The Arctic holds more than a trillion tons of carbon, locked in the frozen soil known as permafrost. That’s more than twice as much carbon as there is in the atmosphere itself, according to a 2013 report from the National Academy of Sciences. And as the climate warms under its growing blanket of human-generated greenhouse gases, thawing permafrost could release some of that carbon into the atmosphere.

That could amplify the effect of global warming by boosting CO2 levels higher than they would otherwise be in a so-called feedback loop and making the consequence of warming, including rising seas, more intense and prolonged heat waves and droughts and distorted weather patterns even more severe. They could also lead to more thawing of permafrost and the release of even more CO2, starting the cycle again.

How much carbon will be released, however, and how much extra warming is likely to result, has been a matter of dispute in recent years. Some researchers have argued that that the effects will be horrific, while others say they’ll be significant but less dramatic. And now, a new report in Nature — the most comprehensive study ever done on the permafrost feedback loop, has come down firmly in the “significant” camp. “The permafrost carbon is not going to explode into the atmosphere catastrophically within just a few years,” Ted Schuur, a permafrost expert at Northern Arizona University, said. “It’s more like it will seep out slowly in small amounts in a very large number of places.”

Those small emissions, however, will add up, By the end of the century, they would add up to between 60 and 80 parts per million (ppm) of carbon dioxide or its equivalent in methane, a more powerful, albeit more short-lived greenhouse gas. That would add nearly 10 percent to the 800 ppm of CO2 that could be in the atmosphere by the end of this century. The 800 ppm alone could warm the planet by as much as 8.6°F above global temperatures during 1986-2005, according to the Intergovernmental Panel on Climate Change (IPCC). The extra CO2 released by thawing permafrost would just make things worse.

While IPCC scientists were aware of the potential for permafrost emissions, the organization didn’t factor them into most recent major report. “The estimates from earlier studies were so uncertain and unreliable that the IPCC wisely chose to leave them out,” said Terry Chapin, a professor of ecology at the University of Alaska, Fairbanks. Chapin was not involved in the Nature paper.

The reason earlier estimates of the permafrost feedback effect have varied so dramatically, Schuur said, is that they’ve been based on studies of very limited areas. “When you’re doing fieldwork, you’re limited by logistics to one place. And if you’re in the lab, you’re limited to samples from a certain region, or or thinking about methane but not CO2. It reminds me of the story of the blind men and the elephant.”