when exposed.265,290,420,421,422
Climate change is also shaping the demographics of the region by spurring the migration of people to the Southwest, primarily from Central America (medium confidence). In 2021, about 13 million people (representing 21.3% of total population) living in the Southwest were foreign born; 21.1% of those foreign born entered the United States in the last 10 years.423 Decreasing agricultural productivity, increasing levels of food insecurity, and adverse climate effects are among the main reasons why people emigrate from the Northern Triangle (Guatemala, Honduras, and El Salvador) to the US (KM 17.2).32,279 About 43% of immigrants apprehended at the Southwest border in 2019 originated from the Northern Triangle.278
Individuals particularly vulnerable to increasing climate change impacts include older adults, outdoor workers, and people with low income (high confidence). Wildfires and the related smoke are affecting a higher number of people, with strong evidence pointing to the most vulnerable populations being at greatest risk.424 Strong evidence further connects rising particulate matter levels to higher risk in already-vulnerable populations, including individuals with low income, Indigenous Peoples, pregnant people, children, and outdoor workers.425,426,427,428,429 It has been well documented that extreme heat disproportionately impacts the health of the most vulnerable populations in the region, including the unhoused,227,228,229 outdoor workers, and migrant farmworkers,230,231,232,233 as well as people with low income,8,234 older adults,235 and pregnant people and babies (particularly with air pollution co-exposures).237,238,239,240
There is high confidence that local, state, and federal initiatives can respond to these climatic and demographic changes by helping people and communities become healthier and more resilient. There is strong evidence that increasing adaptive capacity and resilience across communities, especially the most vulnerable, will reduce the human health impacts of climate change.430 The relative importance of various adaptive strategies will differ across spatial and temporal scales, climatology, and social and behavioral contexts. Improving public health systems, overall health, community infrastructure, and education can reduce health risks that are being exacerbated in the Southwest due to climate change, as well as many health risks in general.429
Changing wildfire dynamics include increases in wildfire size and severity and changing and lengthening fire seasons.18,302,431,432 Extensive research has found that climate change is linked to increases in extreme fire weather,18 wildfire activity,20,302,324 wildfire severity,17,19,21,432 and acreage burned annually.301 Unprecedented high-severity fires have driven ecosystem conversions in many parts of the region.294,310 Recent climate-induced aridification, including loss of snowpack, has also reduced postfire tree seedling and shrub establishment, limiting ecosystem recovery.312,313,441
Projections suggest that future fire activity will continue to degrade ecosystems and alter their structure and function.294,298,317 Increased fire activity,318,319,320 further warming and drying that stress tree seedlings, and model projections of stand-replacing fires at the forest/non-forest boundary in the western US314 have raised the possibility of shifts in species composition or vegetation type.294,315 These projections suggest high variability in ecosystem responses depending on interactions between vegetation type, moisture stress, disturbance regimes, and human alterations.312,317,433,434,435,436
Increasing wildfire risk poses threats to lives and livelihoods in the region. Wildfire and accompanying smoke have led to fatalities caused by the fires themselves,303,325 by smoke from wildfires,326 and by debris flows that occur when heavy rains fall on recently burned slopes.328 Even if not fatal, wildfires have been linked to declines in physical health265,326,437 and mental health.268 Frontline communities, including low-income groups and populations of color, are especially vulnerable to these impacts.231,336 Exposure of people in the Southwest to wildfire risk is also increasing due to population growth in wildland–urban interface areas near fire-prone forests, shrublands, and grasslands.329,330,331 Economic costs to individual households come from structures burned and increased insurance and healthcare costs.438,439 Further costs come from income lost due to fires that affect energy, agriculture, and tourism. 339,351,340,338
Adaptation strategies include reduction in tree density and wildland fuels 356,357,299,353 that can reduce the size and severity of fires when they occur. Integration of Indigenous burning practices with contemporary forest management can mitigate wildfire risk as well.296 Risk is expected to be reduced through public safety power shutoffs initiated by electric utilities when weather conditions suggest wildfire danger is especially high.368
The short-term likelihood of increasing wildfire risks and impacts is very high. What is less clear is the extent to which adaptation strategies such as increasing fuel-reduction treatments and adoption of climate-adapted silviculture will be able to mitigate those impacts. As yet, there has been no reliable way to quantify the degree to which increases in fire area and severity are due to climate change versus past management, non-native species invasions, urbanization, or other factors. To restore forest resilience (KM 7.3), the rate of thinning and fuel reduction needs to be greatly increased, and it is not known whether resources will be available to achieve such intensified management. Fire ecologists point to the need for increasing use of prescribed burning to reduce fuel loads, but it is not certain how much this increase can be achieved, considering the increasing risk of fire escape and public unease about the use of fire as a management tool.359 Geographic factors, forest policies, and public attitudes toward forest management can constrain the rate at which risk-reduction actions can be implemented. Similarly, long-term attitudes toward public safety power shutoffs, restrictions on homebuilding in fire-prone landscapes, and other risk-reduction policies are uncertain.
There is high confidence that the Southwest region is experiencing unprecedented wildfire occurrence and that this change is linked to climate change. Contributing causes include land management policies that have led to high tree densities that increase fuel for fires,295,316 climate-mediated events such as insect and tree disease outbreaks (Box 7.1),317 and increased human population at the forest’s edge,329 all of which interact with climate change in ways that increase wildfire risk and occurrence.18,21
There is high confidence that fires in the region have become larger and more severe. Increased temperatures and changes to precipitation have combined to produce an increase in vapor pressure deficit.331 This, in conjunction with episodes of climatic extremes such as droughts and heatwaves, means it is very likely that these trends will continue in the region’s forests.317,318,319,320 However, there is less certainty about future trends in non-forested areas because of high year-to-year variability in production of the grasses that fuel wildfires,322 with model projections suggesting that climate impacts on plant growth will vary across the region.323
There is high confidence that severe wildfires are placing people, economies, ecosystems, and water resources at risk, and it is very likely that severe wildfires will continue, partially because of climate change impacts. There is consensus among studies from climate models that the Southwest will continue to warm, and there are multiple indicators that the region is becoming more arid, increasing wildfire risk. There are many indicators of costs to human lives, health, and livelihoods due to wildfire, and as the risk of catastrophic wildfire increases, so do those costs.
There is medium confidence that adaptation pathways will reduce wildfire risk and promote ecosystem restoration through forest management and other adaptations such as the application of Indigenous Knowledges. While the adaptation opportunities are known, it is less clear whether society will have the capacity to embrace those opportunities.
