28 October 2009

Paleoclimate and Paleobotany and implication for global warming

Something a little different from my usual political commentary.

I've been reading Emerald Planet, How Plants Changed Earth's History (David Beerling, Oxford), which is a popular treatment of interesting topics in paleobotany and paleoecology.

One of the most interesting things the author discusses is the role of plants in regulating atmospheric composition. Some things I didn't know and I'm quite sure are not common knowledge: 

Apart from a deep and pervasive global glaciation in the 800-600 million years ago range (i.e., before the emergence of complex multicellular life) (during which the planet may possibly have been entirely covered with ice for a time), and a severe glacial epoch probably resulting from cataclysmic volcanic events in roughly Permian time (i.e., before the great age of dinosaurs), the Earth has been mostly ice free. In fact, most of the time the temperature gradients between the polar regions and the equatorial regions have been much less than at present. During the Carboniferous and again during the Eocene (periods of major global warming caused by very high levels of carbon dioxide and other greenhouse gases), the poles were subtropical, with palm trees and vast forests stretching from pole to pole.

Indeed, it is only with the end of the Eocene, roughly 40 million years ago, (possibly triggered by the massive sequestration of carbon by aquatic ferns being buried without releasing their carbon for long periods of time in a stagnant Arctic sea), that a long term global cooling trend, culminating in the Pleistocene glaciations, began. (the changed configuration of the continents and resulting changes in oceanic currents undoubtedly play a major role in this as well). (Check out the Azolla Event in Wikipedia, if you're interested. It's an amazing story).

Generally, through most of the history of multicellular life on Earth, the planet has been significantly warmer than in the last few million years, and in fact, ice caps at both poles have been decidedly rare. During most of their history, Arctic regions and Antarctica have been the sites of forests comparable to the Pacific Northwest today, or even subtropical forests, in warmer eras. And this is not accounted for by their having been in different locations; both the circumpolar lands of the North and Antarctica have been in approximately their current locations since shortly after the breakup of Gondwana and the comparable breakup of Laurasia, roughly 200+ million years ago.

Beerling points out that attempts to model exactly what happened during the Eocene warm period, in particular, in terms of currents, atmospheric circulation, and, especially, the effects of greenhouse gases, particularly methane and CO2, have failed to adequately account for the paleoclimate. (It was warmer than modeling of all these factors can account for, especially at very high latitudes). He concludes, almost as an aside from this, that our understanding of likely effects of significant changes now underway in greenhouse gas composition (including the potential for release of methane in large quantities from ocean floor clathrates), is not adequate to make predictions, but that the effects are likely to be more extreme, not less, than modeling currently available suggests.

This should induce even greater caution, but something tells me that the politicians of the world will remain blissfully unaware of such considerations.

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