Forest

Warmer spring temperatures, earlier snowmelt, wetland drying, and longer growing seasons have already contributed to an increase in wildfires in interior Alaska. There have recently been unprecendented tundra fires in northern and western Alaska associated with sea ice loss. Even if climate warming were curtailed by reducing greenhouse gas emissions, the annual area burned in Alaska is projected to double by mid-century and to triple by the end of the century.

Models of annual mean ground temperature at 1 meter depth project the area of above-freezing ground will expend dramatically during the 21st century under all climate scenarios. This warming is especially pronounced in the second half of the century, covering much of interior Alaska in the higher climate scenarios. By the end of the century, stable permafrost may only exist in the northern third of the state.

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. Increases of 15-25 days are projected for southwestern and south-central parts of the state, with the growing season extending to more than 200 days in a large portion of southwestern Alaska.

There is general agreement between different climate scenarios for future precipitiation projections in Alaska. Generally, the largest increase is projected for the far northwest of the state and the smallest changes in the Inside Passage region. By the end of the century, precipitation increases range from 10% to 35% in different parts of the state. Spring precipitation is actually projected to decrease in some scenarios, but there is generally high uncertainty between future projections.

Alaska has warmed twice as fast as the rest of the U.S. over the past several decades. All climate models agree that temperatures are projected to increase over the 21st century across Alaska, with larger increases projected for later in the century. The greatest warming is expected in northern Alaska, with less warming projected for the southeast part of the state. The greatest warming is expected during winter months, with less dramatic increases expected during the summer.

Historical and projected climate change makes two-fifths (40%) of the region vulnerable to these shifts of major vegetation types or biomes; notably threatened are the conifer forests of southern California and sky islands of Arizona. Douglas-fir, ponderosa pine, and pinyon pine may be most negatively affected in Arizona, Colorado, and New Mexico. Following more frequent fires, forest systems may shift to more early-successional species, open canopy forests, and possibly invasive species.

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.