Fifth National Climate Assessment
28. Southwest

Drought and Aridification

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 needs^5,49 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

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Climate change is projected to reduce snow water equivalent and alter trends in soil moisture and annual runoff.

Figure 28.2. These maps show projected average mid-21st-century (2036–2065; top row) and late-21st-century (2070–2099; bottom row) differences in annual soil moisture, snow water equivalent (the amount of water contained within the snowpack), and runoff over the Southwest region relative to the baseline period, 1991–2020. The data in these maps come from a land-surface hydrological model that simulates different parts of the water and energy balance. The model takes temperature and precipitation data from an ensemble of downscaled Coupled Model Intercomparison Project, Phase 5 (CMIP5) global climate models using an intermediate scenario (RCP4.5)⁵⁷ to create future projections of soil moisture, snow water equivalent, and runoff.⁵⁸ Warming temperatures and precipitation variability are expected to reduce snow water equivalent and alter trends in soil moisture and annual runoff (KM 4.1). The historical record shows that the climatology of 1991–2020 was substantially warmer than the climatology of preceding 30-year periods. Thus, the areas of projected lower soil moisture, snow water equivalent, and runoff in this figure, especially at higher elevations, present marked deficits in comparison to 30-year periods in the 20th century. There are also areas of projected increases in soil moisture and runoff. Some CMIP5 global climate models project increased precipitation over parts of the Southwest, and when these are included in calculating average soil moisture or runoff, the result indicates wetter conditions in some locations, predominantly in Nevada, Utah, southwest Arizona, and southeast California. For more detail on variability, Figures 4.5, 4.6, and 4.7 show data from the same source that illustrate the wet to dry range of projections for the mid-21st century. Figure credit: New Mexico State University; Arizona State University; University of Nevada, Reno; NOAA NCEI; and CISESS NC.

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

Art × Climate

Katelyn Garcia
The 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.

Flooding

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

Disproportionate Impacts

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

Adaptation Pathways

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).

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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.

Ocean Extremes and Adaptation

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

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Pacific marine heatwaves have had coast-wide impacts on ecosystems and fisheries.

Figure 28.4. The 2014–2016 Pacific marine heatwave (MHW) was unusually long and resulted in a variety of impacts for the southwest California coast (a). This MHW was followed by less extensive events in 2018 and 2019–2020 (b). While impacts like this can be expected to continue, they demonstrate the need and potential for adaptive management and mitigation through an integrated ecosystem approach to managing marine habitats and fisheries. EEZ refers to exclusive economic zone. Figure credit: University of California, Santa Barbara; California Department of Transportation; NOAA NCEI; and CISESS NC.

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 isolation^121,122 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 impacts^{132,133,134,135} but are just emerging in California.¹³⁶

Sea Level Rise Impacts and Adaptation Planning

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

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Flooding from sea level rise is expected to affect transportation infrastructure and communities along the California coast, with disproportionate impacts on lower-income communities.

Figure 28.5. These maps show the projected flood risk from 3 feet of sea level rise (SLR), as well as risks to critical infrastructure and surrounding communities, for the San Francisco Bay Area (top row) and the coastline from Los Angeles to San Diego (bottom row). Panels in the left column show transportation infrastructure threatened by flooding with 3 feet of SLR, while those in the right column show the number of hazardous facilities (indicated by circles) and census tracts at risk of flooding, with purple shading indicating the fraction of population in each census tract with income below the poverty level. Flooding from SLR will impact major transportation infrastructure along the coast; given the locations of hazardous facilities and their overlap with lower-income communities, this flooding will have disproportionate impacts on these communities. Flood risk from SLR is consistent with an Intermediate scenario in the year 2100.⁷ Transportation infrastructure includes major airports, highways, and railways. Hazardous facility categories defined by EPA include manufacturing plants, power transmission plants and substations, natural gas pipelines, refineries and oil and gas wells, waste management facilities, landfills and incinerators, and animal operations. Figure credit: Eagle Rock Analytics.

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 California^146,147 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

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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

Impacts to Farming

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

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Monitoring indicators of climate impacts on agriculture can improve understanding and help with adaptation efforts.

Figure 28.6. Climate change impacts to the Southwest’s agriculture include longer growing seasons, a northward shift in plant hardiness zones, expanded areas of heat stress, and higher rates of evapotranspiration, increasing demand for fresh water for irrigation. Monitoring the indicators helps us understand how impacts are experienced and how to adapt to risks. Figure credit: New Mexico State University and Utah State University. See figure metadata for additional contributors.

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

Cascading Impacts of Climate Change to Agriculture

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 nuts^203,204 and legal cannabis.²⁰⁵

Frontline Communities and Food Insecurity

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 for Agriculture

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

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Extreme Heat Impacts

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 income^8,234 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

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With extreme heat events expected to increase in frequency and severity, the ability to perform work outside is projected to decline across parts of the Southwest.

Figure 28.7. The ability to perform work outside—as measured by physical work capacity (PWC)—will decline across large swaths of the Southwest due to heat exposure throughout the century, with the greatest declines expected in southwest Arizona, southeast California, and California’s Central Valley. These impacts on outdoor work will affect agricultural output, as well as earning ability for workers. PWC has a range of 100% (no loss of work capacity) to 0% (complete loss). The maps display the proportion of days in the third quarter (July to September) in which PWC is less than 75%. In historical conditions (a), a few locations in southern Arizona had PWC values less than 75% for half of the quarter. (d) Under a very high scenario (SSP5-8.5) by end of century, most of southern Arizona, southeast California, and some of California’s Central Valley are projected to have less than 75% work capacity for all days in the third quarter. This daily labor loss is calculated based on a given heat load (temperature, humidity, and solar radiation) compared to temperate conditions where there is no thermal effect on work output. To provide a full range of potential impacts, maps are based on representative years for (a) historical/early century (1991–2010); (b) midcentury (2041–2060, SSP1-2.6 [low scenario]); (c) midcentury (2041–2060, SSP5-8.5 [very high scenario]); and (d) end of century (2081–2100, SSP5-8.5). Estimates are based on an individual performing moderate to heavy agricultural work outdoors over a daytime shift (about 7 hours). The PWC is an empirical estimate based on human physiological chamber studies quantifying how PWC changes with the environment heat load based on the wet-bulb globe temperature.^{256,257,258,259} Land areas in white had no crops in the early 21st century. Figure credit: University of Illinois–Urbana Champaign and Arizona State University.

Air Quality and Health Impacts

While the annual average level of fine particulate matter (PM_2.5) has seen decadal decline in the region due to strengthened air quality policies reducing emissions from controllable sources, disparities in PM_2.5 exposure and related health concerns remain high in the region.²⁶⁰ Moreover, the frequency and severity of smoke events with PM_2.5 exceedances of federal air quality standards have increased significantly due to wildfires (Figure 14.3). Since 2015 in Northern California, the annual average PM_2.5 has increased because of wildfire events, which have taken over as the main source of PM_2.5 exceedances.²⁶¹ PM_2.5 in wildfire smoke contributes to adverse health effects for firefighters²⁶² and the public^263,264 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).

Flooding and Disease

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

Impacts to Outdoor Workers

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

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Communities with higher socioeconomic risk factors are expected to be less resilient in the event of climate and weather disasters.

Figure 28.8. The map shows community resilience estimates (CREs) for the Southwest. Community resilience is the capacity of individuals and households to absorb, endure, and recover from the health, social, and economic impacts of a disaster. Individual and household characteristics from the 2019 American Community Survey were modeled, in combination with data from the Population Estimates Program, to create the CREs at the county level. Darker shading indicates a higher proportion of the population at risk. Adapted from US Census Bureau 2021.²⁸¹

Demographic Shifts Related to Climate

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

Adaptation Efforts for Health

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

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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

Impacts on Ecosystems

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

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Climate change is leading to larger and hotter fires and resulting in shifts in vegetation.

Figure 28.9. Data from the states of California, Arizona, Colorado, and New Mexico show that approximately half (about 50%) of vegetation type change (e.g., forests transiting to shrublands or grasslands) is a function of high-severity fire. Adapted from Guiterman et al. 2022²⁹⁴ [CC BY 4.0].

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

Impacts on People and Communities

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).

Art × Climate

Julia Lauer
california
(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

Ecosystem Management Challenges and Adaptation Solutions

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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