A team of scientists studying rock samples in Africa has shown a strong link between falling carbon dioxide levels and the formation of Antarctic ice sheets 34 million years ago.
The results are the first to make the link, underpinning computer climate models that predict both the creation of ice sheets when CO2 levels fall and the melting of ice caps when CO2 levels rise.
The team, from Cardiff, Bristol and Texas A&M Universities, spent weeks in the African bush in Tanzania with an armed guard to protect them from lions to extract samples of tiny fossils that could reveal CO2 levels in the atmosphere 34 million years ago.
Levels of carbon dioxide, the main greenhouse gas, mysteriously fell during this time in an event called the Eocene-Oligocene climate transition.
"This was the biggest climate switch since the extinction of the dinosaurs 65 million years ago," said co-author Bridget Wade from Texas A&M University.
The study reconstructed CO2 levels around this period, showing a dip around the time ice sheets in Antarctica started to form. CO2 levels were around 750 parts per million, about double current levels.
"There are no samples of air from that age that we can measure, so you need to find something you can measure that would have responded to the atmospheric CO2," Paul Pearson of Cardiff University told Reuters.
Pearson, Wade and Gavin Foster from the University of Bristol gathered sediment samples in the Tanzanian village of Stakishari where there are deposits of a particular type of well-preserved microfossils that can reveal past CO2 levels.
"Our study is the first that uses some sort of proxy reconstruction of CO2 to point to the declining CO2 that most of us expected we ought to be able to find," Pearson said on Monday from Cardiff.
He said that CO2, being an acidic gas, causes changes in acidity in the ocean, which absorbs large amounts of the gas.
"We can pick that up through chemistry of microscopic plankton shells that were living in the surface ocean at the time," he explained.
Evidence from around Antarctica was much harder to find.
"The ice caps covered everything in Antarctica. The erosion of sediments around Antarctica since the formation of the ice caps has obliterated a lot of the pre-existing evidence that might have been there."
"Our results are really in line with the most sophisticated climate models that have been applied to this interval," Pearson added. The results were published online in the journal Nature.
"Those models could be used to predict the melting of the ice. The suggested melting starts around 900 ppm (parts per million)," he said, a level he believes could be reached by the end of this century, unless serious emissions cuts were made.Reuse content