Scientists may have finally solved a long-standing mystery about how Earth shifted from a hot, tropical “greenhouse” world to the cooler, ice-covered planet we know today.
According to a new study, a major drop in calcium levels in the ocean over the past 66 million years may have played a key role in cooling the planet after the dinosaurs disappeared.
The research was led by scientists at the University of Southampton and published in the journal Proceedings of the National Academy of Sciences.
The international team found that the amount of dissolved calcium in seawater has fallen by more than half since the end of the age of dinosaurs.
This slow but dramatic change appears to be closely linked to a long-term drop in atmospheric carbon dioxide, one of the main gases that warms the planet.
Dr. David Evans, the study’s lead author and an ocean and Earth scientist at Southampton, explained that calcium levels in the ocean were much higher at the beginning of the Cenozoic Era, shortly after dinosaurs still roamed Earth.
At that time, the oceans behaved differently. With more calcium dissolved in seawater, the ocean stored less carbon, allowing more carbon dioxide to remain in the atmosphere and keeping the planet warm.
As calcium levels gradually declined over millions of years, the oceans became better at pulling carbon dioxide out of the air. This removal of CO₂ reduced the greenhouse effect and led to a major cooling of Earth’s climate—by as much as 15 to 20 degrees Celsius, according to the researchers.
To uncover this history, the team studied fossilized shells of tiny marine organisms called foraminifera, which are preserved in seafloor sediments.
The chemical makeup of these fossils provided the most detailed record yet of how ocean chemistry has changed over deep time. The results revealed a strong connection between seawater calcium levels and atmospheric carbon dioxide.
Computer models helped the scientists understand how marine life fits into this story. When calcium levels were high, organisms such as corals and plankton locked carbon into calcium carbonate in a way that released more CO₂ back into the atmosphere.
As calcium levels fell, these same processes became more effective at trapping carbon in seafloor sediments, keeping it out of the ocean and air for long periods.
The study also linked the calcium decline to changes deep within Earth. The slowdown of seafloor spreading—the process that creates new ocean crust through volcanic activity—reduced chemical exchanges between rocks and seawater. This shift gradually lowered calcium levels in the ocean.
The findings challenge the common idea that ocean chemistry only responds to climate change. Instead, the researchers suggest that slow changes inside Earth itself may have helped “turn down the planet’s thermostat,” shaping Earth’s climate over millions of years.
Source: University of Southampton.
