Forested watershed

For many tree species, the most climatically suited areas will shift from their current locations, increasing vulnerability to insects, disease, and fire in areas that become unsuitable. Projections indicate that 21 to 38 currently existing plant species may no longer find climatically appropriate habitat in the Northwest by late this century. Subalpine forests and alpine ecosystems are especially at risk and may undergo almost complete conversion to other vegetation types by the 2080s.

Forest pests, diseases, and droughts are expected to become more damaging under climate change, and these factors may interact in unpredictable ways. Many impacts will be driven by water deficits, which increase tree stress and mortality, tree vulnerability to insects, and fuel flammability. Higher temperatures and drought stress are contributing to outbreaks of mountain pine beetles that are increasing pine mortality in drier Northwest forests.

Winter snowpack, which slowly melts and releases water in spring and summer, is key to the Northwest’s hydrology and water supplies. Since around 1950, area-averaged snowpack on April 1 in the Cascade Mountains decreased about 20%, spring snowmelt occurred 0 to 30 days earlier, late winter/early spring streamflow increases ranged from 0% to greater than 20% as a fraction of annual flow, and summer flow decreased 0% to 15% as a fraction of annual flow. Under climate change, the largest changes are expected in basins that typically recieve lots of snow.

The freeze-free season is defined as the period of time between the last spring frost (daily minimum temperature below 32 degrees F) and the first fall frost. The length of the annual freeze-free season has been increasing since the 1980s, and all climate models agree that it will continue to increase in the future. The largest increases are projected for the areas west of the Cascades, with some areas increasing by more than 40 days. The rest of the region is expected to have roughly 25-day increases in the growing season.

The largest increases in days per year with more than 1 inch of precipitation is projected for areas east of the Cascades in Oregon and Washington (40% increase). For the rest of the region, projected changes in extreme rainfall are smaller than normal year-to-year variation.

There is general agreement between different climate scenarios for future precipitation projections in the Northwest. Generally, the largest increase is projected for northern Washinton (9-12% increase), with increases becoming smaller toward southern Oregon and Idaho (0-3% increases). Precipitation decreases appear more likely for summer months across the entire region (average decline = 11%), particularly in high-elevation areas. Winter months may experience slightly increased precipitation, and projections for fall and spring are mixed across the region.

Although wildfires are a natural part of most Northwest forest ecosystems, warmer and drier conditions have helped increase the number and extent of wildfires in western U.S. forests since the 1970s. Warmer winters have led to reductions in the mountain snowpack that historically blanketed the region’s mountains, increasing wildfire risk. The warming trend is projected to be accentuated in certain mountain areas in late winter and spring, further exacerbating snowpack loss and increasing the risk for wildfires.

The largest decreases are expected in the inland of the region, particularly in high-elevation areas in central Idaho. The smallest decrease in cold days is expected coastal and southern parts of the region, where these kinds of cold days rarely occur. Similarly, high-elevation areas are expected to have up to 40 fewer days with a minimum temperature below 32 degrees by the middle of the century, particularly in the mountains of Oregon and Washington, and northern Idaho.

The largest simulated increases occur in southern Idaho, with as many as 18 more days above 95 degrees F each year. The smallest increases in hot days occur in high-elevation areas of the Cascades and Rocky Mountains, because projected temperature increases are still not enough to increase the chance of such warm days.

All climate models agree that temperatures are projected to increase over the 21st century across the Northwest, with almost uniform temperature increases across the entire region. The greatest warming is expected in southern Idaho and along the coast. The greatest warming is expected during summer months, particularly in Idaho and Oregon. The warming trend is projected to be accentuated in certain mountain areas in late winter and spring.