Rapidly decomposing Michelle Holihan/Alamy

There’s more bad news on the climate front: soils in cold climates could release far more carbon than expected as the world warms.

For every half a degree Celsius of warming, this extra carbon could be roughly equivalent to roughly one year’s emissions from all human sources. That makes the task of limiting warming to 2°C even harder, because we have four fewer years to slash emissions.

At present, half of all the extra carbon dioxide we are pumping into the atmosphere is being soaked up by the land and sea. Some goes into soils, which are estimated to contain three times as much carbon as the atmosphere.


Any change in how much carbon is taken up or lost from soils can therefore have a big impact on global warming. As the world warms, decomposition will speed up, accelerating carbon loss. But higher temperatures and CO 2 levels will speed up plant growth, increasing carbon take-up.

At present, the soil models used to inform climate models suggest that in high latitudes the growth effect will win out, and carbon storage will increase. But they are wrong, according to a study by Charles Koven of the Lawrence Berkeley National Laboratory in California and colleagues.

“Our work shows a net loss instead,” says Koven. “A quite large one.”

Leaking from soil

Checking soil model predictions is tricky. Scientists cannot travel forward in time, but they can compare cool climes where the soil is sometimes frozen with tropical rainforests.

When Koven’s team compared model predictions to actual observations, they found the models were underestimating the sensitivity of soils in places like the Arctic. “We show that the stronger feedback is more consistent with the observations,” says Koven. “Decomposition speeds up very rapidly when these soils thaw.”

Part of this carbon loss comes from melting permafrost, but Koven’s study looks only at the top metre of soil, not at the deep permafrost that holds vast stores of carbon.

The latest study does not put specific figures on how much more carbon will be lost. However, based on his previous studies Koven estimates that it could be around 10 billion tonnes (10 petagrams) of carbon per half degree of global warming.

“That means there is 40 petagrams of carbon less in our budget [for limiting warming to 2°C] than we think we have,” he says. There are huge uncertainties still, he stresses.

Earlier today the World Meteorological Organization announced that greenhouse gas levels in the air have reached yet another record high. They are now the highest they have been for 800,000 years.

Journal reference: Nature Climate Change, DOI: 10.1038/nclimate3421