THOUSANDS of Arctic lakes are disappearing as a result of global warming. The normally frozen soil, or permafrost, that makes the lake beds watertight is thawing, allowing the water to drain away. The loss of these lakes could endanger native plants and animals, as well as many species of migratory birds that breed and nest around them. It could also threaten the livelihood of people who use the lakes for hunting and fishing.
The Arctic is warming faster than any other region on Earth. Normally its ice sheets reflect much of the sunlight that falls on them back into space, but they are receding and exposing the land and sea underneath. The darker land and sea absorb more light than ice and so heat up even more, causing the ice to melt still faster.
Until now, the effect of this process on the Arctic lakes has been a bit of a puzzle. Some studies have shown that a few Arctic lakes are expanding. 鈥淭he prevailing thought is that in a warming climate, the permafrost will thaw out and the lakes will become more abundant,鈥� says Larry Smith of the University of California, Los Angeles.
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But in 2003, Larry Hinzman and Kenji Yoshikawa of the University of Alaska Fairbanks found that global warming is causing lakes in Council, Alaska, to shrink. They suggested that the permafrost is turning into porous soil, causing the lakes to soak away. However, the study only covered a small region.
To reconcile these findings, Smith, Hinzman and their colleagues looked at a much larger slice of the Arctic 鈥� a vast 515,000-square-kilometre area in Siberia. Comparison of satellite pictures from the 1970s with those from 1997 to 2004 showed that the total surface area of lakes there has decreased by 93,000 hectares. More than 1000 large lakes, originally each bigger than 40 hectares, have shrunk, and 125 others have disappeared (Science, vol 308, p 1429).
鈥淭he loss of these lakes could endanger native plants and animals, as well as many species of migratory birds鈥�
However, when they looked at only the northern part of the Arctic within their survey area, they found that the total surface area of lakes there had increased by 13,300 hectares. Taken as the whole, more lakes are shrinking than are growing in the region. 鈥淭he most interesting thing is that you are seeing changes that one could interpret as conflicting responses, but actually these changes are driven by the same processes,鈥� says Hinzman.
According to the researchers, when the thick northern permafrost melts, deeper layers remain to contain the lakes, so the lakes simply expand. But in the south, where the permafrost is thinner and not continuous, the thawing allows the lake to soak into the soil.
This mechanism explains the patchwork nature of the team鈥檚 findings from satellite pictures. In some cases, drained lakes are found right next to undisturbed ones, says Smith. This suggests that the permafrost can vary in thickness within a small area, and it is this thickness that determines whether a lake shrinks or not. 鈥淚t鈥檚 very abrupt,鈥� says Smith. 鈥淚t鈥檚 not like these lakes are drying up from warmer air temperature.鈥�
Mark Serreze, an expert on Arctic climate at the University of Colorado at Boulder, says he is impressed by the study. 鈥淚t adds to the growing body of evidence that the Arctic is rapidly changing,鈥� he says. 鈥淭he effects of greenhouse gases in the Arctic are beginning to emerge from the noise of natural climate variability.鈥�
