In Earth's long history, its climate has changed many times. This was because orbit parameters altered, continents and oceans shifted, large asteroids fell and volcanoes began to erupt.
In the last decade of the 20th century, the scientific community began to discuss one more possibility for climatic changes, which are long-term in terms of human history and quick in terms of geological time scale.
They called this a “methane catastrophe”, a discharge of large quantity of methane from gas hydrates located in the interior of the Earth. As a result, the climate got warmer within 15-30,000 years, which noticeably changed the late Palaeocene world. However, many researchers assume that the reason for global warming was quick increase of carbonic acid gas concentration in the atmosphere. So, what is to blame: carbonic acid gas or methane?
In 1750 during the pre-industrial epoch, the terrestrial atmosphere contained approximately 1,85 Gt of methane. To explain the warming that occurred 55 million years ago, the methane concentration would have increased by over a hundred times as compared to pre-industrial levels, and the carbonic acid gas concentration by only six times. As there was a sufficient quantity of methane in the interior of the Earth, estimated at between 700 and 30,000 Gt, it is possible that between 1,000 and 2,000 Gt was discharged from the interior of the Earth, of which 400 Gt fell on the Arctic permafrost zone.
Due to the molecular mass difference, carbonic dioxide formation requires 2,75 times more carbon than formation of the same quantity of methane, and as methane is a stronger greenhouse gas than carbonic dioxide, its potential would be 21 times higher than a similar potential of carbonic acid gas. If different absorbent properties of greenhouse gases are taken into account, then greenhouse warming due to methane concentration would have required six times less carbon than the warming due to carbonic acid gas, and, accordingly, much smaller discharge of carbon from the planet interior would have been sufficient.
There are other arguments in favor of the “methane” hypothesis. As the methane concentration in the atmosphere increases, its stability also grows. Thus, in the pre-industrial epoch, the methane molecule lifespan in the atmosphere was 8,4 years, in contemporary conditions - about 10 years, but 55 million years ago, during massive gas discharges, it could exceed 40 years. Besides, in case of high concentration of greenhouse gases in the atmosphere, the water vapour concentration also grows, which increases the greenhouse effect. To achieve the same increase of surface temperature (taking into account the water vapour influence), twice lower methane content in the atmosphere (and less discharge from gas hydrates) would have been sufficient. According to the Russian researchers’ estimates, during the warming period, the annual methane stream into the atmosphere should have been 25 Gt, if the methane molecule lived for 10 years, and 6 Gt, if its lifespan was increased up to 40 years. (In the contemporary world, the major methane leakage from the interior of the Earth occurs owing to natural gas production and makes about 600 Mt per year, and the atmosphere contains 4,6 Gt of methane).
The above estimates demonstrate that quick (in terms of geology) methane-based warming is possible in principle, although it requires considerable methane discharges. Probably, volley methane discharges in the geological past could be the trigger to launch the “quick” warming process. According to the recent palaeontological data, the methane concentration growth normally passes ahead of the temperature growth. Such mechanism explains why some warmings in the history of the Earth had happened earlier than the carbonic acid gas concentration increased.
Nevertheless, the Russian researchers point out that detailed review of the greenhouse phenomenon at the Palaeocene/Eocene border has not been finished yet. It requires further investigation with involvement of geological, geochemical and paleoenvironmental records and contemporary climatic theory models.