Abstract
Abstract. This study not only examined the spatiotemporal variations of active-layer thickness (ALT) in permafrost regions during 1948–2006 over the terrestrial Arctic regions experiencing climate changes, but also identified the associated drivers based on observational data and a simulation conducted by a land surface model (CHANGE). The focus on the ALT extends previous studies that have emphasized ground temperatures in permafrost regions. The Ob, Yenisey, Lena, Yukon, and Mackenzie watersheds are foci of the study. Time series of ALT in Eurasian watersheds showed generally increasing trends, while the increase in ALT in North American watersheds was not significant. However, ALT in the North American watersheds has been negatively anomalous since 1990 when the Arctic air temperature entered into a warming phase. The warming temperatures were not simply expressed to increases in ALT. Since 1990 when the warming increased, the forcing of the ALT by the higher annual thawing index (ATI) in the Mackenzie and Yukon basins has been offset by the combined effects of less insulation caused by thinner snow depth and drier soil during summer. In contrast, the increasing ATI together with thicker snow depth and higher summer soil moisture in the Lena contributed to the increase in ALT. The results imply that the soil thermal and moisture regimes formed in the pre-thaw season(s) provide memory that manifests itself during the summer. The different ALT anomalies between Eurasian and North American watersheds highlight increased importance of the variability of hydrological variables.
Highlights
Permafrost is considered vulnerable to increasing temperatures, and its degradation has the potential to initiate numerous feedbacks, predominantly positive, in the Arctic and global climate system (McGuire et al, 2006)
The simulated total permafrost extent at > 45◦ N cannot directly compare with the permafrost extent derived from the International Permafrost Association (IPA) map, because Iceland and some other northern islands are excluded in this simulation
coupled hydrological and biogeochemical model (CHANGE) tends to overestimate the permafrost at the southern boundary of Siberia, the simulation shows generally good agreement with the IPA map (Fig. 1c)
Summary
Permafrost is considered vulnerable to increasing temperatures, and its degradation has the potential to initiate numerous feedbacks, predominantly positive, in the Arctic and global climate system (McGuire et al, 2006). Most general circulation models project that the warming will continue (IPCC, 2008), increasing the risk of permafrost degradation. It is important to elucidate how soil temperatures are rising, how permafrost is degrading, and the relationships between the two. Permafrost is a useful climate indicator because it integrates processes (e.g., air temperature, precipitation, snow, and vegetation) that occur at and above the ground surface. Anisimov and Nelson (1997) simulated the changes in active-layer thickness (ALT) in the Northern Hemisphere under climate change scenarios and evaluated the risk of ground subsidence in the circum-Arctic permafrost region. Oelke et al (2004) simulated the increasing trends in soil temperature and ALT in the Arctic permafrost regions during 1980–2002 Experiments with temperature warming scenarios have projected poleward shifts in permafrost boundaries relative to the current distribution of permafrost (Chen et al, 2003; Lawrence et al, 2008). Anisimov and Nelson (1997) simulated the changes in active-layer thickness (ALT) in the Northern Hemisphere under climate change scenarios and evaluated the risk of ground subsidence in the circum-Arctic permafrost region. Oelke et al (2004) simulated the increasing trends in soil temperature and ALT in the Arctic permafrost regions during 1980–2002
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