A team of scientists has solved one of geology’s longest-standing puzzles, according to a study published in the journal PNAS. They finally found out how Earth went from a warm, tropical world in the age of dinosaurs to the cooler, ice-capped planet we live on today.
The answer lies in the chemistry of the ocean, specifically, a dramatic decrease in the levels of calcium dissolved in seawater over the past 66 million years.
The research was carried out by a team of researchers from the University of Southampton, UK, in collaboration with scientists from China, the USA, Israel, Denmark, Germany, Belgium and the Netherlands. The team used fossilised remains of tiny marine creatures, called foraminifera, recovered from sediments on the seafloor, to build the most detailed records of ancient ocean chemistry ever constructed.
Their analysis revealed that calcium levels in the ocean have fallen by more than half since the end of the age of dinosaurs. And this shift, the researchers believe, is what drove the planet’s long, slow cooling.
“Our results show that dissolved calcium levels were twice as high at the start of the Cenozoic Era, shortly after dinosaurs roamed the planet, compared to today. When these levels were high, the oceans worked differently, acting to store less carbon in seawater and releasing carbon dioxide into the air. As those levels decreased, CO2 was sucked out of the atmosphere, and the Earth’s temperature followed, dropping our climate by as much as 15 to 20 degrees Celsius,” said lead author Dr David Evans, from the University of Southampton.
In other words, high calcium levels in the ancient ocean meant that marine life (such as corals and plankton) produced and buried less calcium carbonate on the seafloor, leaving more carbon dioxide in the air and keeping the planet warm. As calcium levels fell over millions of years, that process reversed, steadily drawing CO2 out of the atmosphere and cooling the Earth.
“The process effectively pulls carbon dioxide out of the atmosphere and locks it away. This shift could have changed the composition of the atmosphere, effectively turning down the planet’s thermostat,” added co-author Dr Xiaoli Zhou of Tongji University in China.
The researchers also found that the decline in ocean calcium closely matched a slowdown in seafloor spreading, the volcanic process by which new ocean floor is continuously created. As that process slowed, the chemical exchange between rocks and seawater changed, gradually reducing calcium concentrations in the ocean.
“Seawater chemistry is typically viewed as something that responds to other factors that lead to changes in our climate, rather than being the cause itself. But our new evidence suggests that we must look to changing seawater chemistry to understand our planet’s climate history. It may be that changes in these deep Earth processes are ultimately responsible for much of the large climatic shifts that have taken place over geological time,” concluded co-author Professor Yair Rosenthal from Rutgers University in the USA.
Evans D, Rosenthal Y, Erez J, Hauzer H, Cotton LJ, Zhou X, Nambiar R, Stassen P, Pearson PN, Renema W, Saraswati PK, Todd JA, Müller W, Affek HP. The major ion chemistry of seawater was closely coupled to the long-term carbon cycle during the Cenozoic. Proc Natl Acad Sci U S A. 2026 Jan 13;123(2):e2511781122.