even circumpolar scale. Figure 7.4 shows a projection in North America, where permafrost will be thawing in
of future permafrost temperatures for the entire North- practically all areas south of the Brooks Range in Alaska
ern Hemisphere. According to this model, by the end of and in most of subarctic Canada. This is probably due
the 21st century permafrost that is presently discontinu- to the fact that permafrost within continental North
ous with temperatures between 0 and –2.5° C will have America is generally warmer and thinner than in Sibe-
crossed the threshold and will thus be actively thawing. ria. In Russia the most severe permafrost degradation is
The most significant permafrost degradation is expected projected for northwest Siberia and the European North.
Methane emissions from thermokarst lakes Siberia by studying the pattern of bubbles in the lake ice, and
found that the amount of methane emission from lakes in this
Depressions in the irregular thermokarst topography caused region may be five times higher than previously estimated
46

by thawing of ice-rich permafrost are usually occupied by lakes (Figure 7.5). The methane emitted from the thawing edges of
called thermokarst lakes, as meltwater cannot drain away due the lakes in this region was 36 000–43 000 years old, showing
to the underlying permafrost. Active thawing of the permafrost that organic matter previously stored in permafrost for tens of
beneath these lakes releases organic matter into the oxygen- thousands of years is now contributing to methane emissions
deficient lake bottoms, which produces methane as it decom- when permafrost thaws
46
. High rates of methane production
poses. Ninety-five per cent of the methane emitted from these and emission have also been observed in thermokarst lakes in
lakes is released through bubbling
46
. Many of these methane- other regions of the Arctic. The formation of new thermokarst
rich bubbles become trapped in lake ice in the winter as the lakes and expansion of existing ones observed during recent
lake surfaces freeze. Extremely high rates of bubbling from dis- decades has increased methane emissions in Siberia
46,47
. If
tinct points in lake sediments, known as bubbling hotspots, significant permafrost warming and thawing occurs as pro-
can maintain open holes in lake ice even during winter, releas- jected, tens of thousands of teragrams of methane could be
ing methane to the atmosphere year-round. Recently, scien- emitted from lakes, an amount that greatly exceeds the 4850
tists quantified methane emissions from thermokarst lakes in teragrams
48
of methane currently in the atmosphere
49
.
Figure 7.5: Methane bubbles trapped in lake ice form distinct patterns as a result of differing rates of methane bubbling. Meth-
ane emissions from the entire lake are estimated, taking into account the patchiness of bubbling, by surveying the distribution
of bubble patterns in lake ice in early winter.
Photos: Katey Walter
188 GLOBAL OUTLOOK FOR ICE AND SNOW