May 25, 2005— There may have been no Hummers rumbling around in the Jurassic period, but a rapid greenhouse effect some 183 million years ago has now been tracked to fossil fuel emissions nonetheless.

A fresh look at the changes in numbers of tiny pores in the leaves of fossil ginkgos and other plants has revealed a spike in carbon-rich greenhouse gases in the Earth's atmosphere in the latter half of the Jurassic's Toarcian Age.

The leaves, as well as the kinds of carbon in the fossils, point to just one source for the greenhouse effect: the baking of extensive coal beds by molten rock.

"It's like a double whammy," said Jennifer McElwain, the associate curator of paleobotany at the Chicago Field Museum.

A massive upwelling of magma from the depths of the Earth during the Toarcian, in what is today South Africa and Antarctica, released greenhouse gases: the first whammy.

But a lot of the molten rock from that upwelling didn't erupt. Instead, it squeezed into rock layers alongside huge deposits of coal, baking them and causing tremendous amounts of greenhouse gases to escape into the atmosphere, McElwain explained. Her discovery is featured in the May 26 issue of the journal Nature.

Fossil plants through the Toarcian recorded the details of this unusual event with changes in the numbers of pores, called stomata, on their leaf surfaces: fewer stomata when the carbon dioxide levels were high and more stomata when they dropped.

McElwain confirmed that and created a carbon dioxide-stomata index for fossils in the most practical way. She has grown modern ginkgos and descendants of other fossilized plants in chambers with higher and lower amounts of carbon dioxide and counted stomata.

But the stomata alone don't finger the source for the carbon gases. That is revealed by the carbon itself in the fossils. McElwain and her colleagues found a clear rise in carbon-12, the lighter carbon isotope, in the plant fossils during the greenhouse gas spike.

And because plants have always preferred the lighter carbon-12 over the heavier carbon-13, there had to be a huge plant-based source for all the carbon-12 suddenly showing up in the air, said McElwain.

The only source for so much "organic carbon" would be coal beds, she said. Luckily, the burned-out coal beds are easy to locate and can be dated to exactly the same time as the greenhouse event and the rise in carbon-12 in the fossil plants, McElwain said.

From a quick back-of-the-envelope calculation, McElwain estimated that today's rise in CO2 is happening about 90 times more quickly than in the Toarcian.

It's a remarkable tidy package of different data all pointing to the same geological event, said Brad Sageman of Northwestern University.

"I'm a total convert," Sageman said.

In fact, he hopes to apply the stomata method to the Cretaceous. "I think (McElwain) is on the leading edge."