Reading tea leaves won't accurately predict the future, but reading gingko tree leaves can reveal the past. A new study of pores on these fossilized leaves shows that global temperatures have risen and fallen in concert with the concentration of carbon dioxide in the atmosphere for at least the last 300 million years. The discovery confirms a link that had recently been questioned due to reports that greenhouse gases decreased during several warm spells.
An almost overwhelming body of evidence indicates that average global temperatures have been closely linked to the concentration of atmospheric CO2 for at least the last 400,000 years. Pushing the relation farther back in time, however, is difficult. Geochemical measures generally confirm the link between CO2 and temperature over the last few hundred million years, but they also indicate that CO2 levels fell dramatically when the temperature rose during parts of the Jurassic and Cretaceous periods.
Now paleobiologist Greg Retallack of the University of Oregon, Eugene, has tightened the connection again. The densities of stomata--pores that plants use to breathe in CO2--decrease when CO2 concentration rises, mostly because plants need fewer stomata to take in enough CO2. Retallack counted stomata in archival images of fossilized leaves from four closely related genera, including gingko, to estimate CO2 levels during the last 300 million years. This estimate closely tracks global temperatures through each rise and fall, including the warm periods in the Jurassic and Cretaceous, Retallack reports in the 17 May issue of Nature.
Retallack speculates that errors in the models used to interpret geochemical data caused them to incorrectly estimate CO2 levels during the suspect warming periods. "This is exciting because it appears that the stomata are a better way to reconstruct the CO2 record [than existing models]," says paleobotanist Wolfram Kuerschner of Utrecht University in the Netherlands. But Kuerschner would like to see the test confirmed in other plant species. "You have to take care when using plants," he says. "They are not machines that show the CO2 levels directly."