Groups and Networks

An alternative approach is to look for examples in more ancient earth history, of similar phenomena to the present: that is, of sudden, massive outbursts of greenhouse gases into a world that is already warm. At least two have been identified, in the Toarcian Age of the Jurassic Period, some 180 million years ago, and in the early Eocene Epoch, around 55 million years ago.

In both of these, the influx of greenhouse gases has been demonstrated by changes in the ratios of carbon isotopes within fossils. The isotopes themselves do not say whether mainly CO₂ or methane was involved, but plausible scenarios suggest the involvement of both (say, by deriving CO₂ from extraordinary, geologically rare volcanic outbursts, providing initial warming which in turn destabilized methane which had been stored in permafrost or in ocean floor sediments). Whatever the precise mix of gases, the amount of warming is now well established, again from isotope ratios preserved in fossils. Rapid warmings of the order of between 5 and 10 degrees centigrade took place globally, the temperatures declining back to background values over many thousands of years, probably as the excess greenhouse gases were slowly drawn out of the atmosphere by reactions associated with rock weathering.

These geological examples strongly reinforce the modelled scenarios of global warming for later this century. Crucially, such temperature surges show the earth behaving in a non-linear fashion when reacting to environmental stress: that is, it tends to ‘flip’ from one quasi-stable state to another, and this kind of behaviour is inherently difficult to model or to predict. There will very likely be, as the oceanographer Wallace Broecker has said, unpleasant surprises in the greenhouse.

That the earth has been shown to recover eventually is philosophically comforting, but will be of no practical help to many hundreds of human generations.

Sources

• Bowen, G.J. et al. 2004. A humid climate state during the Palecene-Eocene thermal maximum. Nature, vol. 432, pp. 495-499.
• Cohen, A.S. et al. 2004. Osmium isotope evidence for the regulation of atmospheric CO2 by continetal weathering. Geology, vol. 32, pp. 157-160.
• Svensen, H. et al. 2004. Release of methane from a volcanic basin as a mechanism for initial Eocene global warming. Nature, vol. 429, pp. 542-545 (also discussed in the same issue, p. 513-5).
• Hesselbo, S.P. et al. 2002. Massive dissociation of gas hydrate during a Jurassic oceanic anoxic event. Nature, vol. 406, pp. 392-395.
• Hesselbo, S.P. et al. 2002. Terrestrial and marine extinction at the Triassic-Jurassic boundary linked with major carbon-cycle perturbation: a link to initiation of massive volcanism? Geology, vol. 30, pp. 251-254.
• Schrag, D.P. & Alley, R.B. 2004. Ancient lessons for our future climate. Science, vol. 306, pp. 821-822.
• Thomas, D.J. et al. 2002. Warming the fuel for the fire: Evidence for the thermal dissociation of methane hydrate during the Plaeocene-Eocene thermal maximum. Geology, vol. 30, pp. 1067-1070.
• Zachos, J.C. et al. 2003. A transient rise in tropical sea surface temperature during the Paleocene-Eocene thermal maximum. Science, vol. 302, pp. 1551-1554.