Characteristics of the recent warming of permafrost in Alaska

Abstract

[1] Tentative answers are provided to questions concerning the recent warming of permafrost in Alaska, particularly those regarding timing, duration, magnitude, spatial distribution, seasonality, active layer effects, thawing, thermokarst terrain, and causes. Permafrost warmed at most sites north of the Brooks Range from the Chukchi Sea to the Alaska-Canada border, south along a transect from Prudhoe Bay to Gulkana and at sites up to 300 km from the transect. The warming was coincident with the statewide warming of air temperatures that began in 1976/1977 and appears to have occurred statewide with some exceptions. Magnitude of the warming was 3 to 4°C for the Arctic Coastal Plain, 1 to 2°C for the Brooks Range including its northern and southern foothills, and 0.3 to 1°C south of the Yukon River. This suggests a total warming of >6°C at Prudhoe Bay during the last century. The warming was seasonal (primarily in winter) with little change in summer conditions. Consequently, active layer thicknesses did not increase and were not correlated with warming permafrost conditions. Natural thawing at the permafrost surface ($0.1 m/yr) occurred at both a tundra and forest site. Basal thawing at one site was $0.04 m/yr until 2000 when it accelerated to $0.09 m/yr. New thermokarst terrain has been observed in interior and northern Alaska. Probable causes of the warming include increased air temperatures, snow cover effects, and combinations of these. New investigations are needed to further determine the characteristics, especially the causes, of this recent permafrost warming.

Modeling permafrost thaw and ecosystem carbon cycle under annual and seasonal warming at an Arctic tundra site in Alaska

“…This suggests that enhanced permafrost thawing from warming potentially leads to more C loss and may shift the ecosystem C balance toward a weaker sink or a source [Osterkamp and Jorgenson, 2006;Osterkamp, 2007;Schuur et al, 2008]. However, this study suggests that the increased permafrost thaw from …”

“…Site-specific differences such as climate and soil may result in varying responses to the same warming scenario, and sites may also experience temperature increase at different seasons such as winter warming or summer warming. Winter warming has been more pronounced in high-latitude regions during this century [Osterkamp, 2007;Xia et al, 2014]. Several lines of evidence show great seasonal variations in temperature sensitivity of microbial enzyme activity [Brzostek and Finzi, 2012] and nitrogen cycling [Weedon et al, 2012] in high-latitude soils, which will exert a strong control in regulating tundra ecosystem response to warming [Grogan and Chapin, 1999;Chapin et al, 2000].…”

The contribution of snow condition trends to future ground climate

“…Since a snow-covered surface reflects a much large portion of the incoming solar radiation than a snow-free surface, a lengthening or shortening of the snowseason will alter how much incoming solar energy is absorbed by the ground, also affecting soil temperatures. Prior studies, both modeling and observation based, suggest that soil temperature change (at 10-20 m depth) over the latter part of the twentieth and early part of the twentyfirst century can be attributed roughly equally to air temperature and snow depth trends or variations (Zhang et al 2001;Stieglitz et al 2003;Osterkamp 2007b). Osterkamp (2007a) concludes that modeling studies are required to assess the relative role of snow versus air temperature effects on soil temperature trends.…”

On the Arctic near-surface permafrost and climate sensitivities to soil and snow model formulations in climate models

“…Many recent studies (Lemke et al 2007;Osterkamp 2007;Frauenfeld and Zhang 2011) suggest increases in soil temperatures in these permafrost regions, particularly during the last few decades. These increases in soil temperatures are reflected in the increasing active layer thickness (ALT)-defined as the maximum annual thaw depth-and permafrost degradation.…”