Urban Forest Health

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.

Rising temperatures and increases in the duration and intensity of drought are expected to increase wildfire occurrence and also reduce the effectiveness of prescribed fire in the Southeast. While this region experiences the highest number of wildfires in the country, prescribed fire is currently more common than wildfire in Southeastern forests. However, as fire seasons lengthen in the future, the window for prescribed burning may decrease because of increased fuel flammability.

Forest ecosystems throughout the Southeast Region are exposed to a range of natural, introduced, and anthropogenic stressors. Stressors such as invasive plants, forest pests, and diseases are expected to become more damaging under climate change, and these factors may interact in unpredictable ways. The southern pine beetle is already the most destructive pest in the region's forests, and longer growing seasons could allow populations of the pest to expand more rapidly.