Global Warming Effects Around the World

Hetch Hetchy Reservoir, Sierra Nevadas, CA, USA

Top Impact

People (Water use)

Other Impacts

People (Costs)

People (Food)

Declining snowpack and earlier spring snow melt in Sierra Nevada mountains threaten California water supplies

California's Sierra Nevada mountains provide natural storage for much the state's water supply, used for drinking, agriculture, and recreation. Global warming is already causing declining snowpack and earlier spring snowmelt. In coming decades, climate change is expected to put even more stress on California's ability to store and distribute water.1

Key Facts

Meeting California's growing demand for water from the Sierra Nevada mountains can be a challenge as global warming further reduces snowpack. That decline is likely to affect both the timing and availability of water for drinking, agriculture, and recreation.2

  • The Sierra snowpack provides natural water storage equal to about half the capacity of California's major human-made reservoirs.2 Earlier spring runoff typically means a longer dry season and reduced water resources in summer.7
  • By the 2020s, loss of snowpack in the Sierras and Colorado River basin is likely to threaten more than 40 percent of Southern California's water supply.10
  • If our heat-trapping emissions continue to rise unabated, California is projected to face critically dry years up to 50 percent more often, and decreases in water for crops and livestock of 40-50 percent.2,8

Details

Essential for all human, animal, and plant life, water must be carefully managed in California. A warmer climate means declining snowpack in the Sierra Nevada mountains, which is likely to exacerbate existing stresses on the water supply infrastructure and reverberate across many social and economic sectors.2 Although annual precipitation across most of the state rose 5-15 percent from 1958 to 2008, shifts in its timing—and from snow to rain—have implications for drinking water, agriculture, and recreation.2,3

Snowpack is nature's reservoir, storing winter precipitation for use in the drier spring and summer months, when demand is often highest. In most river basins in California and other western states, snow is an even more important means of water storage than artificial reservoirs.4 The Sierra snowpack, for example, provides natural water storage equal to about half the capacity of California's major human-made reservoirs.2

As the climate of the Sierra Nevada warms, more winter precipitation is falling as rain than as snow, and spring snowmelt is occurring earlier.3,5 Spring runoff in the Sierras is also peaking 15-20 or more days earlier.3,6 Given California's current infrastructure, earlier snowmelt can surpass reservoir capacity and flow out of the basin, resulting in a longer dry season and reduced water resources in summer.7

Much of the western United States saw widespread declines in spring snowpack from 1925 to 2000, and especially since the middle of the last century. The largest observed declines—50-75 percent or more—have occurred in Northern California, western Oregon, and western Washington.4

What the Future Holds

If our heat-trapping emissions continue to rise at today's levels, scientists project that snowpack in the Sierra Nevada is likely to decline as much as 40 percent from historical levels by the middle of this century, and as much as 90 percent by the end of the century.2,8 If we make significant efforts to reduce our emissions, the decline by mid-century could be as little as 12 percent.2,8

This loss of natural storage capacity could bring more water shortages for homes, crops, and recreational uses throughout the state. Many municipal water systems in California draw water from the Sierra Nevada. The Bay Area Water Supply and Conservation Agency, for example, relies on water from the Hetch Hetchy reservoir in Yosemite National Park to supply water to over 1.7 million people in the San Francisco Bay Area.9 By the 2020s, loss of snowpack in the Sierras and the Colorado River basin is likely to threaten more than 40 percent of Southern California's water supply.10

Agriculture is the largest water user in California—home to some of the nation's most important crop-producing areas.3 If our carbon emissions continue to rise unabated, scientists project that California is likely to face critically dry years up to 50 percent more often, and 40-50 percent declines in water deliveries to growers.2 At the same time, warmer temperatures typically increase evaporation rates and demand for water for crops.2

Declines in Sierra snowpack can also have widespread implications for winter recreation and tourism.2 If our polluting emissions continue on the same track, snowpack is projected to decline from 40-90 percent in counties from New Mexico to California with major ski resorts.3,8 In this scenario, ski resorts might never have enough snow to operate without snowmaking machines, and could be forced to relocate.2 If we make significant efforts to reduce our emissions, the ski season at lower and middle elevations could shorten by a month.2

Credits

Endnotes

  1. Photograph used by permission. Anthony Dunn, 1999.
  2. Cayan, D., A. Luers, M. Hanemann, G. Franco, and B. Croes. 2006. Scenarios of climate change in California: An overview. Sacramento, CA: California Climate Change Center, Public Interest Energy Research Program. Online at http://www.energy.ca.gov/ 2005publications/ CEC-500-2005-186/ CEC-500-2005-186-SF.PDF. Accessed April 21, 2010.
  3. U.S. Global Change Research Program. 2009. Global climate change impacts in the United States. Edited by T.R. Karl, J.M. Melillo, and T.C. Peterson. Cambridge University Press.
  4. Mote, P.W., A.F. Hamlet, M.P. Clark, and D.P. Lettenmaier. 2005. Declining mountain snowpack in western North America. Bulletin of the American Meteorological Society 86(1):39-49.
  5. Lemke, P., J. Ren, R.B. Alley, I. Allison, J. Carrasco, G. Flato, Y. Fujii, G. Kaser, P. Mote, R.H. Thomas, and T. Zhang. 2007. Observations: Changes in snow, ice and frozen ground. In: Climate change 2007: The physical science basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Edited by S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor, and H. L. Miller. Cambridge University Press, pp. 337-383.
  6. U.S. Geological Survey. 2005. Changes in streamflow timing in the western United States in recent decades. USGS fact sheet 2005-3018. La Jolla, CA: National Streamflow Information Program. Online at http://pubs.usgs.gov/ fs/ 2005/ 3018/.
  7. Peterson, T.C., D.M. Anderson, S.J. Cohen, M. Cortez-Vázquez, R.J. Murnane, C. Parmesan, D. Phillips, R.S. Pulwarty, and J.M.R. Stone. 2008. Why weather and climate extremes matter. In: Weather and climate extremes in a changing climate: Regions of Focus—North America, Hawaii, Caribbean, and U.S. Pacific Islands. Edited by T.R. Karl, G.A. Meehl, C.D. Miller, S.J. Hassol, A.M. Waple, and W.L. Murray. Synthesis and assessment product 3.3. Washington, DC: U.S. Climate Change Science Program, pp.11-34.
  8. The emissions scenarios referred to here are the high-emissions path known as A1FI and the low-emissions path known as B1 from the Intergovernmental Panel on Climate Change.
  9. Hetch Hetchy Water System, Bay Area Water Supply & Conservation Agency. Online at: http://bawsca.org/ water-supply/ hetch-hetchy-water-system/ Accessed April 7, 2011.
  10. Field, C.B., L.D. Mortsch,, M. Brklacich, D.L. Forbes, P. Kovacs, J.A. Patz, S.W. Running, and M.J. Scott. 2007. North America. In: Climate change 2007: Impacts, adaptation and vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Edited by M.L. Parry, O.F. Canziani, J.P. Palutikof, P.J. van der Linden, and C.E. Hanson. Cambridge University Press, pp. 617-652.
Climate Hot Spots
 
Africa
 
Asia
 
Australia & New Zealand
 
Europe
 
Latin America
 
North America
 
Polar Regions
 
Small Islands
 
Key to top impacts color code