In the past 65 million years, the warmest interval occurred between 53 and 51 million years ago. Scientists estimate the annual surface air temperatures were more than 10 Celsius warmer than those of the pre-industrial period. But by 34 million years ago, the Earth had cooled, leading to the development of the continental ice sheet in Antarctica.
Publishing in Nature, a research team led by University of Southampton has linked these temperature shifts to the ebb and flow of atmospheric carbon dioxide levels in the primordial world.
“We cannot directly measure (carbon dioxide) concentrations from that long ago,” said study author Eleni Anagnostou, of the University of Southampton, in a statement. “Instead we must rely on indirect ‘proxies’ present in the geological record.”
Those proxies came in the form of foraminifera, fossils of tiny marine organisms found in ancient ocean sediment. In their study, the researchers substituted the fossils’ isotopes and boron measurements for pH level measurements, and used them to determine atmospheric levels of carbon dioxide during the Eocene epoch.
At the start of the Eocene, carbon dioxide concentrations were around 1,400 parts per million. By the end of the epoch, those concentration levels were halved.
According to NASA, the current global concentration of carbon dioxide in the atmosphere is around 400 parts per million.
“This (carbon dioxide) decline was sufficient to drive the well-documented high- and low-latitude cooling that occurred through the Eocene,” the researchers wrote of the Eocene epoch carbon dioxide reduction.
Looking to the past may help scientists understand how rising carbon dioxide levels could affect the Earth in the future.
“After accounting for changes in vegetation and how the continents were arranged in the past, and correcting for the effect relating to the lack of ice sheets in the Eocene, we found that the sensitivity of the climate system to (carbon dioxide) forcing in the warm Eocene was similar to that predicted by the (Intergovernmental Panel on Climate Change) for our warm future,” said study co-author Gavin Foster, of the University of Southampton.
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