Nearly a quarter of the Northern Hemisphere’s land surface is covered in permanently frozen soil, or permafrost, which is filled with carbon-rich plant debris — enough to double the amount of heat-trapping carbon in the atmosphere if the permafrost all melted and the organic matter decomposed.

According to a paper published Thursday in Science, that melting could come sooner, and be more widespread, than experts previously believed. If global average temperature were to rise another 2.5°F (1.5°C), say earth scientist Anton Vaks of Oxford University, and an international team of collaborators, permafrost across much of northern Canada and Siberia could start to weaken and decay. And since climate scientists project at least that much warming by the middle of the 21st century, global warming could begin to accelerate as a result, in what’s known as a feedback mechanism.

How much this will affect global temperatures, which are currently projected to rise as much as 9°F by 2100, is impossible to say. It all depends on how quickly the permafrost melts, and how quickly bacteria convert the plant material into carbon dioxide and methane gas, and nobody knows the full answer to that. But since climate scientists already expect a wide range of negative consequences from rising temperatures, including higher sea level, more weather extremes and increasing risks to human health, anything that accelerates warming is a concern.

While the rate at which melting permafrost will add carbon to the atmosphere is largely unknown, a study released February 11 in Proceedings of the National Academy of Sciences at least begins to tackle the problem. It shows that when the permafrost does melt, carbon dissolved in the meltwater decomposes faster after it’s been exposed to the ultraviolet component of sunlight.

In any case, there’s no doubt that the permafrost will melt, at least in part, since it’s already starting to do so. In some parts of the Arctic, trees, buildings and roadways have started listing to one side, or even collapsing, as soil that was once hard as a rock has softened from the warming that’s already taken place.

To get an idea of what might be in store for the future, Vaks and his colleagues searched for evidence from the distant past — specifically, from stalagmites and stalactites formed over hundreds of thousands of years in underground Siberian caves. These spiky mineral deposits, known collectively as speleothems, grow layer by layer as surface water percolates through the ground dissolving limestone as it goes, and finally forms droplets that hang from the ceiling of a cave. If the water evaporates before dropping to the floor, it leaves the limestone behind, and over the centuries those bits of limestone grow into a downward-pointing stalactite. If it drops first, then evaporates, the limestone builds up from the floor, creating a stalagmite.

In places without permafrost, this process happens year-in and year-out. Where there’s permafrost, however, water can only drip when the permafrost melts. So Vaks and his colleagues enlisted members of the Arabica Speleological Club in Irkutsk, Russia — amateur cave explorers — to help identify likely caves in a north-south line across Siberia.

Once they’d found the caves, they carefully removed sample speleothems, “preferably from hidden areas, so we wouldn’t spoil the caves’ natural beauty,” Vaks said. The scientists took their samples back to the lab, sliced them lengthwise, and exposed layers laid down over nearly 500,000 years. “By using uranium/thorium dating,” Vaks said, “we could find the layers’ exact ages with high precision.”

They also found that there were long periods when the speleothems didn’t grow at all — certainly not during ice ages, when permafrost locked the soil across most of Siberia, but not even, in the northernmost caves, during warmer interglacial periods, like the one we’re in now when glaciers went into retreat. The last time these northern speleothems showed any growth, in fact, was during an unusually warm period about 400,000 years ago.

At the time, global average temperatures were some 2.5°F warmer than they are today. That sort of temperature increase by itself wouldn’t make an enormous dent in the permafrost, but the Arctic is likely to warm faster than the rest of the globe — as in fact, it has already started to do.

As for the earlier study on carbon and ultraviolet light, environmental scientist Rose Cory, of the University of North Carolina, focused on sites in Alaska where melting permafrost has caused the soil to collapse into sinkholes or landslides. The soil exposed in this way is “baked” by sunlight, and said Cory in a press release, “(it) makes carbon better food for bacteria.”

In fact, she said, exposed organic matter releases about 40 percent more carbon, in the form of CO2 or methane, than soil that stays buried. “What that means,” Cory said, ” is that if all that stored carbon is released, exposed to sunlight and consumed by bacteria, it could double the amount of this potent greenhouse gas going into the environment.”