There is a lot of talk about tipping points when it comes to climate change, alluding to a time in the not-so-distant future where the effects of human-induced global warming are no longer within our control. Melting ice caps and sea level rise are a couple of well-known examples of this, and a new study has revealed yet another way this whole thing could get away from us. Scientists studying carbon stored in the planet's soil have calculated that as temperatures rise, it will escape into the atmosphere in such quantities that it would be the same as adding another United States to the planet's carbon footprint by mid-century.

The stability of carbon stocks in the planet's soil in the face of rising temperatures has been the subject of some debate over the last few decades. But in the view of a team of Yale University researchers, earlier work in the area was painting an incomplete picture, with most of the attention focused on the world's temperate regions, which happen to hold much smaller stocks of carbon.

"Carbon stores are greatest in places like the Arctic and the sub-Arctic, where the soil is cold and often frozen," says study lead author Thomas Crowther. "In those conditions microbes are less active and so carbon has been allowed to build up over many centuries. But as you start to warm, the activities of those microbes increase, and that's when the losses start to happen. The scary thing is, these cold regions are the places that are expected to warm the most under climate change."

So as the temperatures rise and the soil becomes warmer, the carbon that has built up over thousands of years will be unlocked by increasingly busy microbes within.

The team arrived at these conclusions after analyzing raw data on carbon soil stocks collected through dozens of studies over the past 20 years. It predicts that for each degree Celsius of warming, around 30 petagrams of carbon will be ejected from the soil into the atmosphere (a petagram is one trillion kilograms). This is twice the amount emitted annually through human activities.

It is going to take our very best efforts to keep warming to 2° C, the threshold targeted through the Paris Agreement that we are currently well on track to surpass. But assuming best case scenario, the scientists predict that 2° C of warming will bring around 55 trillion kg of carbon out of the soil by mid-century, which makes for a 17 percent increase on projected emissions from human activity for the same period.

The scientists say this work helps to explain why there has been diverging opinions on the impact of carbon soil stocks over the preceding years, with studies indicating that some sites will undergo no change or even increase their capacity for carbon storage.

"The effects are strongly dependent on where you look," says Mark Bradford, professor of terrestrial ecosystem ecology at Yale. "Now that we know this, we can begin to develop more confidence in the idea that this biological feedback is real, and hence likely to accelerate human-induced climate change."

The team also points out that while it calculated the carbon losses likely to result from rising temperatures, there are other factors that could come into play. A recent NASA study showed that rising concentrations of carbon dioxide in the atmosphere are boosting Earth's vegetation coverage, and this is one example of something that could affect how quickly the carbon is released from the soil.

"Getting a handle on these kinds of feedbacks is essential if we're going to make meaningful projections about future climate conditions," said Crowther. "Only then can we generate realistic greenhouse gas emission targets that are effective at limiting climate change."

The research was published in the journal Nature.