finding 25.1 : key-message-25-1

Water for people and nature in the Southwest has declined during droughts, due in part to human-caused climate change (very high confidence). Intensifying droughts (very high confidence) and occasional large floods (medium confidence), combined with critical water demands from a growing population, deteriorating infrastructure, and groundwater depletion, suggest the need for flexible water management techniques that address changing risks over time (high confidence), balancing declining supplies with greater demands.

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:

Research has found that hotter temperatures can make hydrologic droughts more severe. The unprecedented droughts in the Colorado River Basin and California showed that increased temperatures from climate change intensified the severity of the drought.a42c4f5e-f16b-4196-af05-61f117e0491d,ba57f86f-c42f-4bba-83f6-676d6875c176,89e08a41-6091-45fa-a92e-6168a90a8151,3d53beca-0617-4351-a7e9-f5af06a049dc Climate change, more than natural cycles, has reduced snowpack.0d8b090e-e060-4f9d-a442-b7e050608a20 Models project more drought under climate change,a42c4f5e-f16b-4196-af05-61f117e0491d,89e08a41-6091-45fa-a92e-6168a90a8151,d06fadc5-a5e3-463c-85d0-f78c07c6ade9 snowpack and streamflow decline in parts of the Southwest, and decreasing surface water supply reliability for cities, agriculture, and ecosystems.9711f2e3-f3b1-4d25-bc0a-47fd17b56e41

New information and remaining uncertainties:

Projecting future streamflow and hydrologic characteristics in a basin contains many uncertainties. These differences arise because of uncertainty in temperature and precipitation projections due to differences among global climate models (GCMs), uncertainty in regional downscaling, uncertainty in hydrological modeling, and differences in emissions, aerosols, and other forcing factors. Another important uncertainty is differences in the hemispheric and regional-scale atmospheric circulation patterns produced by different GCMs, which generate different levels of snow loss in different model simulations. A key uncertainty is the wide range in projections of future precipitation across the Southwest;9d8a98fa-0338-486a-b902-cd02d43cae87 some projections of higher-than-average precipitation in the northern parts of the Southwest could roughly offset declines in warm-season runoff associated with warming.9d8a98fa-0338-486a-b902-cd02d43cae87

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. Attribution of extreme events, such as the recent California drought to climate change, is an area of emerging science. On the one hand, Seager et al. (2015)4ca5a43c-5fbe-4cb0-8a7d-7ee3acafd7c0 concluded that the California drought was primarily driven by natural precipitation variability. Sea surface temperature anomalies helped set up the high-pressure ridge over California that blocked moisture from moving inland. On the other hand, Diffenbaugh et al. (2015),89e08a41-6091-45fa-a92e-6168a90a8151 Williams et al. (2015),ba57f86f-c42f-4bba-83f6-676d6875c176 and Berg and Hall (2017)2d7d840f-37b6-4484-ba59-aa2d537a8c7c concluded that high temperatures from climate change drove record-setting surface soil moisture deficits that made the drought more severe than it would have been without climate change. Storage of increased precipitation in soils may partially offset increased evaporation, possibly making drought less likely.4fbaaa13-99d2-43df-93db-2be546f18892

In addition to the uncertainties in regional climate and hydrology projections and attribution studies, other uncertainties include potential changes in water management strategies and responses to accommodate the new changing baseline. Additionally, external uncertainties can impact water use in the region via legal, economic, and institutional options for augmenting existing supplies, adding underground storage and recovery infrastructure, and fostering further water conservation, changes in unresolved water rights, and changes to local, state, tribal, regional and national policies related to the balance of agricultural, ecosystem, and urban water use.

Assessment of confidence based on evidence:

The very high confidence in historical droughts derives from the detection and attribution analyses of temperature increases, snow decreases, and soil moisture decreases that have documented hydrologic droughts in California and the Colorado River Basin due to anthropogenic climate change and the conclusions of the Climate Science Special Report (CSSR), Volume I of the Fourth National Climate Assessment.a29b612b-8c28-4c93-9c18-19314babce89 The very high confidence in drought projections derives from the multitude of analyses projecting drought in the Southwest under a range of emissions scenarios and the conclusions of the CSSR.a29b612b-8c28-4c93-9c18-19314babce89 Only medium confidence is found for flood projections due to lack of consensus in the model projections of precipitation. Increasingly arid conditions and the potential for increased water use by people lead to an assessment of high confidence in the need for new ways to address increasing risks of water scarcity. The actual frequency and duration of water supply disruptions will depend on the preparation of water resource managers with drought and flood plans, the flexibility of water resource managers to implement or change those plans in response to altered circumstances,da714e9f-808c-4aae-8d24-aef041988322 the availability of funding to make infrastructure more resilient, and the magnitude and frequency of climate extremes.

This finding was derived from scenario rcp_8_5

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