Arctic permafrost that is thawing due to global warming is releasing greenhouse gases, further compounding the problem of climate change, according to a study released on Thursday.

As the permafrost thaws, changes in the way its soil microbes function and the soil carbon decomposes add to the emissions of carbon dioxide and methane into the atmosphere, according to the study by US and Chinese scientists.

Carbon dioxide and methane are the main greenhouse gases that trap heat and contribute to climate change.

Permafrost is the perennially frozen ground that covers a quarter of the land in the Northern Hemisphere, primarily in the Arctic, said the study published in the monthly Nature Climate Change journal.

Working in Alaska, researchers warmed plots of tundra to thaw the permafrost and after 18 months found numerous changes in the soil microbes, it said.

"This study highlights the critical role that microbes play in mediating carbon losses from Arctic soils," said Susan Natali, a scientist at Woods Hole Research Center in Massachusetts and co-author of the Nature Climate Change paper, in a statement.

"The rapid response of the microbial community to warming suggests that the large store of soil carbon currently contained in permafrost will be highly susceptible to decomposition once it is thawed."

Previous studies have suggested that permafrost could decline by as much as 70 percent by the end of the century, according to the statement.