Ocean Burps and Climate Change?
About 55 million years ago an event known as the Paleocene-Eocene Thermal Maximum (PETM) occurred. This was an episode of rapid and intense warming (up to 7°C at high latitudes) which lasted less than 100,000 years (see Figure 1). Curiously, the PETM was accompanied by an exceptionally large change to the global carbon cycle as indicated by a large drop in the isotopic ratio of 13C to 12C in the ocean and on land.
The ratio of these carbon isotopes generally changes as a function of biological activity, since carbon in living matter tends to be preferentially made up of 12C as opposed to 13C. Thus an increase in biological activity "uses up" more 12C, and therefore the ratio of 13C to 12C in the remaining carbon increases. Conversely, a decrease of biological uptake, leads to a decrease of the isotopic ratio (i.e., it gets "lighter").
The change at the PETM was so large that it would have required a decrease in biological activity equivalent to roughly three times the total present-day terrestrial biosphere. In other words, if all of the terrestrial carbon today (in forests, animals, soils, etc.) were converted to carbon dioxide and returned to the global inorganic carbon pool, the change in the global carbon isotopic ratio would only be a third as big as that observed during the PETM! However, no such event is seen at the PETM, and thus another source for very "light" carbon must be found.
Volcanos are another source of light carbon as carbon dioxide gas within eruptions. But this source would also imply an enormous, and highly unlikely, amount of volcanism to match the observations. In fact, only one source of carbon that is isotopically light and available in large enough quantities has been pinpointed so far, this is the reservoir of methane hydrate deposits (Figure 2) buried on the continental shelves of the oceans (Figure 3).
Bacteria produce methane as they decompose organic matter in the ocean sediments, and in cold, high-pressure environments, methane hydrates will form. This is an ice-like solid that consists of methane surrounded by water molecules in a lattice structure. However, if the temperature warms, or the pressure is reduced (for instance if local sea level decreases), the hydrate will break up and release the methane as gas which can bubble up through the ocean and enter the atmosphere.
What would be the consequences of such a large emission of methane into the atmosphere? At present, methane has a residence time of about 10 years before it is oxidized to carbon dioxide. However, the chemistry of this process is highly non-linear, and as emissions increase, the capacity of the atmosphere to deal with the excess methane decreases and the residence time lengthens. This can lead to quite large increases in the methane concentration. This matters because molecule for molecule, methane is a more powerful greenhouse gas than carbon dioxide. The climate consequences depend very strongly on exactly how long the extra methane hangs around.
Our research (described in Schmidt and Shindell 2003) has examined these chemical and radiative effects. Based on plausible scenarios for what may have occurred at the PETM, we tried to estimate the history of the methane and carbon dioxide concentrations. Using radiation modeling we estimated how strong the climate forcing would be for each scenario, and then ran general circulation models to see how that forcing would change the climate.
We found that for some scenarios, the methane levels can stay high enough and remain long enough to play the dominant role in the subsequent climate warming. The temperature changes are close enough to those observed through the PETM to support both the hypothesized scenario and our modelling efforts. While there are huge uncertainties in almost every aspect of this study, this research shows that we can "connect the dots" from a methane hydrate forcing to the observed global warming.
Methane plays a large role in present day climate forcing (see "Trends of Measured Climate Forcing Agents" for more details) and has more than doubled in concentration since the pre-industrial period. This study goes a long way in quantifying its role in paleoclimate variability as well.
Schmidt, G.A., and D.T. Shindell 2003. Atmospheric composition, radiative forcing, and climate change as a consequence of a massive methane release from gas hydrates. Paleoceanography 18, no. 1, 1004, doi:10.1029/2002PA000757.
Please address all inquiries about this research to Dr. Gavin Schmidt.