February 27, 2009
CO2 Drop Caused Greenhouse-To-Icehouse Shift
A team of Yale geologists has a new perspective on the greenhouse-to-icehouse shift where global climate changed from an ice-free world to one with massive ice sheets in the Antarctic nearly 34 million years ago.
The study, which is detailed in the February issue of Science, disproves a long-held theory that massive ice growth was accompanied by very little global temperature change.
"Previous reconstructions gave no evidence of high-latitude cooling," said senior author Mark Pagani, professor of geology and geophysics at Yale. "Our data demonstrate a clear temperature drop in both hemispheres during this time."
Computer modeling suggests that the cooling was caused by a reduction of greenhouse gases in the atmosphere.
The conclusions are based on "temperature proxies" which are calculations of temperature based on the distribution of organic molecules from ancient plankton that only lived at certain temperatures.
These ancient plankton, found in ocean sediments, were examined by the Integrated Ocean Drilling Program (IODP) by coring deep-ocean sediments.
"Temperatures in some regions, just before the Antarctic glaciers formed, were surprisingly higher than current climate models predicted, suggesting that these models underestimate high-latitude warming under high CO2 conditions," said Zhonghui Liu, lead author.
At this time the Earth was warm, and wet. The north and south poles even experienced subtropical climates.
The substantial cooling suggests a decline in CO2 levels, rather than localized changes in ocean circulation, he added.
The ice formed over 100,000 years, which is considered a quick shift in geological terms.
The study also refuted a theory saying that ice-expansion occurred in the Northern Hemisphere during this period. The Yale scientists say this theory was supported by poor physical evidence.
According to Pagani, there are about 70 meters of vertical sea level rise shown in the Antarctic ice sheets. There are many questions about the glacier's stability, and what thresholds would cause radical glacier melting.
"Our findings point to the difficulty of modeling accurate temperatures under higher CO2 in this critical region," Pagani added.
The National Science Foundation helped fund the research.
Image 2: Projection of the what the first Antarctic ice sheet might have looked like as the global climate cooled about 33.5 million years ago. Antarctica is in gray, with the ice sheet shown in meters of ice thickness. The ice sheet is continental in scale, but somewhat smaller than today. The estimate is based on prior modeling work of DeConto and Pollard and is supported by this new data study. Credit: DeConto & Pollard/ Nature
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