A new study by researchers from the University of Southampton reveals the key force that pushed global climate shifts between 53 and 34 million years ago: atmospheric carbon dioxide concentration. The findings are the first the pinpoint the relationship between carbon dioxide (CO2) and climate change during the "Eocene epoch," a time period on Earth when global temperatures were 14 degrees Celsius warmer than today.
The team used ancient ocean sediments to create new records of past CO2 levels, supporting the idea that increased atmospheric CO2 levels were connected to the high temperatures during the Eocene, as well as suggesting that the decrease in CO2 levels that came afterwards is tied to the cooling the created the polar ice sheets we still see today.
"We cannot directly measure CO2 concentrations from that long ago," said Eleni Anagnostou, a postdoctoral researcher at the University of Southampton and lead author of the study. "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."
These marine fossils, also called foraminifera, stem from creatures that lived near the ocean surface during the Eocene epoch and possess shells that harbor the chemical composition of the seawater that they called home.
"Fossil foraminifera have beautiful and intricate shells," said Eleanor John, a postdoctoral researcher at Cardiff University and co-author of the study. "We can identify and separate the various species, including crucially those that lived in the topmost layer of the ocean, where the chemistry is controlled by atmospheric CO2."
The team used geochemical techniques to analyze isotopes of the element boron in the shells to determine the pH, which in turn was used to determine atmospheric CO2 levels. The results revealed that CO2 levels were cut in two between the early and late Eocene, which they determined was the result of a relationship between sea surface temperature and the CO2 levels at various latitudes.
"The new data can be used to determine how Earth will respond to increasing CO2 levels and better understand the effects of climate change and global warming," said Gavin Foster, a professor at the University of Southampton and co-author of the study. "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 CO2 forcing in the warm Eocene was similar to that predicted by the IPCC [Intergovernmental Panel on Climate Change] for our warm future."
"This confirms that the Eocene world really was a greenhouse world, with the main difference to now being the higher CO2 level," Anagnostou added. "The comparison gives us more confidence in our predictions of future climate warming in the face of rapid anthropogenic CO2 increase."
The findings were published in the April 25 issue of Nature.
