Major fall in CO2 levels dramatically cooled Earth’s climate 30 million years ago

New research suggests a 3C fall in temperature on land and in the oceans was a key driver of a pivotal transition from ‘hot greenhouse’ to a ‘cold icehouse’ world, writes Harry Cockburn

The fossilised remains of a tree stump in lignite deposits in Antarctica (Vittoria Lauretano)

Around 34 million years ago, a huge transformation in the Earth’s climate driven by a decline in the concentration of atmospheric carbon dioxide (CO2), turned the climate from a “warm greenhouse” to a “cold icehouse”, new research has confirmed.

However, this change, which occurred over millions of years, could be “partly reversed” over the coming centuries due to human-made greenhouse gas emissions, scientists have warned.

The transition meant that Antarctica, which had been covered by extensive lush forests, became the snow-bound wasteland covered with thick ice sheets we know today.

However, the key drivers of the transition from greenhouse to icehouse, and the role of CO2 concentration, have been widely debated. There are many unknowns because there are so few remaining clues as to how the climate changed on land.

But a new technique in which scientists from the University of Bristol examine molecular fossils preserved in ancient coal deposits, has helped reconstruct land temperatures across the transition period, which is thought to have occurred within a time frame of around six million years.

The international team, led by Dr Vittoria Lauretano and Dr David Naafs, assessed the distribution of bacterial lipids preserved in ancient wetland deposits.

Paper co-author Rich Pancost, from the University of Bristol’s School of Chemistry, said: “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.”

To reconstruct how the global average temperatures changed across the greenhouse to icehouse transition, the team applied their new approach to coal deposits from the southeast Australian Gippsland Basin.

These extensive deposits contain geological information spanning more than 10 million years of Earth history, the scientists said.

The new data suggests that land temperatures cooled during the same time the oceans also became cooler and by a similar magnitude  – with long-term cooling of about 3C.

In order to then explore the causes behind the temperature decline, the team used advanced climate modelling simulations.

“Crucially, only simulations that included a decline in atmospheric CO2 could reproduce a cooling consistent with the temperature data reconstructed from the coals,” the team said.

“These results provide further evidence that atmospheric CO2 plays a crucial role in driving Earth’s climate, including the formation of the Antarctic ice sheet,” they added.

Previous research on sediment cores taken from the west Antarctic shelf has revealed forest soils, roots, pollen and spores, indicating the continent had an “exceptionally warm climate” during the mid-Cretaceous period when dinosaurs roamed the Earth.

Researchers suggested the rainforest in Antarctica would have resembled the forests in New Zealand today.

The new research is published in the journal Nature Geoscience.