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So scientists have been paying increasing attention to the geologic past, searching for possible lessons from ancient episodes of warming driven by natural processes which also might include emission of greenhouse gases. The research, discussed in a symposium at the 2006 Annual Meeting of the American Association for the Advancement of Science (AAAS) in St. Louis, suggests there could be some dramatic, potentially abrupt changes in store.
"There's an awful lot of knowledge we can gain from past climates," says Mark Chandler, an atmospheric scientist at Columbia University. He has been studying a warming episode 3 million years ago during a period called the Middle Pliocene.
"If you go back in time, you have to go back to the middle Pliocene before you get to see a climate that was as warm as what we are going to see in the next 50 to 100 years in our own time," Chandler said.
Ocean temperatures rose substantially during that warming episode--as much as 7 to 9 degrees Celsius (about 12 to 16 degrees Fahrenheit) in some areas of the North Atlantic. But scientists are puzzled. The carbon dioxide levels at that time--inferred from geochemical data--were roughly comparable to our own time, approaching 400 parts per million. Today's computer models do not predict the sort of temperature rises that occurred during the middle Pliocene, Chandler said.
That raises the possibility that human-induced global warming could trigger even more rapid climate changes--- rising sea levels, melting ice caps, disturbed agricultural regions--- than currently projected.
Scientists have been studying various other periods of rapid warming, one interval about 55 million years ago, at the very beginning of the Eocene geological period. At that time, temperatures rose rapidly by as much as 10 degrees C (18 degrees F) at high latitudes in the Arctic and Antarctic. Tropical oceans and deep ocean waters warmed by 4 to 6 degrees C (about 7 to 11 degrees F).
There were dramatic effects on plants and animals during the event, called the Paleocene-Eocene Thermal Maximum (PETM). Scott Wing of the Smithsonian Museum of Natural History and his colleagues reported recently in the journal Science that some plants, including relatives of poinsettia and sumac, migrated from the Gulf Coast to Wyoming, a distance of about 1,000 miles, in 10,000 years or less. Those conclusions were drawn from an examination of plant fossils from the Bighorn Basin of northwestern Wyoming.
Scientists think that the fast-changing climate at the beginning of the Eocene was driven by a natural release of carbon-containing greenhouse gases analogous to what is occurring with the release of carbon dioxide and other gases since the start of the industrial revolution. "What we are looking at is an occurrence that is similar to what we see in fossil fuel burning," said Ellen Thomas, a Yale University paleoclimate scientist.
The natural trigger for the PETM warming episode remains a subject of intense debate, but Thomas said a leading hypothesis involves the release of huge amounts of methane gas that had been trapped in ice compounds called methane hydrates. Methane produces less carbon dioxide than coal when it is burned as a fuel, and it is a powerful greenhouse gas. Most methane hydrates in the present oceans are frozen in sediments in the deep oceans, but some are associated with permafrost soils in the Arctic. Release of the methane can occur through natural processes, including underwater landslides or other seismic events, or by warming of ocean waters. Once released, the gas can induce atmospheric warming that has a positive feedback effect, releasing still more gas as ocean waters and permafrost regions begin to warm. The process can reach a tipping point where it starts to rapidly accelerate.
"You don't want to exaggerate, you don't want to sound like a doomsayer," says Thomas. "But if the hydrates hypothesis is valid, there could be nasty feedbacks in the climate system." Climate change in the next few centuries may not be a smooth, linear process. Warming induced by human-caused emissions of carbon dioxide and other gases could, in turn, trigger release of methane from natural sources in the Arctic permafrost and elsewhere.
"You may get a runaway greenhouse effect with dramatic results," Thomas said. "Once the threshold is crossed, the climate may change very rapidly but recovery will be much slower, as we see in records from the Paleocene-Eocene Thermal Maxiumum, where recovery took about 100,000 years."
Chandler said the Pliocene event 3 million years ago also gives cause for concern. "You have to take some warning from the Pliocene," he said. Even in the absence of huge amounts of carbon dioxide as a forcing mechanism, he said, there still appear to be trigger points that, once passed, can produce rapid warming through feedbacks such as changes in sea ice and the reflectivity of the Earth's surface.
Scientists have been looking at techniques for easing the impact of greenhouse gases, including schemes to sequester industrial emissions of carbon dioxide in deep ocean waters. But Chandler said his study of paleoclimates suggests that mitigation of global warming--once it is well underway--may be difficult. "I don't think you can reverse things as easily as some suggest," Chandler said.
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