One way climate scientists try to understand the consequences of increased atmospheric carbon is to study the relationship between greenhouse gas concentrations and temperature in previous periods of global warming. The most similar event to current and predicted warming took place almost 56 million years ago, during the Palaeocene-Eocene Thermal Maximum (PETM).

During this event, the average temperature rose almost 6°C over about twenty thousand years—the most rapid (documented) warming in the past. However, according a recent paper in Nature Geoscience, the current rate of carbon release to the atmosphere is almost ten times higher than the highest rate during the PETM. Such rapid changes may alter the environment more quickly than ecosystems can adapt.

In order to calculate the rate of carbon release, the researchers analyzed rock cores containing ocean sediments from Spitsbergen, Norway, to determine the ratio of carbon isotopes in calcium carbonate (from the shells of marine life). The two stable isotopes of carbon, 12C and 13C, occur in different ratios depending on the source of carbon dioxide (methane or organic matter); a release of carbon dioxide into the atmosphere causes a shift in this ratio. The ocean then absorbs a significant amount of this carbon dioxide, with some carbon ending up in the carbonate incorporated by the marine biosphere.

Using the carbon isotope information combined with the sample depth, the group built a set of data points to represent the way the carbon isotope content changed over time. They used this data set with an Earth system computational model to work backwards and find both the necessary atmospheric greenhouse gas content that would cause such a shift, and the global temperature increase those greenhouse gasses would cause. This is a different approach than previous researchers, who performed guess-and-check calculations.

After performing multiple simulations and sensitivity analyses, the team found two scenarios that can explain the experimental data: a peak release of methane at 0.3 Petagrams of carbon per year (Pg C/yr) or a larger release of carbon dioxide at 1.7 Pg C/yr. Unfortunately, with the data collected so far, the authors are unable to determine the more likely scenario, but they say that the carbon dioxide release better matches the starting conditions as they are currently understood. In addition, the resulting warming from the carbon dioxide scenario, approximately 6.5°C vs. 2.1°C for methane, more closely agrees with previously known warming results.

The total amount of carbon released to the atmosphere during the PETM (3,500-13,000 Pg C) is comparable to the current and past fossil fuel reserves, so scientists consider this event the best historical analog for the present global warming situation. However, these new results show that the peak rate of carbon release to the atmosphere in this period was about ten times less than the rate at which we current emit carbon, which is approximately 9 Pg C/yr. That suggests that we are altering our environment in an unprecedented (and unpredictable) way.

Nature Geoscience, 2011. DOI: 10.1038/ngeo1179 (About DOIs).

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