finding 25.2 : key-message-25-2

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 (high confidence). Greenhouse gas emissions reductions, fire management, and other actions can help reduce future vulnerabilities of ecosystems and human well-being (high confidence).

This finding is from chapter 25 of Impacts, Risks, and Adaptation in the United States: The Fourth National Climate Assessment, Volume II.

Process for developing key messages:

The authors examined the scientific literature in their areas of expertise. The team placed the highest weight on scientific articles published in refereed peer-reviewed journals. Other sources included published books, government technical reports, and, for data, government websites. The U.S. Global Change Research Program issued a public call for technical input and provided the authors with the submissions. The University of Arizona Center for Climate Adaptation Science and Solutions organized the Southwest Regional Stakeholder Engagement Workshop on January 28, 2017, with over 70 participants at the main location in Tucson, AZ, and dozens of participants in Albuquerque, NM, Boulder, CO, Davis, CA, Los Angeles, CA, Reno, NV, and Salt Lake City, UT, all connected by video. Participants included scientists and managers. The author team met the following day for their only meeting in person. Subsequently, authors held discussions in regular teleconferences. Many chapter authors met at the all-author meeting March 26–28, 2018, in Bethesda, MD.

Description of evidence base:

Scientific research in the Southwest has provided many cases of detection and attribution of historical climate change impacts. Detection is the finding of statistically significant changes different from natural cycles. Attribution is the analysis of the relative contribution of different causes and whether greenhouse gas emissions from human sources outweigh other factors. Published field research has detected ecological changes in the Southwest and attributed much of the causes of the changes to climate change. Wildfire across the western United States doubled from 1984 to 2015, compared to what would have burned without climate change, based on analyses of eight fuel aridity metrics calculated from observed data, historical observed temperature, and historical modeled temperature from global climate models.de4a77df-03ba-4319-a13f-7fdefbb353a5 The increased heat has intensified droughts in the Southwest,a42c4f5e-f16b-4196-af05-61f117e0491d,ba57f86f-c42f-4bba-83f6-676d6875c176 reduced snowpack,0d8b090e-e060-4f9d-a442-b7e050608a20,247874c4-0ebc-4fc6-9e45-b5f0de315261 and advanced spring warmth.56447233-ad64-46b3-8371-925de98e78c0 These changes have dried forests,e126059c-67f3-4522-8381-ae2499296312,fc6e40e3-32d7-4d49-82c5-8fe3d9845886 driving the wildfire increase.de4a77df-03ba-4319-a13f-7fdefbb353a5,80cf45a9-2066-4434-ae46-0e8f53b2427d Tree death across the western U.S. doubled from 1955 to 20079c23a870-58cf-49f6-9c6f-01cb94e4bb5a likely due to increased heat,f03117be-ccfe-4f88-b70a-ffd4351b8190 wildfire,de4a77df-03ba-4319-a13f-7fdefbb353a5 and bark beetle infestations,df19cf82-b4eb-4281-a379-2f1863e7142f,0346508c-1b13-4e3e-a95d-33acaac2b2c1 all of which are mainly attributable to climate changede4a77df-03ba-4319-a13f-7fdefbb353a5,df19cf82-b4eb-4281-a379-2f1863e7142f,0346508c-1b13-4e3e-a95d-33acaac2b2c1 more than to other factors such as fire exclusion or competition for light and water.9c23a870-58cf-49f6-9c6f-01cb94e4bb5a In the Yosemite National Park biome shift,e353701d-b2bf-4ddd-af78-6bced072e963 the research analyzed the relative contributions of temperature, precipitation, and the Pacific Decadal Oscillation. The researchers found that “Minimum temperature was the main effect related to accelerating annual branch growth in krummholz whitebark pine and initiation of pine invasion into formerly persistent snowfield openings.” In the Yosemite National Park small mammal range shift,0327141f-d0f2-4c78-833d-61c47136242d the locations of the monitoring sites allowed relative isolation of climate change factors. Moritz et al. (2008)0327141f-d0f2-4c78-833d-61c47136242d state, “The transect spans YNP [Yosemite National Park], a protected landscape since 1890, and allowed us to examine long-term responses to climate change without confounding effects of land-use change, although at low to mid-elevations there has been localized vegetation change relating to seral dynamics, climate change, or both.”

Cutting emissions through energy conservation and renewable energy can reduce ecological vulnerabilities. Under high emissions, projected climate change could triple burned area in the Sierra Nevada, but under low emissions, fire could increase just slightly.8dfecf8b-f8a8-4f03-8d68-551b13794a1d Projections of biome shifts37982de0-0e01-476f-b522-b8162d709134,5f6e8f59-7b2c-434e-b211-fa2c0ebc20a7 and wildlife range shifts83e4ade9-14c6-40b4-ae82-508e72e0597f,e028e561-0d0d-4ebd-acc0-5aa92fc73750,5aa9463f-dfab-4ff9-9e39-0edd90c5eae7,67d3c1b0-6da6-4c9c-a3ce-12724e4406c5,f0366470-ff07-4d4f-b785-8bea5f8e54c2 consistently show lower vulnerabilities with lower emissions. Extensive research on, and practice of, fire management show that allowing naturally ignited fires to burn in wilderness and using low-severity prescribed burns can reduce fuels and the risk of high-severity fires under climate change.3def47b9-0e32-440b-bef1-f9bc176a7dd0,82c292bd-b80a-425b-8300-23330807b4f6,c387ad96-7868-4751-89f7-d0d62911b346 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.54acf6dd-9fd1-418f-ae0f-39f17afb79b0,93e74b72-dff9-4cc6-96e2-2dfd76d9c418,2e3ab1ec-834f-40c6-8946-b4869c2673a1,76524700-c1fa-4024-a639-246de55b38d8 Numerous research results have identified climate change refugia for plants and animals.9f272ed1-62dd-4393-89d3-f0d8b0dcb7a6,b284e9ea-c2d2-446b-9ba3-c5ada472ee0c,4401b714-c4aa-4e90-af15-4153b3c6880a

New information and remaining uncertainties:

Because climate model projections often diverge on whether precipitation may increase or decrease, two broad types of fire futures391560e0-40c1-4f9d-b063-e87d18c87e02 could be 1) dry-fire future—hotter and drier climate, increased fire frequency, fire limited by vegetation, potential biome change of forest to grassland after a fire due to low natural regeneration, and high carbon emissions; or 2) intense-fire future—hotter and wetter climate, more vegetation, increased fire frequency and intensity, fire limited by climate, and higher carbon emissions. These two broad categories each encompass a range of fire conditions. On the ground, gradients of temperature, precipitation, and climate water deficit (difference between precipitation and actual evapotranspiration) generate gradients of fire regimes. Because climate change, vegetation, and ignitions vary across the landscape, potential fire frequency shows high spatial variability. Therefore, future fire types could appear in patches across the landscape, with different fire future types manifesting themselves in adjacent forest patches. Changes in aridity may shift some plant and animal species ranges downslope to favorable combinations of available moisture and suitable temperature, rather than upslope.9743c446-fef0-44f4-82bd-7f2ff1614205 Plants and animals may respond to changing climate, and have been shown to do so, through range shifts, phenology shifts, biological evolution, or local extirpation. Thus, no single expected response pattern exists.820ced23-71ae-4607-8353-74e3881db2a1

Assessment of confidence based on evidence:

Field evidence provides high confidence that human-caused climate change has increased wildfire, tree death, and species range shifts. Projections consistently indicate that continued climate change under higher emissions could increase the future vulnerability of ecosystems, but that reducing emissions and increasing fire management would reduce the vulnerability, providing high confidence in positive benefits of these actions.

This finding was derived from scenario rcp_8_5

References :

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