The study, published today in Nature, is the first to resolve the relationship between carbon dioxide (CO₂) and global temperatures during the period known as the ‘Eocene epoch’. This is an important step in understanding ancient climate and thus helping scientists better predict future climate change.

The UK-wide research team developed new records of past CO₂ levels by analysing ancient ocean sediments and compared them to previously generated data on ocean temperatures. The results support previously unsubstantiated theories 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 Bristol team includes Professor Richard Pancost from the University of Bristol and Director of the Cabot Institute, as well as Professor Dan Lunt, Professor Andy Ridgwell, Dr Kirsty Edgar and Dr Gordon Inglis.

“We cannot directly measure CO₂ concentrations from that long ago,” says Dr Eleni Anagnostou, lead author and postdoctoral researcher at the University of Southampton. “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.”

The fossils, called foraminifera, were once tiny marine animals that lived at shallow depths in the ocean during the Eocene epoch; their shells capture the chemical makeup of the ambient seawater they lived in. Applying pioneering geochemical techniques – developed at the University of Southampton and the University of Bristol over the past decade – the team used isotopes of the element boron as a proxy for pH (a measure of acidity), which changes in surface waters as a function of atmospheric CO₂.

They found that between the early Eocene and the late Eocene, CO₂ levels approximately halved. Based upon 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. Professor Gavin Foster says: “After accounting for changes in vegetation, the positions of continents, and 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 for our warm future.”

Dr Inglis adds: “This complements previous work, most of which has focused on times when the Earth was either colder or slightly warmer than it is today.  Seeing a similar response from a much warmer climate interval gives us more confidence in our predictions of warming due to the current anthropogenic CO₂ increase.”

This collaborative study involving the University Bristol and the Cabot Institute, is an output from ‘Descent into the Icehouse’, one of the four research projects under the umbrella programme ‘The Long-term Co-Evolution of Life and the Planet’ funded the by the Natural Environment Research Council (NERC).

Dr Anagnostou, Professor Foster and Professor Lunt speak in further detail about the present study and the wider research project in the ‘Descent into the Icehouse’ video. 

Paper

'Changing atmospheric CO₂ concentration was the primary driver of early Cenozoic climate' by Eleni Anagnostou, Eleanor John, Kirsty Edgar, Gavin Foster, Andy Ridgwell, Gordon Inglis, Richard Pancost, Dan Lunt and Paul Pearson in Nature