Atmospheric carbon dioxide (CO2) concentration was the major driver behind the global climatic shifts that occurred in the "Eocene epoch" between 53 and 34 million years ago, says a new study.
The results support the view that elevated CO2 was responsible for the extreme warmth of the early Eocene and that CO2 decline was responsible for the subsequent cooling that ultimately led to the establishment of today's polar ice sheets.
The researchers believe that the findings, published in the journal Nature, could help scientists better predict future climate change.
For the study, the research team developed new records of past CO2 levels by analysing ancient ocean sediments.
"We cannot directly measure CO2 concentrations from that long ago," said study lead author Eleni Anagnostou, postdoctoral researcher at University of Southampton in Britain.
"Instead we must rely on indirect 'proxies' present in the geological record. In this study, we used the chemical composition of marine fossils preserved in sediments to reconstruct ancient CO2 levels," Anagnostou noted.
Applying pioneering geochemical techniques - developed at the University of Southampton over the past five years - the team used isotopes of the element boron in the shells as a proxy for pH (a measure of acidity), and used that to determine the atmospheric CO2 levels.
They found that between the early Eocene and the late Eocene, CO2 levels approximately halved.
Using our current understanding of the relationship between sea surface temperature and CO2 at different latitudes, they also demonstrated that the changes in CO2 concentration can explain the majority of the cooling that occurred.
This research can also be used to gain a better understanding of how the Earth will respond to increasing levels of CO2 in the future, the scientists said.
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