Over the past 50 years, Alaska has warmed at more than twice the rate of the rest of the United Statesâ€™ average. Its annual average temperature has increased 3.4Â°F, while winters have warmed even more, by 6.3Â°F.501 As a result, climate change impacts are much more pronounced than in other regions of the United States. The higher temperatures are already contributing to earlier spring snowmelt, reduced sea ice, widespread glacier retreat, and permafrost warming.220,501 These observed changes are consistent with climate model projections of greater warming over Alaska, especially in winter, as compared to the rest of
Climate models also project increases in precipitation over Alaska. Simultaneous increases in evaporation due to higher air temperatures, however, are expected to lead to drier conditions overall, with reduced soil moisture.90 In the future, therefore, model projections suggest a longer summer growing season combined with an increased likelihood of summer drought
Average annual temperatures in Alaska are projected to rise about 3.5 to 7Â°F by the middle of this century. How much temperatures rise later in the century depends strongly on global emissions choices, with increases of 5 to 8Â°F projected with lower emissions, and increases of 8 to 13Â°F with higher emissions.91 Higher temperatures are expected to continue to reduce Arctic sea ice coverage. Reduced sea ice provides opportunities for increased shipping and resource extraction. At the same time, it increases coastal erosion522 and flooding associated with coastal storms. Reduced sea ice also alters the timing and location of plankton blooms, which is expected to drive major shifts of marine species such as pollock and other commercial fish stocks.527
Longer summers and higher temperatures are causing drier conditions, even in the absence of strong trends in precipitation.
Between 1970 and 2000, the snow-free season increased by approximately 10 days across Alaska, primarily due to earlier snowmelt in the spring.503,504 A longer growing season has potential economic benefits, providing a longer period of outdoor and commercial activity such as tourism. However, there are also downsides. For example, white spruce forests in Alaskaâ€™s interior are experiencing declining growth due to drought stress505 and continued warming could lead to widespread death of trees.506 The decreased soil moisture in Alaska also suggests that agriculture in Alaska might not benefit from the longer growing season.
Insect outbreaks and wildfires are increasing with warming.
Climate plays a key role in determining the extent and severity of insect outbreaks and wildfires.506,507 During the 1990s, for example, south-central Alaska experienced the largest outbreak of spruce beetles in the world.243,506 This outbreak occurred because rising temperatures allowed the spruce beetle to survive over the winter and to complete its life cycle in just one year instead of the normal two years. Healthy trees ordinarily defend themselves by pushing back against burrowing beetles with their pitch. From 1989 to 1997, however, the region experienced an extended drought, leaving the trees too stressed to fight off the infestation.
Prior to 1990, the spruce budworm was not able to reproduce in interior Alaska.506 Hotter, drier summers, however, now mean that the forests there are threatened by an outbreak of spruce budworms.509 This trend is expected to increase in the future if summers in Alaska become hotter and drier.506 Large areas of dead trees, such as those left behind by pest infestations, are highly flammable and thus much more vulnerable to wildfire than living trees.
The area burned in North Americaâ€™s northern forest that spans Alaska and Canada tripled from the 1960s to the 1990s. Two of the three most extensive wildfire seasons in Alaskaâ€™s 56-year record occurred in 2004 and 2005, and half of the most severe fire years on record have occurred since 1990.510 Under changing climate conditions, the average area burned per year in Alaska is projected to double by the middle of this century.507 By the end of this century, area burned by fire is projected to triple under a moderate greenhouse gas emissions scenario and to quadruple under a higher emissions scenario.91 Such increases in area burned would result in numerous impacts, including hazardous air quality conditions such as those suffered by residents of Fairbanks during the summers of 2004 and 2005, as well as increased risks to rural Native Alaskan communities because of reduced availability of the fish and game that make up their diet. This would cause them to adopt a more â€œWesternâ€ diet,511 known to be associated with increased risk of cancers, diabetes, and cardiovascular disease.512
Lakes are declining in area.513,514 A continued decline in the area of surface water would present challenges for the management of natural resources and ecosystems on National Wildlife Refuges in Alaska. These refuges, which cover over 77 million acres (21 percent of Alaska) and comprise 81 percent of the U.S. National Wildlife Refuge System, provide breeding habitat for millions of waterfowl and shorebirds that winter in the lower 48 states. Wetlands are also important to Native peoples who hunt and fish for their food in interior Alaska. Many villages are located adjacent to wetlands that support an abundance of wildlife resources. The sustainability of these traditional lifestyles is thus threatened by a loss of wetlands.
Thawing permafrost damages roads, runways, water and sewer systems, and other infrastructure.
Permafrost temperatures have increased throughout Alaska since the late 1970s.149 The largest increases have been measured in the northern part of the state.515 While permafrost in interior Alaska so far has experienced less warming than permafrost in northern Alaska, it is more vulnerable to thawing during this century because it is generally just below the freezing point, while permafrost in northern Alaska is colder.
Land subsidence (sinking) associated with the thawing of permafrost presents substantial challenges to engineers attempting to preserve infrastructure in Alaska.516 Public infrastructure at risk for damage includes roads, runways, and water and sewer systems. It is estimated that thawing permafrost would add between $3.6 billion and $6.1 billion (10 to 20 percent) to future costs for publicly owned infrastructure by 2030 and between $5.6 billion and $7.6 billion (10 to 12 percent) by 2080.230 Analyses of the additional costs of permafrost thawing to private property have not yet been conducted.
Thawing ground also has implications for oil and gas drilling. As one example, the number of days per year in which travel on the tundra is allowed under Alaska Department of Natural Resources standards has dropped from more than 200 to about 100 days in the past 30 years. This results in a 50 percent reduction in days that oil and gas exploration and extraction equipment can be used.220,245
Thawing permafrost can push natural ecosystems across thresholds. Some forests in Alaska are literally toppling over as the permafrost beneath them thaws, undermining the root systems of trees (see photo next page).
When permafrost thaws, it can cause the soil to sink or settle, damaging structures built upon or within that soil. A warming climate and burial of supports for the Trans-Alaska Pipeline System both contribute to thawing of the permafrost around the pipeline. In locations on the pipeline route where soils were ice-rich, a unique above-ground system was developed to keep the ground cool. Thermal siphons were designed to disperse heat to the air that would otherwise be transferred to the soil, and these siphons were placed on the pilings that support the pipeline. While this unique technology added significant expense to the pipeline construction, it helps to greatly increase the useful lifetime of this structure.519