Last 100150 Years

Although there is clear evidence that thermokarst activity during the last 100-150 years has spread and intensified, not all of it can be attributed to global warming. One of the clearest examples where climate warming has exacerbated thermokarst is an abrupt increase in ice-wedge melting since 1982 in continuous permafrost of northern Alaska (Jorgenson et al. 2006). The melting probably resulted from record high summer temperatures between 1989 and 1998, and was initiated by extreme hot and wet summer weather in 1989, leading to unusually deep thaw of the active layer. This thermokarst activity coincided with a 2-5°C increase in mean annual ground temperature, partially melting ice wedges that had previously been stable for thousands of years.

Further south, in warm discontinuous permafrost of sub-Arctic regions, increased thermokarst activity since the Little Ice Age is well-established (see review in Jorgenson and Osterkamp 2005), but the causes are complex. For example, thermokarst activity in the Tanana Flats, central Alaska, has transformed large areas of birch forest into fens and bogs (Jorgenson et al. 2001). Thermokarst here probably began in the mid-1700s, associated with climate warming. But thermokarst activity during the succeeding ~250 years has been enhanced in part by (1) convec-tive heat transfer by movement of relatively warm (2-4°C year-round) groundwater through the fens and underlying outwash gravel, (2) fires, and (3) increased snow depths. Isolating the influence of fire, snow and climate warming is difficult, because an increase in fire frequency may correlate with an increase in summer temperatures (Jorgenson and Osterkamp 2005), and because warmer winter temperatures may correlate with increased snowfall and therefore warmer MAGTs (cf. Osterkamp 2007).

In western Siberia, climate warming since the early 1970s is thought to have driven thermokarst activity in two different ways (Smith et al. 2005). In the continuous permafrost zone, the number of lakes has increased substantially, whereas in discontinuous, isolated and sporadic permafrost it has decreased. This disparity supports a conceptual model in which initial warming of cold, continuous permafrost favours thermokarst activity and lake expansion, followed by lake drainage as permafrost degrades further. In central Siberia, climate warming since the 1980s has led to increased water temperature in the Lena River and its tributaries, in turn leading to increased rates of fluvial thermal erosion along their banks (Costard et al. 2007); significantly, this recent climate warming followed a period of cooling in the mid twentieth century, when thermal erosion rates along the coast of the Laptev sea tended to decrease (Are 1983). Finally, increases in mean annual air temperature and summer air temperatures between 1992 and 2001 at Yakutsk have coincided with (1) thermokarst subsidence beneath stable inter-alas meadows and (2) flooding of young thermokarst basins, enhancing thermokarst activity at a nearby permafrost monitoring site (Fedorov and Konstantinov 2003).

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