DENVER -- Fifty-five million years ago, the Earth was on average 4 to 6 degrees Celsius warmer than it is today, but contemporary climatologists have had difficulty explaining that fact using models that rely solely on the effects of greenhouse gases.
Research presented earlier this week at the 1999 meeting of the Geological Society of America in Denver points to methane gas as a possible reason why the paleoclimate in the Cenozoic era was so warm.
The geologists conducting the research believe their findings may help scientists understand the mechanisms of global climate change today.
During the Cenozoic, geologic evidence shows that Earth's polar ice caps did not exist. They had melted away because temperatures at the poles may have been two times or more warmer than the global average at the time.
Lisa Sloan, an Earth scientist at the University of California at Santa Cruz (UCSC), said new climate models she and her colleagues have generated indicate that the indirect effects of high concentrations of methane gas in the atmosphere probably played a major role in warming the Earth, particularly the polar regions.
Previous climate models showed that an abundance of greenhouse gases in the atmosphere prevented incoming solar radiation from escaping back into space. But in those models, greenhouse gases could account for warming in the tropics, not at the poles.
"We wanted to know what was driving the climate change on an ice-free Earth," Sloan said.
Methane, a byproduct of abundant bacteria thriving on an already warm and wet Earth between 50 and 60 million years ago, produced stratospheric clouds of water vapor that can only form in the shadowy polar regions. This held in solar heat, warmed and melted ice at the poles and, in turn, warmed the Earth, she said.
"The warmer, wetter Earth produced greater sources of methane," said Sloan. "This is the first time we have looked at this indirect effect on climate change."
The rapid climate warming had profound effects on countless marine and terrestrial organisms, according to research by William C. Clyde of the Department of Earth Sciences at the University of New Hampshire. These included mass extinction among certain species. Evidence shows that this epoch of global warming lasted less than 150,000 years.
Could such a runaway warming process happen today? Not likely, said James Zachos, also at UCSC. An overly warm ocean triggered the methane release. When the ocean reached a certain temperature, a critical threshold was crossed that allowed a catastrophic release of methane hydrates in the sea floor.
Today's oceans, said Zachos, are too cold for that to occur. Nevertheless, another researcher, Scott Wing of the Smithsonian Institution in Washington, D.C. said, "If we can understand what's going on in such an extreme world of 50 million or so years ago, then maybe we can better understand the general [climate] rules."
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