Forest

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.

Global sea level rise is projected to rise between 1 and 4 feet by the end of the century. Sea level rise and related increases in storm surges pulsing farther inland will continue to exacerbate ongoing land loss in low-lying coastal areas and may result in excessive saltwater inundation of coastal forests. The number of Category 4 and 5 hurricanes has increased since the 1980s, and this trend can be attributed both to natural variability and climate change.