What ended Earth's greenhouse age? New clues from the ocean
Earth didn’t always look like it does now. After the dinosaurs disappeared 66 million years ago, the planet spent long stretches in a much warmer “greenhouse” state – no big ice sheets, higher sea levels, and a more tropical feel in many places.
Over time, though, Earth cooled dramatically, eventually building permanent ice in Antarctica and later in the Northern Hemisphere. Scientists have argued for decades about what drove that slow but huge shift.
Seawater calcium declines
An international team led by the University of Southampton reports that dissolved calcium concentrations in the ocean have fallen by more than half over the last 66 million years.
In their view, that chemical change may have helped pull carbon dioxide out of the atmosphere and keep it locked away, gradually “turning down” the planet’s temperature.
Study lead author David Evans described how the shift may have unfolded.
“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,” said Evans.
“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.”
Calcium levels and carbon storage
At first glance, calcium doesn’t sound like a climate lever. But in the ocean, calcium is central to the carbonate system – the chemistry that controls how carbon is stored in seawater and how much can end up as CO2 in the air.
Many marine organisms – plankton, corals, and others – use calcium and carbonate to build calcium carbonate shells and skeletons. When they die, some of that material sinks to the seafloor, where sediments bury it.
That burial removes carbon from circulation over the long term. If conditions favor more burial (or different styles of burial), the atmosphere can end up with less CO2 over geological time.
If conditions swing the other way, the ocean-atmosphere system can “leak” more CO2 back into the air.
The team argues that when calcium levels were higher, the ocean tended to store less carbon in seawater in ways that keep CO2 low, and more carbon ended up back in the atmosphere.
As calcium levels declined through the Cenozoic, the balance shifted toward processes that lock carbon away, lowering atmospheric CO2 and helping cool the planet.
Reconstructing ancient seas
Nobody has a jar of seawater from 50 million years ago, so the researchers relied on proxies – physical traces left behind by ancient life.
They analyzed fossilized shells of foraminifera, tiny ocean-dwelling organisms preserved in seafloor sediments.
The chemistry of these fossils can record the chemistry of the seawater they lived in.
By collecting foraminifera from sediment cores and analyzing their chemical signatures, the team built what they describe as the most detailed record yet of how ocean calcium changed across the last 66 million years.
That record showed a strong link between seawater calcium and atmospheric CO2 levels inferred from other lines of evidence.
Correlation isn’t proof, so the team also leaned on computer modelling to test whether the calcium changes were big enough to plausibly push carbon cycling – and therefore climate – in the direction suggested by the fossil record.
The “carbon lockbox” idea
In the team’s modeling work, calcium levels influence how efficiently marine life fixes carbon and how much ends up buried as calcium carbonate on the seafloor.
The logic: change the ingredients available to marine organisms, and you change the ocean’s long-term carbon bookkeeping.
“The process effectively pulls carbon dioxide out of the atmosphere and locks it away,” said study co-author Xiaoli Zhou from Tongji University.
“This shift could have changed the composition of the atmosphere, effectively turning down the planet’s thermostat.”
This is the kind of mechanism that wouldn’t cause a sudden flip in climate, but could help explain a slow, persistent trend.
That pattern is exactly what we see when Earth moves from a warm early Cenozoic world toward the cooler conditions that eventually allowed ice sheets to grow.
A shift in seawater chemistry
The study also tries to explain why calcium fell. The team points to deep Earth geology – specifically, changes in seafloor spreading.
Mid-ocean ridges constantly create new ocean crust. That fresh crust interacts chemically with seawater, and those reactions help set the ocean’s long-term composition.
The researchers say their calcium decline lines up with evidence that seafloor spreading slowed over time.
If the production of new seafloor declines, the scale and nature of chemical exchange between rock and seawater changes too, potentially driving calcium downward over millions of years.
Study co-author Yair Rosenthal from Rutgers University noted that this flips the usual framing on its head.
“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,” Rosenthal explained.
“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.”
Climate change over millions of years
This isn’t claiming calcium alone explains everything about Earth’s long cooling story.
Climate over tens of millions of years is shaped by lots of moving parts: tectonics, volcanic outgassing, mountain building and weathering, ocean circulation shifts, biological evolution, and more.
But the study argues that ocean chemistry isn’t just a passive “diary” of climate change – it can be one of the drivers.
If the team is right, then one reason Earth eventually became an icy planet is that the ocean slowly changed its internal rules about carbon storage.
The calcium decline didn’t just happen alongside cooling; it may have helped make cooling possible by steadily lowering CO2 in the atmosphere and keeping it lower.
And that’s the intriguing part: a huge climate transformation, playing out over tens of millions of years, might partly trace back to something as unglamorous as how much calcium the ocean carried.
The study is published in the journal Proceedings of the National Academy of Sciences.
Image Credit: University of Southampton
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