A regime shift in lake ice is leading to permafrost thawing below shallow Arctic lakes as a result of changing winter climate, a new study has found. Warmer winters combined with an increase in snowfall during the last 30 years have limited the growth of seasonal lake ice. In response, lakebed temperatures of Arctic lakes less than one metre deep have warmed by 2.4 degrees Celsius during the past three decades, and during five of the last seven years, the mean annual lakebed temperature has been above freezing. These rates of warming are similar to those observed in terrestrial permafrost, yet those soils are still well below freezing and thaw is not expected for at least another 70 years. However, a regime shift in lake ice is leading to sub-lake permafrost thaw now. Since permafrost underneath lakes is generally warmer than the surrounding terrestrial permafrost, rising temperatures in the lakebeds make permafrost thaw sooner than beneath surrounding dry land. These lakes may cover 20 to 40 per cent of the landscape in vast areas of Arctic lowlands, researchers said. "During the 1970s, late winter lake ice thickness measurements commonly exceeded 2 metres in northern Alaska," said Christopher Arp, from the University of Alaska. "Such thick ice growth helps to limit sub-lake permafrost thaw by freezing the sediments solid each winter. However, during winter field surveys over the last decade, lake ice has typically only grown to 1.5 metres thick, and has been as thin as 1.2 metres," said Arp. These drastic reductions in lake ice, caused by changes in winter climate, are the primary reason that shallow lakebed temperatures are warming and the permafrost below them is thawing. Interactions and feedbacks among climate, permafrost, and hydrology underscore the complexity of forecasting change in the Arctic. For example, thinner lake ice may help fish overwintering, or it may help the oil industry since they need lake water to build winter ice roads. However, sub-lake permafrost thaw will likely unlock a portion of the permafrost carbon pool and potentially release this carbon in the form of greenhouse gases. "With further thawing of sub-lake permafrost there is a good chance that the ground will subside, increasing the lake depth and accelerating further permafrost thawing," said Vladimir Romanovsky of the University of Alaska Fairbanks Geophysical Institute. "In contrast, the warming on the land may increase the protective vegetation layer and delay thawing of permafrost outside of lakes," said Romanovsky. With increasingly warmer and snowier winters yielding thinner lake ice, shallow lakes will likely continue to warm, Arp said. The study was published in the journal Geophysical Research Letters.