During the last deglaciation, between roughly 21,000 and 10,000 years ago, there was a rise in atmospheric carbon. This surge brought CO 2 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 CO 2 . 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 CO 2 levels seen in ice core measurements—suggesting that carbon released by melting permafrost contributed to the rise of CO 2 .

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.

The researchers set out to build a model of the Earth that accounts for carbon from permafrost, in addition to other sources. They took an existing model, CLIMBER-2, and added their own simplified model of permafrost carbon to it. Without the addition, CLIMBER-2 simulates the exchange of carbon between the atmosphere, the ocean, and life.

With that model in hand, the researchers ran two tests: the first only taking into account the ocean, land, and atmosphere (OLA); the second, which takes into account permafrost, as well as the other three (POLA). OLA reproduced the results of previous studies but couldn’t account for the δ13C data. POLA came closer, due to the inclusion of the permafrost, but something still wasn’t quite right.

So the researchers used a trial-and-error method, putting POLA through many iterations, making it a better and better match for the data. The resulting model was called POLA FWF . The main difference between FWF and the regular POLA experiment was that FWF includes an influx of fresh water into the ocean from the melting ice. Carbon is released into the atmosphere through this process, so that would have an important impact on the atmospheric carbon levels.

POLA FWF turns out to be an excellent match for the data from the last deglaciation, even predicting the timing of a downturn in the data roughly 17,000 years ago. It also generally agrees with data from the sea level record. Not only that, but the model is a better match for temperature anomalies during that period than previous models.

Consequently, the researchers conclude that the melting permafrost is probably a major factor in carbon fluxes during glacial periods.

Around 10,000 years ago, the simulation begins to diverge from the data. The researchers attribute this to the influence of yet another terrestrial carbon source: peatlands. Not included in the model, peatlands are estimated to have incorporated more than five billion tons of carbon, as they are extremely efficient carbon sinks.

The simulations also estimated the effects of this permafrost feedback on the present-day environment and into the future and determined that the effect should increase the amount of future emissions by about 10 percent to 40 percent. This increase is dependent on how much humans contribute to warming—the more we heat things up, the more permafrost melts, and thus the more CO 2 is released.

Without humans’ effects on the CO 2 levels, the simulation predicts the soil will absorb more carbon, the permafrost would expand, and the global climate would cool slightly.

In any case, the authors feel confident that their simulations emphasize the importance of the permafrost carbon sink. “We propose that permafrost plays a major role in the carbon cycle, especially during fast warming events affecting high latitudes,” the researchers conclude in their paper.

Nature Geoscience, 2015. DOI: doi:10.1038/NGEO2793 (About DOIs)