Fresh analysis of ancient shells suggest carbon had a pronounced effect on Earth's climate during the Eocene Epoch, some 35 million years ago.

The findings, published this week in the journal Nature Communications, suggest that as the planet continues to warm, the climate will become more sensitive to changes in carbon levels.

At the outset of the Eocene Epoch, which lasted from 56 to 34 million years ago, temperatures were 14 degrees Celsius greater than they are today. Rainforests extended throughout Antarctica, and croc-filled swamps stretched across Europe and North America.

Throughout the second half of the Eocene Epoch, Earth's climate rapidly cooled and glaciation spread across the planet's poles. Until now, the relationship between carbon levels and climate change during the Eocene wasn't well understood.

To trace the rise and fall of atmospheric carbon levels during the Eocene, researchers measured ratios of boron isotopes inside the fossilize shells of ancient marine plankton deposited in marine sediment layers.

Scientists used the measurements to estimate water acidity and calcite saturation during the Eocene, with the estimates allowing them to model changes in the atmospheric carbon concentrations during the Eocene.

The analysis revealed a strong link between carbon concentrations and global warming, showing Earth hosted large concentrations of carbon in its atmosphere and oceans at the beginning of the Eocene Epoch.

However, the fossil shells show a combination of volcanism, weathering of rocks and burial of organic material helped drastically reduce carbon levels, leading to a prolonged cooling effect.

"These methods really allow us to get a unique insight into not only how the climate system varied in the past, but why," study co-author Gavin Foster, professor of isotope geochemistry in ocean and earth science at the University of Southampton, said in a news release.

"It's this ability to accurately reconstruct atmospheric CO2 in the past that means we can determine climate sensitivity millions of years ago, providing a powerful test of the understanding that is encapsulated in the state-of-the-art climate models that are key to predicting our future warmth," Foster said.

The findings could have implications for the models climate scientists rely on to predict the influence of carbon on sea level rise and global warming, as well as shifts in precipitation patterns and other climatic effects.

"Now that we have demonstrated that the climate is more sensitive when it is warm, like it was during the Eocene, the next step is to work out why this is and make sure this behavior is well represented in the climate models that are used to predict our future climate," said study co-author Tali Babila, post-doctoral researcher at Southampton.

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