Permafrost—a vast, frozen subsurface layer of soil—covers nearly a quarter of the land in the northern hemisphere. It contains centuries worth of carbon in the form of plants that have died since the last ice age but remained frozen rather than decomposing.
Now scientists are learning that the "perma" part of its name may no longer be accurate.
As the Arctic heats up at a rate twice that of the rest of the globe and as sea ice and glaciers turn to water, the permafrost is also thawing. A recent review article in the journal Nature found that as the unfrozen organic matter decays, vast stores of carbon in the permafrost could be released into the atmosphere. This will trigger an irreversible feedback system and nullify existing calculations of just how much carbon humans can burn and keep the globe within a relatively safe degree of warming.
Kevin Schaefer, a permafrost scientist with the National Snow and Ice Data Center at the University of Colorado in Boulder and an author of the article, calls the thawing of the permafrost a "true climatic tipping point." Scientists are still trying to pinpoint when it will happen, but Schaefer said that a likely point is around the middle of this century, when the Arctic changes from a carbon sink to a carbon source. When that happens, it will trigger a centuries-long, unstoppable feedback system, in which warming will release carbon, which will trigger more warming, which will release more carbon.
The authors of the Nature article found that if humans continue on the current path of energy use, the permafrost could release 92 gigatons of carbon into the atmosphere by the end of this century. That represents nearly 18 percent of what the world has emitted since the start of the Industrial Revolution—or more than one third of what can be safely burned and still keep global warming within 2 degrees Celsius.
And that's only part of it. The authors reported that 59 percent of total permafrost emissions would occur after 2100.
The scientific understanding of the permafrost is new—so new, in fact, that it wasn't ready in time for the latest round of climate assessment reports from the Intergovernmental Panel on Climate Change, the world's largest scientific body on global warming.
The 2014 IPCC report estimated that to hold global warming below 2 degrees Celsius, worldwide carbon dioxide emissions would have to be cut by 40 percent to 70 percent by 2050, and then drop to nearly zero by the end of the century. This is a tall order on its own, and it does not take into account additional emissions from permafrost thawing.
"This is not a minor feedback," Schaefer said. "It's still small compared to fossil fuels, but it is not negligible either. If you don't account for it, you'll overshoot this 2 degree target."