Ancient fossils reveal window into the future

Experts from the School of Earth and Environmental Sciences are among a group of UK researchers who have, for the first time, revealed the detailed relationship between atmospheric carbon dioxide (CO2) and the climate during a period of intense global warming around 53 million years ago.

The team used ancient fossils of sea creatures to develop new records documenting the levels of CO2 in the atmosphere during the ‘Eocene epoch’ – a period spanning between 53 and 34 million years ago that started with extreme warmth and, through gradual cooling, ended with the formation of the polar ice sheets that still exist today.

An analysis of the fossils revealed that between the warm climate in the early Eocene and the colder climate in the late Eocene, CO₂ levels approximately halved and could therefore explain the majority of the cooling that occurred around this time.

By discovering how the climate reacted to a drastic change in levels of atmospheric CO2 in the past, the researchers believe the results can be used to gain a better understanding of how the Earth will respond to future increasing levels of CO2 in the face of a rapid rise in anthropogenic emissions.

The UK-wide team, which also included researchers from the University of Southampton and the University of Bristol, arrived at their results by analysing ancient ocean sediments containing fossils of sea creatures.

The fossils, called foraminifera, were once tiny marine creatures that lived near the ocean surface during the Eocene epoch; their shells capture the chemical makeup of the seawater they lived in.

Applying pioneering geochemical techniques, 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.

"Fossil foraminifera have beautiful and intricate shells," said Dr Eleanor John, postdoctoral researcher at Cardiff University, a partner in 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.”

Dr Eleni Anagnostou, the lead author of the study and a postdoctoral researcher at the University of Southampton, said: “This confirms that the Eocene world really was a greenhouse world, with the main difference to now being the higher CO2 level. The comparison not only suggests that the Earth’s temperature response to changing CO₂ is not very dependent on how warm the overall climate is, but also gives us more confidence in our predictions of future climate warming in the face of rapid anthropogenic CO₂ increase.”

The findings have been published in the journal Nature.