Permafrost covers almost a quarter of the land of the Northern Hemisphere. A temperature rise of 2˚C above pre-industrial levels would thaw about 40% of this permafrost, releasing billions of tonnes of carbon into the atmosphere.

The permafrost – the soil that has historically remained frozen year-round – is more sensitive to climate change than previously thought, finds a study published in the journal Nature Climate Change.

Warming of 2˚C is the official limit for the end of the century targeted in the Paris Agreement, but even this increase would release much of the carbon locked up in the permafrost.

"In the permafrost in total there's around 1,000 gigatonnes of carbon," study author Sarah Chadburn of the University of Exeter told IBTimes UK.

Once permafrost thaws, the carbon in dead organic matter – which may have been frozen for tens of thousands of years – begin to decay. This releases carbon dioxide and methane, also a potent greenhouse gas, exacerbating climate change.

Melting permafrost also means that the buildings, roads and other infrastructure built on this ground is at risk. The process of collapsing roads and structures is already underway in much of the Arctic.

"This would also have consequences for the ecosystems. Those are going to definitely going to change.

One way they are already doing so is through the collapse of coastlines of Arctic nations. These are held fast by earth that is frozen solid, but as they melt they slough off into the oceans, affecting the availability of nutrients and populations of marine creatures there.

Thawing permafrost in northern Norway. Sebastian Westermann

The study used historical data on air temperatures dating back to 1900, and data on the extent of permafrost dating from the 1960s, to see the effect of global warming on the permafrost. They then used climate models to forecast how this relationship would run in the future given various levels of climate change.

They found that about 4 million sq km of permafrost thaws for every 1C increase in global temperatures. How much carbon and methane would be released from this thawed soil is difficult to predict, Chadburn said, as some areas of permafrost lock up more carbon than others.

The only way to limit the thaw of the permafrost is to reduce carbon emissions, the authors said.

A collapsing Siberian coastline due to melting permafrost
permafrost map
A map showing the Arctic's permafrost Hugo Ahlenius, UNEP/GRID-Arendal