The heat was on

时间:2019-03-07 10:17:00166网络整理admin

By Jeff Hecht GLOBAL warming can be a killer. Fossil leaves show that the mass extinction at the end of the Triassic period, which wiped out 95 per cent of plant species, was caused by soaring temperatures. Geologists know that this extinction was marked by frequent volcanic eruptions, as the supercontinent Pangaea split apart. Now, by analysing plants from fossil beds in Greenland and Sweden that span the end of the Triassic and the beginning of the Jurassic 206 million years ago, Jenny McElwain and her colleagues at the University of Sheffield have confirmed suspicions that this volcanism caused a jump in carbon dioxide levels in the atmosphere. McElwain counted stomata, the openings in the leaves that let in CO2, in dozens of fossilised species. She found that the leaves of most species living after the extinction had a much lower density of stomata compared with the species that lived before it. Stomatal densities change in response to changing levels of CO2, so the reduced density is likely to have been caused by a rise in CO2 in the atmosphere. Based on the stomata counts, McElwain estimates that CO2 concentrations rose from 600 parts per million to between 2100 and 2400 ppm. That would make temperatures rise by up to 4 °C (Science, vol 285, p 1386). Another sign that the climate became warmer is that large leaves vanished after the Triassic period, to be replaced by small or multi-lobed leaves. McElwain says that small leaves could have dissipated the extra heat, while large leaves would have become too hot for photosynthesis. Although plants suffered most in the Triassic extinction, half of all land animal species perished as well. Overheating could have killed animals directly, or destroyed the plants they ate. Experts are enthusiastic about the results. “This is the first direct evidence that carbon dioxide itself, or carbon dioxide warming, is implicated in extinctions,” says Paul Olsen,