Two eras in the distant past create cause for concern over our environmental future.

Other than The Day After Tomorrow, climate scientists have very few blueprints for how the world may change if global warming continues for several decades. In the search for concrete examples of how events might progress, some researchers are looking back at ancient time periods that mirror modern day with respect to radical temperature shifting.

Two eras are particularly relevant: the Mid-Pliocene, which dates back to three million years ago, when the temperatures were about 2 to3&deg C warmer than current temperatures; and the Paleocene-Eocene Thermal Maximum (PETM) episode, of 55 million years ago, which featured a gradual temperature spike of about 5° C.

Projections for the next century typically predict a rise of about 2 to3° C. Mark Chandler, a climate scientist at Columbia’s Earth Institute, said that the analogous temperature shift in the Mid-Pliocene era coincided with a drastic reduction of polar ice caps and a 15 to 25-meter rise in the world’s sea level. Today, such a rise would inundate coastal areas, including all of Florida and many low-lying countries.

“I don’t want to turn this into a panic issue because it probably takes thousands of years to produce a response like the Mid-Pliocene,” Chandler said, “But even a much smaller increase of a meter or two would be devastating.”

While the Mid-Pliocene saw similar temperature shifts, greenhouse gases present in the atmosphere were at relatively low levels, only 5 to 10% higher than current levels.

Some scientists see this as an indication that greenhouses gases, the levels of which have risen 35% in the last two centuries, didn’t cause the Mid-Pliocene warming, and therefore we should be less concerned by their presence in our own atmosphere.

“The complete opposite way to look at it is to say, ‘Boy, we’ve really underestimated the impact of greenhouse gases,’” said Chandler. “We don’t have to double the amount of greenhouse gases in the air, maybe we just have to increase them by 50%.”

During the PETM, four to five trillion tons of carbon was released into the atmosphere naturally, causing radical warming, said James Zachos, a professor of Earth sciences at University of California, Santa Cruz. Currently, we are in the process of emitting similar amounts of carbon into the air, he said, but, in our case, we’re burning fossil fuels and releasing the gas a lot faster—over the course of 300 to 400 years, instead of 10,000 years as in the ancient, analogous era.

During the PETM, Zachos said, the ocean absorbed much of the carbon, dissolving it in the surface layer of water. But given our current rate of emission, the surface layer will saturate quickly, its pH levels will drop and ocean absorption will not be possible.

“The rate of release [during the PETM] was gradual enough to prevent a huge swing in ocean pH, specifically in the surface ocean,” Zachos said. “The rate of release at present is too fast for this buffering mechanism to prevent a large drop in ocean pH and to allow the ocean to absorb the carbon dioxide.”

Both Zachos and Chandler presented their research at the annual meeting of the American Association for the Advancement of Science, which took place last week in St. Louis.

Originally published February 22, 2006


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