Coral reefs have shaped Earth's climate for 250 million years

For a quarter of a billion years, coral reefs have been quietly steering the planet's most fundamental cycles. Not just responding to climate shifts—actively shaping how fast Earth recovers from them.

A new study published in Proceedings of the National Academy of Sciences reveals that shallow-water reefs have been flipping Earth between two distinct climate modes, each with its own rhythm of carbon recovery. When reefs flourish, the planet heals slowly. When reefs decline, it heals faster. The researchers traced this pattern across 265 million years of Earth's history, combining plate tectonics data, climate records, and ocean ecology into vast computer simulations.

"Reefs didn't just respond to climate change—they helped set the tempo of recovery," says Tristan Salles, a geoscientist at the University of Sydney who led the work.

How reefs rewire the carbon cycle

When shallow, warm waters are extensive and reefs thrive, they lock away calcium carbonate—the chemical backbone of reef structures. This calcium sequestration starves the ocean, leaving less of the compound available to absorb carbon dioxide. The result: when volcanic eruptions or other events spike atmospheric CO2, the atmosphere takes hundreds of thousands of years to balance itself.

When reefs shrink or vanish—through tectonic shifts, sea-level changes, or other disruptions—calcium floods back into the open ocean. Now the water can absorb carbon dioxide far more efficiently. But there's more happening beneath the surface. That calcium and carbonate also carries nutrients that trigger plankton blooms. These microscopic algae feast on carbon at the surface, then sink to the ocean floor when they die, burying carbon deep in sediment where it stays locked away for millennia.

The fossil record shows something striking: during reef-poor periods, entirely new types of plankton evolved rapidly. When reefs flourished and nutrients stayed scarce, evolutionary change slowed. This means reefs don't just influence carbon cycles—they set the pace of life itself in the ocean.

"We're seeing a deeply intertwined feedback cycle between life and climate," says Alexander Skeels, an evolutionary biologist at the Australian National University. "Species don't just change because of Earth's climate. More and more, we're finding that biological species directly influence the climate, creating a co-evolving feedback loop."

The modern crisis

Half of Earth's coral reefs have vanished since 1950. Today's human-driven carbon emissions are warming the ocean and acidifying it at speeds the planet hasn't seen in millions of years, killing both corals and plankton simultaneously. We're disrupting a system that took quarter-billion years to calibrate.

Salles is clear about what this means: "From our perspective on the past 250 million years, we know the Earth system will eventually recover from the massive carbon disruption we are now entering. But this recovery will not occur on human timescales. Our study shows that geological recovery requires thousands to hundreds of thousands of years."

The research doesn't offer false comfort. It offers clarity. The planet will heal—just not in time for us to see it.