Fossil leaves reveal Earth's history

CARBON DIOXIDE has been a greenhouse gas for at least the past 300 million years, according to a new study of pores in the leaves of fossilized plants. Although carbon dioxide is widely recognized as being involved in warming the globe today, its role as an agent for climate change past and potentially therefore, in the future - has recently been contested.

Carbon and oxygen isotopes in marine organisms and ancient soils indicate that high levels of atmospheric carbon dioxide have not always accompanied global temperature peaks. This hints that other factors could have regulated global climate during Earth's long history.

But after reading ancient leaves, Gregory Retallack of the University of Oregon in Eugene concludes in Nature that carbon dioxide and climate have been `coupled' since the time of the dinosaurs. He has produced the first non-geological record of ancient acrbon dioxide going back as far as many geological measures.

The finding, says Retallack, puts carbon dioxide back in pole position as the major agent of global climate change. "Think again, folks, carbon dioxide is a player," he says. Plants absorb carbon dioxide through pores, called stomata, on the undersides of their leaves. Just as we pant in oxygen-poor high altitudes, plants develop more stomata to breathe when there is less carbon dioxide in the atmosphere.

Retallack studied the density of stomata on fossilized leaves from trees of the Ginkgo family and their more ancient fern-like relatives from Russia, China and the United States. He calibrated his fossil CO2 detectors by comparing the density of their stomata with that in living Ginkgo trees grown in greenhouses filled with carbon dioxide at varying concentrations.

Stomata density peaks in Retallack's leaves coincide with the last major ice ages - when carbon dioxide was at an all-time low. This agrees with previous geological data. However, where temperatures appeared to be high but carbon dioxide low in the geological climate record, stomata density suggests that carbon dioxide was in facthigh.

The stomata density peaks also record dips in global carbon dioxide corresponding to about five known extinction events, including the `end-Cretaceous' event when an asteroid slammed into the Earth. "The ecosystem was whacked, and whacked badly," says Retallack. Thure Cerling, who studies climate using isotope ratios in ancient soils at the University of Utah in Salt Lake City, agrees.

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