Forest [FAR1]

Temperatures in Hawai'i have warmed by 0.76 degrees F over the past 100 years. Periods of warming and cooling have occurred over the past century, with a statistically significant trend toward warmer temperatures. Warming is occurring at both high and low elevations, although higher elevation areas have been warming faster than lower elevations over the past 30 years. Much of the warming can be attributed to higher minimum temperatures.

Changing temperature and precipitation regimes may shift suitable habitat for many tree species and ecosytems, and the effects of wildfires, pests, and diseases will be more harmful to some species than to others. Also, fundamental changes may make some areas less suitable for forests. For example, permafrost degradation is widespread in central Alaska, shifting ecosystems from birch forests to wetland types such as bogs and fens. If broad-scale water balances become increasingly negative, peatlands may begin to support upland forest species.

Alaska cedar is declining and experiencing elevated mortality in large areas. Alaska cedar decline has been attributed to earlier snowmelt, which exposes roots to damage from lower temperatures. Projected future warming is expected to cause additional mortality from freezing-induced root damage, because more snow will be delivered as rain and winter snowpacks will be thinner and more variable. Additionally, warmer winters may allow pests and diseases to expand into previously unavailable territory in Alaska.

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.