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The Arctic has begun releasing more carbon than it is absorbing. The change is a result of climate change thawing frozen ground, and the problem is expected to get much worse as the world warms further.

Previously it appeared some sites in the Arctic had already flipped from being carbon sinks into sources of emissions, but research now shows the phenomenon has happened across the region as a whole.

Vegetation growth in the far north absorbs carbon dioxide across summer, and we had thought that negligible amounts of CO₂ were escaping from frozen soil in the long winter months, as the cold temperatures prevented thawing.

Now, it appears the region has warmed enough to change that. Observations for 2003 to 2017 show that between October and April, the Arctic emitted 1.66 gigatonnes of CO₂ a year, outweighing the 1.03 gigatonnes soaked up over the rest of the year.

鈥淕iven that the Arctic has been taking up carbon for tens of thousands of years, this shift to a carbon source is important because it highlights a new dynamic in the functioning of the Earth system,鈥� says Susan Natali at Woods Hole Research Center in Massachusetts. A recent report by the UN鈥檚 Intergovernmental Panel on Climate Change聽didn鈥檛 make it explicit that this threshold had been passed.

Natali and her colleagues compiled data from ground measurements of CO₂ flows collected over past decades across the Arctic, along with observations of other variables that can drive those flows, including soil temperature. They used the data to build a machine-learning model and project what the future holds as temperatures rise.

The model paints a worrying picture. Under the worst-case global scenario, by 2100 the CO₂ emissions from the Arctic鈥檚 thawing permafrost will climb by 41 per cent. That would be the equivalent of adding the annual emissions of a country such as the UK.

However, if countries hold temperature rises below 2掳C as the Paris climate deal demands, the increase will be limited to 17 per cent. Some level of soil warming and permafrost thaw in the Arctic is now unavoidable, but Natali says the work shows we can greatly reduce the impact by cutting global emissions.

Hanna Lee at Norwegian research centre聽NORCE says the study challenges assumptions that the region is still too cold to release CO₂ and shows the importance of monitoring winter emissions.

One aspect the research didn鈥檛 examine was whether those emissions might be partially offset in coming decades by higher CO₂ levels driving increased plant growth. The study also didn鈥檛 look at methane emissions from permafrost, which researchers fear could abruptly skyrocket as temperatures rise.

In the future, another thing to watch for is whether fires, such as the unprecedented blazes across the Arctic this summer, will make the region an even bigger emitter of CO₂, as they strip off the trees and vegetation that insulate the permafrost.