CO2 the culprit in southern hemisphere temperature drop millions of years ago

Our planet is warming and its atmosphere is changing, but what impact will these changes have on its different ecosystems? To find answers, some scientists have been investigating climates from millions of years ago. A study supported in part by the EU-funded TGRES project focused on temperatures in the southern hemisphere from the middle Eocene to the early Oligocene epoch, about 41 to 33 million years ago. During this period, Earth’s climate changed dramatically. While in the Eocene epoch Antarctica was an ice-free, rainforest-covered region, by 34 million years ago it was covered with ice. This climatic transition from greenhouse to icehouse is well documented by sea surface temperature records that point to noticeable long-term cooling. However, to be able to tell what drove this change, scientists need to know whether this cooling also happened on land.

Looking for answers in bacterial biomarkers

In this study, the research team used bacterial molecular fossils preserved in south-east Australian lignite to create a record of terrestrial temperatures spanning a period of around 9 million years, from the middle Eocene to the early Oligocene. Study co-author Dr Richard Pancost of TGRES project host University of Bristol explains the use of these bacterial biomarkers in a news item posted on ‘Sci-News’: “These compounds originally comprised the cell membranes of bacteria living in ancient wetlands, with their structures changing slightly to help the bacteria adapt to changing temperature and acidity. Those compounds can then be preserved for tens of millions of years, allowing us to reconstruct those ancient environmental conditions.” The results showed that mean annual temperatures in south-east Australia gradually dropped from about 27 °C during the middle Eocene to around 22 to 24 °C in the late Eocene. This was followed by an approximate 2.4 °C drop across the Eocene–Oligocene Boundary, about 33 to 34 million years ago.

What drove the drop in temperatures on land and sea?

The similarity between this trend and other temperature records in the southern hemisphere pointed to a likely common driving mechanism, namely CO2. This led the researchers to conduct climate model simulations to confirm their results. They found that only simulations including a decline in atmospheric CO2 led to a cooling in south-east Australia, which was consistent with the temperature data from the lignite. The results add to the current body of evidence that atmospheric CO2 is a major driver of change in Earth’s climate. “Our data form an important benchmark for testing climate model performance, sea–land interaction and climatic forcings at the onset of a major Antarctic glaciation,” the authors conclude in their study. The study has been published in the journal ‘Nature Geoscience’. The TGRES (The Greenhouse Earth System) project ended in 2018.