This website is the digital version of the 2014 National Climate Assessment, produced in collaboration with the U.S. Global Change Research Program.

For the official version, please refer to the PDF in the downloads section. The downloadable PDF is the official version of the 2014 National Climate Assessment.

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Welcome to the National Climate Assessment

The National Climate Assessment summarizes the impacts of climate change on the United States, now and in the future.

A team of more than 300 experts guided by a 60-member Federal Advisory Committee produced the report, which was extensively reviewed by the public and experts, including federal agencies and a panel of the National Academy of Sciences.

Explore the effects of climate change
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Southwest

Increased heat, drought, and insect outbreaks, all linked to climate change, have increased wildfires. Declining water supplies, reduced agricultural yields, health impacts in cities due to heat, and flooding and erosion in coastal areas are additional concerns.

Explore how climate change is affecting the Southwest.

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Introduction

The Southwest region includes the states of Arizona, California, Colorado, Nevada, New Mexico, and Utah. The Highlights section below offers a high-level overview of climate change impacts on this region, including the five Key Messages and selected topics. (see Ch. 20: Southwest)

Key Message: Reduced Snowpack and Streamflows

Snowpack and streamflow amounts are projected to decline in parts of the Southwest, decreasing surface water supply reliability for cities, agriculture, and ecosystems.

Key Message: Threats to Agriculture

The Southwest produces more than half of the nation’s high-value specialty crops, which are irrigation-dependent and particularly vulnerable to extremes of moisture, cold, and heat. Reduced yields from increasing temperatures and increasing competition for scarce water supplies will displace jobs in some rural communities.

Key Message: Increased Wildfire

Increased warming, drought, and insect outbreaks, all caused by or linked to climate change, have increased wildfires and impacts to people and ecosystems in the Southwest. Fire models project more wildfire and increased risks to communities across extensive areas.

Key Message: Sea Level Rise and Coastal Damage

Flooding and erosion in coastal areas are already occurring even at existing sea levels and damaging some California coastal areas during storms and extreme high tides. Sea level rise is projected to increase as Earth continues to warm, resulting in major damage as wind-driven waves ride upon higher seas and reach farther inland.

Key Message: Heat Threats to Health

Projected regional temperature increases, combined with the way cities amplify heat, will pose increased threats and costs to public health in southwestern cities, which are home to more than 90% of the region’s population. Disruptions to urban electricity and water supplies will exacerbate these health problems.

Southwest

The Southwest is the hottest and driest region in the U.S., where the availability of water has defined its landscapes, history of human settlement, and modern economy. Climate changes pose challenges for an already parched region that is expected to get hotter and, in its southern half, significantly drier.

Southwest image

Heat, drought, and competition for water supplies will increase in the Southwest with continued climate change.

Increased heat and changes to rain and snowpack will send ripple effects throughout the region, affecting 56 million people – a population expected to increase to 94 million by 20502 – and its critical agriculture sector. Severe and sustained drought will stress water sources, already over-utilized in many areas, forcing increasing competition among farmers, energy producers, urban dwellers, and ecosystems for the region’s most precious resource.

The region’s populous coastal cities face rising sea levels, extreme high tides, and storm surges, which pose particular risks to highways, bridges, power plants, and sewage treatment plants. Climate-related challenges also increase risks to critical port cities, which handle half of the nation’s incoming shipping containers. The region’s rich diversity of plant and animal species will be increasingly stressed. Widespread tree death and fires, which already have caused billions of dollars in economic losses, are projected to increase. Tourism and recreation also face climate change challenges, including reduced streamflow and a shorter snow season, influencing everything from the ski industry to lake and river recreation.

Firefighters and wildfire

Climate change contributes to increasing fires.

More than half of the nation’s high-value specialty crops, including certain fruits, nuts, and vegetables, come from the Southwest. A longer frost-free season, less frequent cold air outbreaks, and more frequent heat waves accelerate crop ripening and maturity, reduce yields of corn, tree fruit, and wine grapes, stress livestock, and increase agricultural water consumption.5,6,7,8 These changes are projected to continue and intensify, possibly requiring a northward shift in crop production, displacing existing growers and affecting farming communities.9,10

Winter chill periods are projected to fall below the duration necessary for many California trees to bear nuts and fruits, which will result in lower yields.11

Once temperatures increase beyond optimum growing thresholds, further increases, like those projected beyond 2050, can cause large decreases in crop yields and hurt the region’s agricultural economy.

Longer Frost-free Season Increases Stress on Crops

Longer Frost-free Season Increases Stress on Crops

Graph shows significant increases in the number of consecutive frost-free days per year in the past three decades compared to the 1901-2010 average. This leads to further heat stress on plants and increased water demands for crops. Warmer winters can also lead to early bud burst or bloom of some perennial plants, resulting in frost damage when cold conditions occur in late spring. Higher winter temperatures also allow some agricultural pests to persist year-round, and may allow new pests and diseases to become established.1 (Figure source: Hoerling et al. 20132).

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Selected Responses Selected Responses Details/Download

Climate change is exacerbating the major factors that lead to wildfire: heat, drought, and dead trees.12,13,14,15 Between 1970 and 2003, warmer and drier conditions increased burned area in western U.S. mid-elevation conifer forests by 650%.16 Climate outweighed other factors in determining burned area in the western U.S. from 1916 to 2003.17 Winter warming due to climate change has exacerbated bark beetle outbreaks by allowing more beetles, which normally die in cold weather, to survive and reproduce.18 More wildfire is projected as climate change continues,15,19,20,21,22 including a doubling of burned area in the southern Rockies,21 and up to 74% more fires in California.22 For more on fire in the Southwest see pages 53-54.

References

  1. Abatzoglou, J. T., and C. A. Kolden, 2011: Climate change in western US deserts: Potential for increased wildfire and invasive annual grasses. Rangeland Ecology & Management, 64, 471-478, doi:10.2111/rem-d-09-00151.1.

  2. Baldocchi, D., and S. Wong, 2008: Accumulated winter chill is decreasing in the fruit growing regions of California. Climatic Change, 87, 153-166, doi:10.1007/s10584-007-9367-8.

  3. Battisti, D. S., and R. L. Naylor, 2009: Historical warnings of future food insecurity with unprecedented seasonal heat. Science, 323, 240-244, doi:10.1126/science.1164363.

  4. Bonfils, C., B. D. Santer, D. W. Pierce, H. G. Hidalgo, G. Bala, T. Das, T. P. Barnett, D. R. Cayan, C. Doutriaux, A. W. Wood, A. Mirin, and T. Nozawa, 2008: Detection and attribution of temperature changes in the mountainous western United States. Journal of Climate, 21, 6404-6424, doi:10.1175/2008JCLI2397.1. URL

  5. Frisvold, G., L. E. Jackson, J. G. Pritchett, and J. Ritten, 2013: Ch. 11: Agriculture and ranching. Assessment of Climate Change in the Southwest United States: A Report Prepared for the National Climate Assessment, G. Garfin, A. Jardine, R. Merideth, M. Black, and S. LeRoy, Eds., Island Press, 218-239. URL

  6. Gonzalez, P., R. P. Neilson, J. M. Lenihan, and R. J. Drapek, 2010: Global patterns in the vulnerability of ecosystems to vegetation shifts due to climate change. Global Ecology and Biogeography, 19, 755-768, doi:10.1111/j.1466-8238.2010.00558.x. URL

  7. Hoerling, M. P., M. Dettinger, K. Wolter, J. Lukas, J. Eischeid, R. Nemani, B. Liebmann, and K. E. Kunkel, 2013: Ch. 5: Present weather and climate: Evolving conditions. Assessment of Climate Change in the Southwest United States: A Report Prepared for the National Climate Assessment, G. Garfin, A. Jardine, R. Merideth, M. Black, and S. LeRoy, Eds., Island Press, 74-97. URL

  8. Jackson, L., V. R. Haden, S. M. Wheeler, A. D. Hollander, J. Perlman, T. O’Geen, V. K. Mehta, V. Clark, and J. Williams, 2012: Vulnerability and Adaptation to Climate Change in California Agriculture. A White Paper from the California Energy Commission’s California Climate Change Center (PIER Program). Publication number: CEC-500-2012-031. 106 pp., Sacramento, California Energy Commission. URL

  9. Krawchuk, M. A., M. A. Moritz, M. A. Parisien, J. Van Dorn, and K. Hayhoe, 2009: Global pyrogeography: The current and future distribution of wildfire. PLoS ONE, 4, e5102, doi:10.1371/journal.pone.0005102. URL

  10. Litschert, S. E., T. C. Brown, and D. M. Theobald, 2012: Historic and future extent of wildfires in the Southern Rockies Ecoregion, USA. Forest Ecology and Management, 269, 124-133, doi:10.1016/j.foreco.2011.12.024.

  11. Littell, J. S., D. McKenzie, D. L. Peterson, and A. L. Westerling, 2009: Climate and wildfire area burned in western US ecoprovinces, 1916-2003. Ecological Applications, 19, 1003-1021, doi:10.1890/07-1183.1.

  12. Lobell, D. B., C. B. Field, K. N. Cahill, and C. Bonfils, 2006: Impacts of future climate change on California perennial crop yields: Model projections with climate and crop uncertainties. Agricultural and Forest Meteorology, 141, 208-218, doi:10.1016/j.agrformet.2006.10.006.

  13. Luedeling, E., E. H. Girvetz, M. A. Semenov, and P. H. Brown, 2011: Climate change affects winter chill for temperate fruit and nut trees. PLoS ONE, 6, e20155, doi:10.1371/journal.pone.0020155. URL

  14. Medellín-Azuara, J., R. E. Howitt, D. J. MacEwan, and J. R. Lund, 2012: Economic impacts of climate-related changes to California agriculture. Climatic Change, 109, 387-405, doi:10.1007/s10584-011-0314-3.

  15. Moritz, M. A., M. A. Parisien, E. Batllori, M. A. Krawchuk, J. Van Dorn, D. J. Ganz, and K. Hayhoe, 2012: Climate change and disruptions to global fire activity. Ecosphere, 3, 1-22, doi:10.1890/ES11-00345.1. URL

  16. Purkey, D. R., B. Joyce, S. Vicuna, M. W. Hanemann, L. L. Dale, D. Yates, and J. A. Dracup, 2008: Robust analysis of future climate change impacts on water for agriculture and other sectors: A case study in the Sacramento Valley. Climatic Change, 87, 109-122, doi:10.1007/s10584-007-9375-8.

  17. Raffa, K. F., B. H. Aukema, B. J. Bentz, A. L. Carroll, J. A. Hicke, M. G. Turner, and W. H. Romme, 2008: Cross-scale drivers of natural disturbances prone to anthropogenic amplification: The dynamics of bark beetle eruptions. BioScience, 58, 501-517, doi:10.1641/b580607. URL

  18. Wei, M., J. H. Nelson, M. Ting, C. Yang, J. Greenblatt, and J. McMahon, 2012: California’s Carbon Challenge. Scenarios for Achieving 80% Emissions Reductions in 2050. Lawrence Berkeley National Laboratory, UC Berkeley, UC Davis, and Itron to the California Energy Commission. URL

  19. Wei, M., N. H. James, G. B. Jeffery, M. Ana, J. Josiah, T. Michael, Y. Christopher, J. Chris, M. M. E. James, and K. M. Daniel, 2013: Deep carbon reductions in California require electrification and integration across economic sectors. Environmental Research Letters, 8, 014038, doi:10.1088/1748-9326/8/1/014038. URL

  20. Westerling, A. L., H. G. Hidalgo, D. R. Cayan, and T. W. Swetnam, 2006: Warming and earlier spring increase western U.S. forest wildfire activity. Science, 313, 940-943, doi:10.1126/science.1128834.

  21. Westerling, A. L., B. P. Bryant, H. K. Preisler, T. P. Holmes, H. G. Hidalgo, T. Das, and S. R. Shrestha, 2011: Climate change and growth scenarios for California wildfire. Climatic Change, 109, 445-463, doi:10.1007/s10584-011-0329-9.

  22. Williams, A. P., C. D. Allen, C. I. Millar, T. W. Swetnam, J. Michaelsen, C. J. Still, and S. W. Leavitt, 2010: Forest responses to increasing aridity and warmth in the southwestern United States. Proceedings of the National Academy of Sciences, 107, 21289-21294, doi:10.1073/pnas.0914211107. URL

The National Climate Assessment summarizes the impacts of climate change on the United States, now and in the future.

A team of more than 300 experts guided by a 60-member Federal Advisory Committee produced the report, which was extensively reviewed by the public and experts, including federal agencies and a panel of the National Academy of Sciences.

United States Global Change Research Program logo United States Global Change Research Program participating agency logos