During the last deglaciation, between roughly 21,000 and 10,000 years ago, there was a rise in atmospheric carbon. This surge brought CO₂ levels up to where they were in preindustrial times and contributed to the warming that ended the glacial period. But there’s a significant item missing from this picture: we don’t know where the carbon came from.

Researchers had suggested that changes in the distribution of ice, driven by alterations in Earth’s orbit and tilt, altered the ocean’s capacity to absorb CO₂. But a new paper performed a model-driven analysis of past changes in carbon levels and come up with a somewhat different answer. The authors’ simulations showed that, when a permafrost carbon component was included, it was possible to reproduce the atmospheric CO₂ levels seen in ice core measurements—suggesting that carbon released by melting permafrost contributed to the rise of CO₂.

Carbon accounting

Data from the ice cores can help narrow down the possibilities, because it records something called δ¹³C (delta-thirteen-C), which is essentially a measure of the ratio of carbon-13 to carbon-12 in the atmosphere. (It’s mathematically a bit more complicated, but that’s the basic idea). As this ratio is influenced by biological activity, it can give some clues about the carbon’s source. Even with these clues, however, previous simulations have failed to narrow down the possibilities. The researchers suspected that was because these weren’t taking into account an important mechanism: change in permafrost.