Report from the International Permafrost Association: State of Permafrost in the First Decade of the 21st Century

摘要

Very similar permafrost temperature dynamics have been observed in Alaska, northwest Canada, the European north of Russia and northwest Siberia during the last 20 to 30 years (Figure 1A-D). The general trend is a substantial increase in permafrost temperatures in the 1990s compared to the 1970s or the beginning of the mid-1980s. Most sites show almost no trend during the first decade of the 21st century (late 1990s for the Russian sites) but at several, there is an indication of a slight increase in permafrost temperatures during the last one or two years. Not all permafrost observatories show warming trends over the past 20 years. The data from interior Alaska (Figure 1A), northwest Canada (Figure 1B), northwest Siberia (Figure 1C) and central Asia (Figure 1F) show that the rate of warming decreases significantly when permafrost temperatures approach 0℃. At many sites this increase ceases completely when temperatures are within a few tenths of a degree of 0℃. Good examples of this are the Livengood and Gulkana sites in Alaska (Figure 1A) and several sites in Canada (Figure 1B) and western Siberia (Figure 1C). All these sites are in fine-grained, ice-rich sediments that can hold significant amounts of unfrozen water at temperatures close to but below 0℃. It was suggested earlier (Smith et al., 2005; Romanovsky et al., 2007, 2008b) that partial melt of ice within the upper few tens of metres of permafrost is responsible for the apparent stagnation of permafrost temperatures. During this transition period of 'internal' thawing of permafrost, temperature may not be the best indicator of changes in permafrost. Measuring the unfrozen water content in the upper permafrost may provide a better understanding of changes that are occurring in this near-isothermal permafrost.