Rising CO2 to Spur a New Eocene?
Lowenstein and colleagues calculated ancient carbon dioxide levels by examining sodium carbonate minerals in Colorado, Wyoming, China, and Turkey.
They report that the Eocene's atmospheric carbon dioxide concentration was greater than 1,125 parts per million (ppm) by volume. Today's levels are only about 380 ppm, but that number is up from an estimated 280 parts per million before the industrial revolution.
Climate models vary widely, but those used by the United Nations' Intergovernmental Panel on Climate Change suggest that levels could range from 400 to 1,000 ppm by century's end.
The amount of fossil fuels humans use, and at what rate, are critical unknowns in any model of future CO2 levels.
"Depending on how much [fossil fuel] we burn, [Eocene carbon dioxide levels] would be very close to the upper range predicted for the next hundred years," Lowenstein said.
"We knew [the Eocene] was warm, and now we have evidence that the CO2 was high," he continued. "So this does prove a link between high CO2 levels and the warmest era on record."
"I think this is very interesting. Any new information that we can get on ancient CO2 levels is very important," Schrag said.
Risky Business
Modeling climate is a notoriously tricky business, and climatologists caution that many variables remain unknown.
"Back then there were no glaciers on the poles, the ocean currents may have been different, the world was a different world," Lowenstein said. "So as far as predictions, it's not fair to say that this is exactly what the climate will be like in a hundred years."
Harvard's Schrag agrees. But he suggests that the uncertainty inherent in current climate-modeling techniques could mean that current models underestimate future warming.
"CO2 alone is probably not enough to explain some aspects of the ancient climate," he said.
"There [were] palm trees in Wyoming, and palm trees don't grow where it gets cold in the winter. When you take a climate model designed for the modern climate and raise the CO2 levels this high, you don't get [Wyoming] winter temperatures warm enough for palm trees to survive.
"That tells us that there is something missing from the models," Schrag said.
Factors absent from current models—such as warming induced by cloud cover—may have amplified warming in the past.
"There's no reason we'd know about [cloud feedback under Eocene-like CO2 conditions] in the modern climate, because we haven't seen such high levels of CO2," he said.
"But it's important for thinking about future warming. It says that it's quite possible that current models are underestimating warming—and we may be in for some surprises in the future."
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