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Global warming and mass extinction at the Triassic-Jurassic boundary

There have been several major extinctions in the past 500 million years, some involving the demise of all but a small fraction of existing organisms. The greatest known extinction was at the end of the Permian (245 m.y.). There was another at the end of the Cretaceous (65 m.y.) which led to the extinction of most of the dinosaurs. Both of these are widely (although not universally) believed to have been related to the impact of very large meteorites. There was also a major extinction at the end of the Triassic period - some 202 m.y. ago.

Biologists at the University of Sheffield (UK) have studied fossil plant remains from Greenland and Sweden which represent the last few m.y. of the Triassic and the first few m.y. of the Jurassic. They present data showing that the Triassic-Jurassic change in climate resulted in two specific changes in the nature of tree leaves:

a decrease in the density of stomata (pores), and
a decrease in the average size of leaves,

The authors have concluded from these results that the Triassic-Jurassic extinction was related to a significant sharp increase in the carbon dioxide content of the atmosphere - from around 600 ppm in the late Triassic to as much as 2400 ppm in the early Jurassic. (The current level is around 380 ppm.) An increase in CO₂ of this magnitude would have had an overall warming effect of 3 to 4° C.

Both of the adapatations described above would have enabled the newly evolved plants to survive under conditions of higher CO₂ levels and higher temperatures.

It is suggested that the change in atmospheric CO₂ was a result of a substantial increase in rift-associated volcanism associated with the break-up of Pangea. The amount of CO₂ released from new sites of sea-floor volcanism and spreading would have been sufficient to produce the observed increase in CO₂.

It cannot be determined from the available data how quickly the CO₂ levels rose, but it is likely to have been a period of millions of years. Over the past 100 years we have witnessed (and presumably caused) a CO₂ increase of close to 100 ppm (from 280 to 380 ppm). If we carry on at this rate we will see a total increase of the same order of magnitude as the Triassic-Jurassic one within another 200 years. We are also witnessing extinctions - for all sorts of reasons, most of which relate to human activities - on a scale which is not observed anywhere in the fossil record.

Reference: McElwain, J., Beerling, D. and Woodward, F., Fossil plants and global warming at the Triassic-Jurassic boundary, Science, Vol. 285, 27 August 1999, p. 1386-1390.