An episode at the beginning of the Eocene epoch about 55 million years ago caused an abrupt (on geological time scales) global warming of 5-10ºC. At the same time, an enormous amount of carbon dioxide, evidently due to oxidation of hydrocarbons, entered the atmosphere/ocean component of the carbon cycle (Dickens, 2004). This carbon dioxide, estimated to be 1,500 to 3,000 Gt carbon equivalent, was of organic origin and entered the carbon cycle over a period of less than 20,000 years. Svensen et al. (2004) have identified a region of the North Atlantic sea floor off the Norwegian coast that includes a complex of vents capable of releasing such enormous amounts of greenhouse gases. Estimates of anthropogenic carbon releases to the atmosphere in the coming centuries range from 3,000 to 4,000 Gt. A better understanding of the cause and consequences of this abrupt warming event likely will shed light on what can be expected in the current global warming.
References
Dickens, G.R., 2004: Hydrocarbon-driven warming. Nature, 429,
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Svensen, H., S. Planke, A. Malthe-Sorenssen, B. Jamtveit, R. Myklebust, T.R.
Eidem,, and S.S. Rey, 2004: Release of methane from a volcanic basin as a
mechanism for initial Eocene global warming. Nature, 429, 542-545.