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Climate Change

Northwest

Climate Impacts in the Northwest

Map of the Northwest including: Oregon, Washington, Idaho, and the western portion of Montana.
Key Points
  • Climate change will likely result in reduced snowpack and lower summer streamflows, worsening the existing competition for water.
  • Higher temperatures, changing streamflows, and an increasing presence of pests and disease would threaten forests, agriculture, and salmon populations.
  • Sea level rise is projected to increase coastal erosion and land loss, including loss of beaches.

The Northwest is bordered by the Pacific Ocean to the west and Canada to the north. The region includes the cities of Seattle, Portland, Spokane, Boise, and Tacoma. The Cascade Mountain Range runs north-south through Washington and Oregon, splitting the region. The climate on each side of the mountain range is very different. West of the mountains, temperatures are mild year-round (days below freezing or above 90°F are relatively rare), winters are wet, and summers are dry. East of the mountains, it is typically sunnier and drier over the course of the year, winters are colder, and summers can be significantly hotter. [1]

The climate of the Northwest is changing. Over the last century, the average annual temperature rose by 1.5°F, with increases in some areas up to 4°F. [2] Changes in snowpack, streamflows, and forest cover are already occurring. Future climate change will likely continue to influence these changes. Average annual temperature in the region is projected to increase by 3-10°F by the end of the century. [2] Winter precipitation is projected to increase while summer precipitation is projected to decrease, though precipitation projections are less certain than those related to temperature. [2] Future climate change impacts would be compounded by pressures related to the region's rapidly growing population.

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Impacts on Water Resources

A line graph shows the average streamflow over the course of the year for the Quinault River in Washington. One line shows historical data for the early 1900s. This line shows low average streamflows in August through October with a peak in December, a slight decline until April, and a second peak in June. The second line shows the historical data for the late 1990s - the monthly average flows follow roughly the same pattern, except that the winter peak is in January, not December, and the June peak is lower than the early 1900s data point. The third line shows a range of projected average flows for the 2040s. This line projects that the stream flow peak will be significantly higher than the 1900s data. Additionally, instead of having a second peak in the summer, the average flow continues to decline throughout the spring and summer months. This graph shows that the projected average flow of the Quinault River will change significantly. View enlarged image

Peak streamflow in the Quinault River is projected to peak earlier in the year.
Source: USGCRP (2009)

A reliable water supply is crucial for energy production, agriculture, and ecosystems in the Northwest. Much of the region's water is stored naturally in winter snowpack in the mountains. The snowpack melts and runs off into streams and rivers in the late spring and summer, a time when there is very little rainfall. Climate change will likely threaten this natural storage, with important consequences for the timing of runoff and amount of water available in streams and rivers (streamflow) throughout the year. [2]

  • Higher winter temperatures are projected to cause more precipitation to fall as rain instead of snow. This would decrease snow accumulation. The April 1st snowpack, an indicator of natural water storage available for the warm season, is projected to decline by as much as 40% in the Cascades by the 2040s. [2] This reduction in available snowpack (and thus water) could increase the risk of drought during normally dry summers.
  • Higher temperatures will likely contribute to earlier snowmelt and major changes in the timing of runoff. As a result, the peak of spring runoff is projected to shift 20 to 40 days earlier by the end of the century. [2]
  • Warmer temperatures are projected to cause more precipitation to fall as rain. Overall winter precipitation is projected to increase. These changes would increase winter flood risks. [2]
  • Changing streamflows would strain water management and worsen existing competition for water. Competing demands for water in the Northwest currently include hydropower, agricultural irrigation, municipal and industrial uses, and protection of ecosystems and threatened or endangered species. Increasing temperatures and population could increase demand and further stress urban water supplies. Meanwhile, the climate change impacts listed above could decrease supply. [2]
  • About 70% of electricity in the Northwest is supplied by hydroelectricity. Decreasing summer streamflows would reduce hydroelectric supply and stress electricity supplies. Meanwhile, rising temperatures would increase electricity demand for air conditioning and refrigeration. [2]

For more information on climate change impacts on water, please visit the Water Resources Impacts page.

For more information on climate change impacts on energy, please visit the Energy Impacts page.

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Impacts on Forests, Agriculture, and Food Supply

Higher temperatures, changing streamflows, and increases in pests and disease threaten forests, agriculture, and salmon populations in the Northwest. [2]

  • Projected climate change impacts threaten forests due to higher forest fire risk, decreasing tree growth, and increasing insect attacks. Higher summer temperatures, earlier spring snowmelt, and potential reductions in summer soil moisture would contribute to wildfire risk. Drought stress and higher temperatures would likely impede tree growth, though high-elevation forests may experience increased growth in the short term. These climate impacts would also contribute to increased frequency and intensity of attacks from mountain pine beetles and other insects. These attacks would worsen fire risk and reduce timber production. Projected climate changes will likely cause shifts in the composition, range, and even existence of Northwest tree species. [2]
  • Decreasing supplies of water for irrigation, increasing incidence of pests and disease attacks, and growing competition from weeds threaten Northwest agriculture, particularly the production of tree fruits such as apples. [2]
Threatened Salmon Populations
Photograph of salmon swimming upstream.

Salmon swimming upstream.
Source: US Fish and Wildlife Service

Human activities already threaten Northwest salmon populations. These activities include dam building, logging, pollution, and overfishing. Climate change impacts would further stress these salmon populations. Salmon are particularly sensitive due to their seasonally timed migration upstream to breed. Higher winter streamflows and earlier peak flows due to climate change would damage spawning nests, wash away incubating eggs, and force young salmon from rivers prematurely. Lower summer streamflows and warmer stream and ocean temperatures are less favorable for salmon (and other cold-water fish species). These climate change impacts facilitate the spread of salmon diseases and parasites. Many salmon species are already considered threatened or endangered under the Federal Endangered Species Act. Studies show that by 2100, one third of current habitat for Northwest salmon and other coldwater fish would be unsuitable. This is because warming temperatures are projected to pass key temperature thresholds. [2]

For more information on climate change impacts on forests, please visit the Forests Impacts page.

For more information on climate change impacts on agriculture and food supply, please visit the Agriculture and Food Supply Impacts page.

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Impacts on Coastal Resources

Map of Olympia, Washington showing that four feet of sea level rise would result in roughly one third of the developed land to be covered during a high tide. View enlarged image

Regions of Olympia and Harbor Island that will likely be lost due to sea level rise by the end of the century.
Source: USGCRP (2009)

Climate change, particularly sea level rise, is projected to increase coastal erosion and loss of beaches in the Northwest. [2] The heavily populated south Puget Sound region, which includes Olympia, Tacoma, and Seattle, is one of the most sensitive regions in the Northwest to coastal erosion. [2]

  • Some climate models project that changing wind patterns, combined with sea level rise, could accelerate coastal erosion in this region. [2]
  • Relative sea level rise in the Puget Sound basin is estimated at about 13 inches by 2100. Some areas of the basin could experience rises of up to 50 inches (in more rapidly subsiding areas). [2]
  • Heavier winter rainfalls could increase the risk of landslide on coastal bluffs. The combination of saturated soils and sea level rise could increase the number and severity of landslides, especially in areas with high development or already unstable slopes. [2]

For more information on climate change impacts on coastal resources, please visit the Coastal Resources page.

To learn more about what the Northwest is doing to adapt to climate change impacts, please visit the adaptation section of the Northwest Impacts and Adaptation page.

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References

[1] U.S. Fish and Wildlife Service (2011). Climate Change in the Pacific Northwest .

[2] USGCRP (2009). Global Climate Change Impacts in the United States . Karl, T.R., J. M. Melillo, and T. C. Peterson (eds.). United States Global Change Research Program. Cambridge University Press, New York, NY, USA.

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