The case of the disappearing lakes
Hundreds of lakes across a swath of north central Siberia either shrank or disappeared between 1973 and 1998, according to a satellite analysis presented in the 3 June issue of Science. The study team, led by Laurence Smith (University of California, Los Angeles), attributes the shrinkage to a multistage process related to the melting of permafrost.
The region studied is about as large as Colorado and Wyoming combined. In 1973 it included more than 10,000 lakes that each spanned more than 40 hectares (100 acres). By 1997-98 the total lake-covered acreage had dropped by 6%, and 125 lakes had completely vanished. Satellite photos show that vegetation in each of the former lakebeds persisted through at least 2004, suggesting that the lakes are unlikely to return.
Smith and colleagues identified two distinct belts of lake change. In the northern tier, where permafrost is continuous, the total lake area increased by 12%, while lake coverage declined in areas to the south where permafrost melt is more prevalent. The decline appears to contradict previous work that found lakes multiplying due to thermokarsts (rain- and snow-catching basins produced by local breaks in thermofrost). Smith and his coauthors reconcile the discrepancy with a continuum tied to climate warming. As permafrost first begins to degrade and form thermokarsts, the lake area increases; then it decreases as more widespread permafrost melting sets in across lakebeds and the lakes drain into the soil.