The integrity of Southwest forests and other ecosystems and their ability to provide natural habitat, clean water, and economic livelihoods have declined as a result of recent droughts and wildfire due in part to human-caused climate change. Greenhouse gas emissions reductions, fire management, and other actions can help reduce future vulnerabilities of ecosystems and human well-being.
The forests and other ecosystems of the Southwest region that provide natural habitat and essential resources for people have declined in fundamental ways due in part to climate change. Vast numbers of trees have died across Southwest forests and woodlands,143,144,145,146 disproportionately affecting larger trees.147 Tree death in mid-elevation conifer forests doubled from 1955 to 2007 due in part to climate change.146 Field measurements showed that changes attributable, in part, to climate change, including increases in temperature, wildfire,7 and bark beetle infestations,148,149 outweighed non-climate factors such as fire exclusion or competition for light.146
Wildfire is a natural part of many ecosystems in the Southwest, facilitating germination of new seedlings and killing pests. Although many ecosystems require fire, excessive wildfire can permanently alter ecosystem integrity.150,151 Climate change has led to an increase in the area burned by wildfire in the western United States.7,152 Analyses estimate that the area burned by wildfire from 1984 to 2015 was twice what would have burned had climate change not occurred (Figure 25.4).7 Furthermore, the area burned from 1916 to 2003 was more closely related to climate factors than to fire suppression, local fire management, or other non-climate factors.152
Climate change has driven the wildfire increase,7,153 particularly by drying forests and making them more susceptible to burning.154,155 Specifically, increased temperatures have intensified drought in California,14 contributed to drought in the Colorado River Basin,12,13 reduced snowpack,46,49,156 and caused spring-like temperatures to occur earlier in the year.101 In addition, historical fire suppression policies have caused unnatural accumulations of understory trees and coarse woody debris in many lower-elevation forest types, fueling more intense and extensive wildfires.150,157
Wildfire can threaten people and homes,159 particularly as building expands in fire-prone areas. Wildfires around Los Angeles from 1990 to 2009 caused $3.1 billion in damages (unadjusted for inflation).159 Respiratory illnesses and life disruptions from the Station Fire north of Los Angeles in 2009 cost an estimated $84 per person per day (in 2009 dollars).160 In addition, wildfires degraded drinking water upstream of Albuquerque with sediment, acidity, and nitrates161,162 and in Fort Collins, Colorado, with sediment and precursors of cancer-causing trihalomethane, necessitating a multi-month switch to alternative municipal water supplies.163,164
Ecosystems can naturally slow climate change by storing carbon, but recent wildfires have made California ecosystems and Southwest forests net carbon emitters (they are releasing more carbon to the atmosphere than they are storing).6,144,165 Wildfire has also exacerbated the spread of invasive plant species and damaged habitat. For example, repeated wildfire in sagebrush in Nevada and Utah has caused extensive invasions of cheatgrass, reducing habitat for the endangered sage-grouse.64,166
Post-wildfire erosion damages ecosystems by denuding hillsides, such as occurred in Valles Caldera National Preserve in New Mexico when the 2011 Las Conchas Fire generated the biggest local erosion event in 1,000 years.167 In New Mexico, consecutive large wildfires degraded habitat and reduced abundance of six out of seven native coldwater fishes and some native insects, although nonnative fishes were less affected.168
With continued greenhouse gas emissions, models project more wildfire across the Southwest region.169,170,171,172,173 Under higher emissions (SRES A2)174 (see the Scenario Products section of App. 3), fire frequency could increase 25%,172 and the frequency of very large fires (greater than 5,000 hectares) could triple.169 The Santa Ana winds and other very dry seasonal winds increase fire risk in California175 and Mexico.176 Under higher emissions (SRES A2), sediment flows after fires would double in one-third of western U.S. watersheds modeled,177 with the sediment potentially damaging ecosystems, homes, roads, and rail lines (Ch. 12: Transportation; Ch. 17: Complex Systems). Under the higher scenario (RCP8.5), cumulative firefighting costs for the Southwest could total $13 billion from 2006 to 2099 (in 2015 dollars, discounted at 3%).178
Reducing greenhouse gas emissions can reduce ecological vulnerabilities to wildfire.179 For example, under a higher emissions scenario (SRES A2), climate change could triple burned area (in a 30-year period) in the Sierra Nevada by 2100, while under a lower emissions scenario (SRES B1174), fire would only slightly increase.173
Allowing naturally ignited fires to burn in wilderness and preemptively setting low-severity prescribed burns in areas of unnatural fuel accumulations can reduce the risk of high-severity fires under climate change.180,181,182,183,184 These actions can naturally reduce or slow climate change because long-term storage of carbon in large trees can outweigh short-term emissions.185,186 Proactive use of fire in Yosemite, Sequoia, and Kings Canyon National Parks has improved the resilience of giant sequoias and other trees to severe fires and protected their stores of carbon.187,188,190,191
Climate change has also contributed to increased forest pest infestations, another major cause of tree death in Southwest forests and woodlands (Ch. 17: Complex Systems, Box 17.4). Bark beetle infestations killed 7% of western U.S. forest area from 1979 to 2012,148,149 driven by winter warming due to climate change103,192 and by drought.193 Tree death from bark beetles in Colorado increased organic matter in local streams, elevating precursors of cancer-causing trihalomethane in local water treatment plants194 to levels that exceed the maximum contaminant levels for drinking water specified by the U.S. Environmental Protection Agency.195 Without greenhouse gas emissions reductions, further increases in heat and drought could kill many more trees,143,196,197 especially affecting piñon pine,198 whitebark pine,199 and tall old-growth trees.200 Drought hastens tree mortality over a wide range of temperatures.201 On the Colorado Plateau in Utah, five years of hotter temperatures in experiments killed microbial biocrusts, which conserve soil fertility and protect soils from erosion.202,203,204 In addition, grasslands205,206 and desert plants207,208 are vulnerable to increased plant death.
Field research in Southwest ecosystems has detected geographic shifts (Ch. 7: Ecosystems) of both plant and animal species, partly attributable to climate change. In Yosemite National Park, forest shifted into subalpine meadows from 1880 to 2002,209 and small mammals shifted 1,600 feet (500 m) upslope from 1914 to 2006,210 with climate change outweighing other factors as the cause.209,210 Across the United States, including the Southwest, birds shifted northward between 0.1 and 0.5 miles (0.2 to 0.8 km) per year from 1975 to 2004, and analyses attribute the shift to climate change.211,212
Continued climate change would cause north–south or upslope shifts of biomes (major vegetation types) in the Southwest as vegetation follows cooler temperatures.213 Areas highly vulnerable to such biome shifts include the Arizona Sky Islands214 and the Sierra Nevada.215 Potential shifts of suitable habitat for individual species include the shifting of Joshua tree habitat out of much of Joshua Tree National Park,207,216 American pika habitat shifting off of mountain tops,217,218 and upslope or northward shifts of numerous birds and reptiles across the Southwest.219,220,221 Climate change may also cause shifts in the timing of plant and animal life events (phenology), including flower blooming, plant leafing, and breeding time of birds and other animals.222,223,224 The arrival of migrating broad-tailed hummingbirds in Colorado advanced five days between 1975 and 2011.225 Plant species that provide essential food (nectar) for the hummingbirds also shifted in phenology (Ch. 7: Ecosystems), but much more than the birds, potentially jeopardizing breeding success.
To prepare for potential future ecological changes, U.S. federal agencies have begun to integrate climate change science into resource management planning in the Southwest. For example, the U.S. National Park Service has developed park plans with specific actions for managing resources under climate change.226 On private lands, planning that integrates native plants and wildlife into working landscapes such as farms, orchards, and ranches can promote conservation outside of protected areas and provide valued ecosystem services, as demonstrated for rangelands by the Malpai Borderlands Group in Arizona and New Mexico.227,228 In response to severe wildfires, the City of Flagstaff, Arizona, enacted a bond to provide funds to thin forest around the town perimeter.229,230 Ecosystem restoration provides an opportunity to integrate climate change considerations into natural resource management.231 Desert research scientists have developed the ability to grow microbial biocrusts and are testing whether translocating biocrusts that are adapted to thrive at higher temperatures can restore the soil-stabilizing, nutrient-fixing, and other services that these organisms provide in many Southwest desert ecosystems.232,233,234 Finally, conservation of forests, especially coast redwoods, which have the highest carbon densities of any ecosystem in the world,235 can slow or reduce climate change by naturally removing carbon from the atmosphere.6