Forested watershed

Invasive plants, forest pests, diseases, droughts, and floods are expected to become more damaging under climate change, and these factors may interact in unpredictable ways. Drought and increased temperatures due to climate change have caused extensive tree death across the Southwest. In addition, winter warming due to climate change has exacerbated bark beetle outbreaks by allowing more beetles, which normally die in cold weather, to survive and reproduce. Wildfire and bark beetles killed trees across 20% of Arizona and New Mexico forests from 1984 to 2008.

Water is central to the region’s productivity. Winter snowpack, which slowly melts and releases water in spring and summer, when both natural ecosystems and people have the greatest needs for water, is key to the Southwest’s hydrology and water supplies. Over the past 50 years across most of the Southwest, there has been less late-winter precipitation falling as snow, earlier snowmelt, and earlier arrival of most of the year’s streamflow.

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 interior of California.

The Southwest has already experienced a modest increase in some measures of heavy precipitation, thought not to the extent of other regions. For example, storms in the 99th-percentile category increased by 10% from 1958 to 2016 across the region. Intense rainfall is expected to continue to occur more frequently in the future, even though overall precipitation is expected to decline. For example, large rain events that historically had an expected return of 5 years are expected to occur twice as often by the end of the century, under a high emission scenario (RCP 8.5).

There is uncertainty between different climate scenarios for future precipitation projections in the Southwest. Generally, there is a south-to-north gradient in annual precipitation projections, with decreases projected from central California, Nevada, Utah, and Colorado to the south. Decreases are largest in southern California, Arizona, and New Mexico (3-12%). The northern half of the region is projected to experience no change or a slight (0-3%) increase in annual precipitation.

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.