During the last deglaciation, between about 21,000 and 10,000 years ago, there was an increase in carbon in the atmosphere. This wave brought CO2 levels to where they were in pre-industrial times and contributed to the warming that ended the Ice Age. But there’s an important item missing from this photo: We don’t know where the carbon came from.
Researchers had suggested that changes in ice distribution, driven by changes in Earth’s orbit and tilt, altered the ocean’s ability to absorb CO2. But a new paper performed a model-driven analysis of past carbon content changes and came up with a slightly 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 has contributed to the rise in CO2.
Carbon accounting
Data from the ice cores can help narrow down the possibilities because it registers 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 a bit more complicated mathematically, but that’s the basic idea). Because this ratio is influenced by biological activity, it can provide some clues about the carbon source. However, even with these clues, previous simulations haven’t narrowed down the possibilities. The researchers suspected this was because they failed to account for an important mechanism: change in permafrost.
The researchers wanted to build a model of Earth that takes into account carbon from permafrost, among other sources. They took an existing model, CLIMBER-2, and added their own simplified model of permafrost carbon. 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 took into account only the ocean, land, and atmosphere (OLA); the second, which takes permafrost into account, as well as the other three (POLA). OLA reproduced the results of previous studies, but could not explain the δ13C data. POLA came closer with the addition of the permafrost, but something was still not right.
So the researchers used a trial-and-error method, with POLA going through many iterations, getting it to match the data better and better. The resulting model was called POLAFWF. The main difference between FWF and the regular POLA experiment was that FWF involves an inflow of fresh water into the ocean from the melting ice. This process releases carbon into the atmosphere, so that would have an important impact on the carbon content in the atmosphere.
POLAFWF turns out to be an excellent match with the data from the last deglaciation, even predicting the timing of a downturn in the data about 17,000 years ago. It is also generally consistent with sea level record data. Not only that, but the model is more resistant to temperature anomalies in that period than previous models.
Consequently, the researchers conclude that the melting permafrost is likely an important factor in carbon fluxes during ice ages.
About 10,000 years ago, the simulation begins to deviate from the data. The researchers attribute this to the influence of yet another terrestrial carbon source: peat bogs. Peatlands not included in the model are estimated to contain more than five billion tons of carbon, as they are highly efficient carbon sinks.
The simulations also estimated the effects of this permafrost feedback on the current 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 depends on how much humans contribute to warming – the more we warm things up, the more permafrost melts and therefore more CO2 has been released.
Without the effects of humans on the CO2 levels, the simulation predicts that the soil will absorb more carbon, the permafrost would expand and the global climate would cool slightly.
In any case, the authors are confident that their simulations highlight the importance of the permafrost carbon sink. “We propose that permafrost plays an important role in the carbon cycle, especially during rapid warming events affecting high latitudes,” the researchers conclude in their paper.
Natural Geosciences2015. DOI: doi:10.1038/NGEO2793 (About DOIs)