Populations of microscopic algae, called diatoms, have been absent from the lake Kaffeklubben Sø for over 2,000 years. But a new study has found that the diatoms are returning, thanks to global warming.

"It's a pure climate change story," said study co-author Bianca Perren, a paleoecologist at the University of Franche-Comté in Besançon, France, who specializes in Arctic environmental change (see pictures).

Diatoms were once abundant in Kaffeklubben Sø, which was formed about 3,500 years ago after glacial retreats created numerous small lakes on the coastal plain.

As surrounding temperatures cooled, diatom populations decreased until they vanished some 2,400 years ago, Perren explained.

"Until about 1920, [the lake] was basically in a deep freeze," she said.

Ice completely covered its surface, cutting off