Urban Forest Health

Although many ecosystems require fire, excessive wildfire can permanently alter ecosystem integrity. Climate change has led to an increase in the area burned by wildfire in the western United States. Climate change has driven the wildfire increase particularly by drying forests and making them more susceptible to burning. With continued greenhouse gas emissions, models project more wildfire across the Southwest region.

Projections of precipitation are less certain than those for temperature, however projections indicate that the southwestern U.S. may experience chronic future precipitation deficits, particularly in the spring. With continued greenhouse gas emissions, higher temperatures would cause more frequent and severe droughts in the Southwest. Higher temperatures sharply increase the risk of megadroughts—dry periods lasting 10 years or more. Snowpack supplies a major portion of water in the Southwest, but with continued emissions, models project substantial reductions in snowpack.

The largest decreases are expected in the northern half of the region, particularly in high-elevation areas. The smallest decrease in cold days is expected coastal and southern parts of the region, where these kinds of cold days rarely occur. High elevation areas could have up to 60 fewer days each year with minimum temperatures below 32° F (0°C) by the mid century, when compared to the period 1976 - 2005.

The largest simulated increases occur in Arizona and New Mexico, with as many as 25-30 more days above 95 degrees F each year. These hot days are already common in these parts of the region. The smallest increases in hot days occur in high-elevation areas, because projected temperature increases are still not enough to increase the chance of such warm days.

Southern parts of the region could have up to 45 more days each year with maximum temperatures of 90°F (32°C) or higher, by mid century, when compared to the period 1976-2005.

All climate models agree that temperatures are projected to increase over the 21st century across the Southwest, with almost uniform temperature increases across the entire region. The greatest warming is expected in the northern part of the region, especially Nevada, Utah, and Colorado. The greatest warming is expected during summer months, particularly in Utah and Colorado. Warming is generally smaller in winter, and along the Pacific coast.

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.