Located at 83° 37' north, on the coastal plain of northern Greenland, the 48-hectare Kaffeklubben Sø looks out over the Arctic Sea. "It's kind of the end of the earth," says Bianca Perren of the University of Franche-Comté in Besançon, France.

One to two metres of ice cover the lake year-round, but a "moat" of water forms around the edge of the lake in summer when average temperatures rise to 1.6 °C.

The lake formed about 3500 years ago when local precipitation increased, says Perren. A few species of silica-shelled algae called diatoms lived in the young lake, but their populations declined as regional temperatures cooled, and they vanished entirely 2400 years ago. All that survived under the ice were hardy cyanobacteria, which require little light and can survive even under several metres of ice.

A couple of brief summer thaws allowed diatoms to return briefly, but the lake remained nearly barren until around 1960, when the first diatom species returned. The latest water samples, collected by Perren and her colleagues, contain some 20 species.

Other high Arctic lakes that are not quite as remote show similar changes, says study co-author Alex Wolfe at the University of Alberta in Edmonton, Canada.

Climate-induced bloom

Last year, Wolfe and colleagues reported that nitrates and industrial pollutants may have encouraged the tiny organisms to bloom in other lakes. However, there are no traces of nitrates in Kaffeklubben, indicating its recolonisation by diatoms was driven purely by climate change.

Other, more dramatic changes may be on the way. Alexandre Guertin-Pasquier, a palaeontologist at the University of Montreal, Canada, has recently discovered fossil evidence of a lush forest, containing pine, willow and spruce, on Bylot Island in Baffin Bay, Canada, about 3 million years ago. The annual average temperature there at the time was about 0 °C, considerably warmer than today.

It isn't clear how the forest coped with the challenging conditions – the Arctic winter would still have involved months of darkness. But Guertin-Pasquier told the Canadian Paleontology Conference in Toronto last month that the forests could return if Arctic conditions continue to warm – although with average annual temperatures on Bylot Island currently at -15 °C, it will be some time before they do.

Perren and Wolfe's study may have implications for the opposite side of the world. "Ice-covered lakes in the dry valleys of Antarctica are a good analogue of what we had in Kaffeklubben [before 1960]," says Perren. As temperatures there warm, more species may return to these lakes.

The subglacial lakes beneath Antarctica, including Lake Vostok, are less likely to be so prone to warming, though, because they have been buried beneath several kilometres of ice for tens of millions of years.

Journal reference: Geology, DOI: 10.1130/G33621.1