Researchers from the University of St Andrews have shed new light on the fundamental role played by carbon dioxide (CO₂) in Earth’s geological warming and cooling.

Atmospheric carbon dioxide is Earth’s most important greenhouse gas. It absorbs heat and radiates much of it back to Earth, so when atmospheric CO₂ rises, Earth’s climate warms. However, while the role CO₂ plays in driving recent climate change is well understood, its role in the geological past has been “locked in rocks.”

For the first time, new research led by a team from the School of Earth and Environmental Sciences at the University of St Andrews and published in Nature Geoscience on Monday 6 January 2025 has unlocked an important part of the geological record that captures one of the biggest climatic transitions witnessed by complex life.

The new findings provide crucial evidence that for hundreds of millions of years, CO₂ has been regulating climate and environmental conditions on Earth, playing a pivotal role as the world moves in and out of ice ages.

Lead researcher from St Andrews, Dr. Hana Jurikova, worked with nine other universities and institutions around the world to reconstruct how CO₂ changed during the Carboniferous and Permian periods, between 335 and 265 million years ago, a time known as the Late Paleozoic Ice Age, when Earth’s climate cooled dramatically.

Researchers used the chemical fingerprints stored in fossil brachiopod shells, ancient clam-like organisms that are among the oldest existing animals and still inhabit oceans today. These shells are preserved across all time periods of the fossil record, archiving clues to how Earth’s climate and environment has evolved. By combining multiple chemical fingerprints, the team was able to precisely calculate how much CO₂ was in Earth’s atmosphere in the past and how it changed.

“This showed that the Late Paleozoic Ice Age had prolonged low CO₂ levels, unprecedented in Earth’s history. Atmospheric CO₂ then rose abruptly, 294 million years ago, due to large-scale volcanic eruptions, warming the planet and melting back the ice,” explains Dr. Jurikova. “The end of the Late Paleozoic Ice Age was a turning point in the evolution of life and the environment, leading to the rise of reptiles. Now we know it was paced by carbon dioxide.”

This research provides new insights into how CO₂ and Earth’s climate evolved on long, geological timescales, demonstrating a central role for CO₂ in major climate transitions. Low atmospheric CO₂ in the Carboniferous period caused an extensive ice age, while elevated CO₂ in the beginning of the Permian period drove global warming and ice sheet melt.

“This time interval gives a potential geological analog for the impact of CO₂ on our planet,” explains Dr. James Rae, who co-authored the study. “CO₂ emissions in the past caused major global warming and sea level rise, and if left unchecked, they will do so again in the future.”