Scientists have long known that wetlands produce large amounts of methane and had thought it unlikely that greenhouse gases escaped from beneath frozen tundra, said Torben Christensen, a biogeochemist at Lund University in Sweden.

"Mother Nature is showing us something that is really surprising," Christensen, who led the study published in the journal Nature, said in a telephone interview.

"Nobody would expect to have loads of gas seeping out from a frozen environment."

Human activities such as burning of fossil fuels are pumping greenhouse gases linked to global warming into the atmosphere and scientists say it is imperative to understand all the sources for the emissions.

"This study improves our understanding of how nature is connected to the climate," Christensen said. "In doing that it gives us a better understanding on how climate change is affecting arctic ecosystems."

The research team stayed on at Zackenberg Valley research station in northeast Greenland two months into the winter season. This allowed them for the first time to take measurements from frozen tundra and compare the production of methane to the summer months.

After measuring atmospheric methane concentrations when the soil began freezing in late September, they found that the levels of the greenhouse gas increased significantly as the freeze began -- about as much as in the summer months.

The findings do not change overall estimates of methane emission from high northern latitudes but do revise the view of the distribution of the greenhouse gas during the year, the researchers added.

"The assumption has always been that a frozen, snowed-under environment is not active in terms of a greenhouse effect," Christensen said. "It turns out this was a wrong assumption."