Issues: Global Warming

Recent scientific studies illustrate the connection between global warming and water shortages in the American West. Current evidence suggests that without action to stem the heat-trapping pollution that causes global warming, Western states can expect longer, more frequent drought and even greater difficulties meeting future water needs.

Reporting in the January 31, 2003 issue of Science, researchers from the National Oceanic and Atmospheric Administration (NOAA) say the current drought in the American West is caused at least partly by global warming-induced rises in western Pacific and Indian ocean temperatures:

• Unprecedented sea surface temperature patterns in the Pacific can explain the persistence of today's western U.S. drought. Climate modeling has shown that warm water in the western Pacific coupled with cool water in the eastern Pacific causes a shift in large-scale circulation patterns that reduces precipitation in the western United States.
  
  
• The warming of the western Pacific and Indian oceans since 1950 (roughly 1 degree Celsius) "is beyond that expected of natural variability and is partly due to the ocean's response to increased greenhouse gases."
  
  
• Large-scale climate modeling suggests the same global warming-induced conditions that contributed to the current drought will increase the risk of extended drought in the western United States.¹
  
  

Researchers Find Reduced Snowpack, Earlier Runoff and Decline in Water Supplies

Despite some increases in winter precipitation, much of the mountainous West has experienced declines in spring snowpack, according to a February 2004 study by scientists at the universities of Washington and Colorado.

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• From 1950 to 1997, in Oregon, western Washington and northern California, snowpack shrank by 50 to 75 percent. Decreases in the northern Rockies during that period ranged between 15 and 30 percent.
  
  
• The dominant factor in declining spring snowpack was global warming, not changes in land use, forest canopy or other factors.
  
  
• In most mountain ranges, the largest relative losses occur in areas at lower elevation with warmer mid-winter temperatures.
  
  
• Estimated warming rates for the West are in the range of 4 to 9 degrees Fahrenheit (2 to 5 degrees Celsius) over the next century, which the researchers warn "will have profound consequences for water use in a region already contending with the clash between rising demands and increasing allocations of water for endangered fish and wildlife."²
  

The January 2004 issue of the journal Climatic Change published a number of new papers on global warming, precipitation and water in the West. Findings include:

• Average snowpack in the Colorado River basin will decrease by 30 percent by mid-century. Total water demand for the Colorado River basin will exceed available supply.
  
  
• Decreases in relative humidity will increase the risk of severe wildfires.
  
  
• The date of peak spring runoff will continue to advance, coming more than a month earlier in many Western rivers by the end of the 21st century.
  

Study Links Global Warming to Changing Western Precipitation

Global warming will have significant impacts on precipitation patterns in the western United States, according to a February 2004 study by L. Ruby Leung at the U.S. Department of Energy's Pacific Northwest National Laboratory:

• Global warming will reduce the amount of water stored as snow by 30 to 70 percent over the next 50 years across a region stretching from California's Sierra Nevada Mountains in the south to the Cascade range in the Pacific Northwest.
  
  
• Some coastal mountain ranges have already lost 60 percent of their snowpack over the last 50 years.³
  
  
• The reduction in Western mountain snow cover will lead to increased fall and winter flooding, and severe spring and summer drought. These effects mean trouble for western agriculture, fisheries and hydropower.
  
  
• Rising temperatures will push snowlines in the mountains up from 3,000 feet to higher than 4,000 feet over the next 50 years.
  

Warming Seas Will Cause Drying and Water Deficits in the West

Anticipated reductions in Arctic sea ice due to global warming would force a significant drying of western North America, according to a March 2004 study in Geophysical Research Letters.

• "...decreased Arctic sea ice causes drying of western North America."
  
  
• Shrinking Arctic ice would shift storm tracks northward, causing a 30 percent drop in winter precipitation (15 centimeters) and annual precipitation (25 centimeters) from the Gulf of California north to southern British Columbia, and as far inland as the Rocky Mountains.
  
  
• Water deficit, measured as evaporation minus precipitation, may increase by up to 50 percent (0.5 to 2 millimeters/day) over coastal western North America.⁴
  
  
• Continued warming in the West combined with shrinking Arctic ice will "exacerbat(e) the water crisis in western North America."⁵
  

Global Warming Exacerbates Water Shortage Problems

Global warming impacts on snowpack and spring runoff have been and will continue to be adverse in the West, according to a February 2004 study by University of Washington climate scientist Edward Miles. The study found:

• Since 1950, global warming has been accompanied by an increase in precipitation, but more now comes as rain rather than snow, which means earlier spring runoff and longer summer droughts.
  
  
• The western United States faces decades of chronic water shortages, according to a combination of computer models and hundreds of measurements in the Cascades, Sierras and Rockies.
  
  
• Even with only modest temperature increases, water content of critical snowpack will drop about 60 percent by 2050.
  
  
• Water storage systems in the Northwest and northern California are not designed for the predicted changes in runoff, making it harder to meet the competing needs of fisheries, farmers and power generators.⁶
  

Forecasts from the Rocky Mountain/Great Basin Regional Assessment on global warming by the federal government's U.S. Global Change Research Program include:

• The date of peak flow in three Rocky Mountain rivers (Boise, Humboldt and Yellowstone) will come 10 to 15 days earlier than during the 20th century.
  
  
• Seasonal temperature will increase 4 to 14 degrees Fahrenheit over the 21st century, with the largest increases in winter.
  
  
• Total precipitation will increase, but with a significant shift from snow to rain. This could have a severe impact on natural snowpack and man-made water storage and management systems. In this region, 85 percent of the water used by people is surface water, and 85 percent of this originates as run-off from mountain snowpack.
  
  
• One model projects that skiing could be eliminated from the region by 2070.⁷

last revised 12.9.04

Notes

1. Martin Hoerling and Arun Kumar, "The Perfect Ocean for Drought," Science , Volume 299 (January 31, 2003): pp. 691-694.

2. Robert F. Service, "As the West Goes Dry," Science, Volume 303 (February 20, 2004): pp. 1124-1127.

3. Pacific Northwest National Laboratory, "Global Warming to Squeeze Western Mountains Dry by 2050," www.pnl.gov/news/2004/04-03.htm (January 2004).

4. Jacob O. Sewall and Lisa Cirbus Sloan, "Disappearing Arctic Sea Ice Reduces Available Water in the American West," Geophysical Research Letters , Volume 31 (March 2004), www.agu.org/pubs/crossref/2004/2003GL019133.shtml.

5. Ibid.

6. Edward Miles, "Climate Change in the Pacific Northwest," presented at 2004 AAAS Climate Change Dialogue, Seattle, Washington, February 13, 2004, http://www.cses.washington.edu/cig/outreach/files/AAAS_2004.shtml.

7. F.H. Wagner, ed., Rocky Mountain/Great Basin Regional Climate-Change Assessment: Report for the U.S. Global Change Research Program (Logan, UT: Utah State University, 2003), www.cnr.usu.edu/publications/book.pdf.