Earth has a natural thermostat and scientists finally know how it works

New research points to a previously overlooked connection between sea level and the availability of phosphate in the ocean. Changes in global temperature affected the size of polar ice sheets, which altered sea level. Those shifts then influenced how much phosphate reached the open ocean, how much carbon became buried in marine sediments, and how much carbon dioxide remained in the atmosphere.

Together, these processes helped determine whether Earth became warmer or cooler over long stretches of time.

Sea Level and Earth's Carbon Cycle

The new study was co-authored by Zunli Lu, professor of Earth and environmental sciences in Syracuse University's College of Arts and Sciences. It examines how changing sea levels and ocean oxygen conditions affected phosphate availability and atmospheric carbon dioxide over the past 60 million years.

The findings were published in Proceedings of the National Academy of Sciences.

"We know that atmospheric carbon dioxide decreased substantially as Earth cooled over the last 60 million years, but we have had remarkably little understanding of where that carbon ended up," says lead author Ros Rickaby, professor of Earth sciences at the University of Oxford, in a department news article. "Our results suggest that enhanced burial of organic carbon in marine sediments played a much more important role than was previously appreciated."

Phosphate as a Hidden Climate Regulator

At the center of the study is phosphorus, especially phosphate, a nutrient that marine organisms need to grow. The researchers describe phosphate as a previously "invisible" part of the climate puzzle.

When sea levels were high, shallow continental shelves covered a larger area. These shelves trapped phosphate in coastal sediments, leaving less of the nutrient available in the open ocean.

With less phosphate in the water, marine productivity slowed. Fewer organisms grew, less organic carbon sank to the seafloor, and less carbon became buried in sediments. Ocean waters also became more oxygen-rich, while carbon dioxide accumulated in the atmosphere.

The result was a warmer planet.

Falling Seas Triggered a Carbon Feedback

When sea levels dropped, the process moved in the opposite direction.

As continental shelves shrank, more phosphate entered the water. That extra nutrient supported a surge in marine life. When organisms died, their remains sank and decomposed, consuming oxygen in the surrounding water.

Over time, low-oxygen zones formed in the ocean. When those zones reached carbon-rich sediments on continental shelves, they activated a powerful feedback process.

Low oxygen caused sediments to release even more phosphate. That additional phosphate encouraged more marine growth, which led to greater burial of organic carbon on the seafloor. As more carbon was removed from the ocean and atmosphere, atmospheric CO2 declined.

"Our co-author, Christian Bjerrum, studied the connection among sea level, ocean oxygen and phosphate with a computer model two decades ago," Lu says. "We finally pieced together the geologic records necessary to test this hypothesis."

A Sea Level Sweet Spot for Carbon Burial

The researchers found that this feedback reached its greatest strength when sea level stood roughly 10 to 40 meters above its modern level.

At this sea level "sweet spot," low-oxygen waters overlapped with organic-rich continental shelf sediments. That combination allowed unusually large amounts of carbon to become buried for millions of years.

The team compared this pattern with 60 million years of geological evidence. The data included carbon isotope records, measurements of phosphorus accumulation in deep-sea sediments, and a newer iodine-to-calcium method for reconstructing ancient ocean oxygen levels.

Reading Ancient Ocean Oxygen

Lu's laboratory performed the iodine-to-calcium measurements.

The method examines the chemistry of ancient foraminifera, microscopic marine organisms whose remains are preserved in seafloor sediments. Their chemical composition allows scientists to estimate how much oxygen was present in the water when they lived.

The samples were analyzed with a mass spectrometer at Syracuse University. The instrument was funded by the National Science Foundation.

Why the Eocene Stayed Warm

The Eocene epoch, which lasted from roughly 56 to 34 million years ago, provides a clear example of what happened when the carbon burial feedback was largely inactive.

During that period, sea levels were extremely high and broad continental shelves were flooded. Phosphate became trapped in shallow sediments, leaving the open ocean relatively nutrient-poor.

Marine productivity remained lower, the ocean became highly oxygenated, and less organic carbon was buried. With the feedback mechanism effectively switched off, carbon dioxide accumulated in the atmosphere and Earth remained warm.

A Climate System That Became More Stable

The researchers propose that the zones where carbon burial occurs have gradually narrowed over geological time as low-oxygen waters moved deeper.

This long-term shift may have helped stabilize both atmospheric oxygen and carbon dioxide. The swings between carbon burial and carbon accumulation in the atmosphere became less extreme, making Earth's climate system more resistant to disruption.

Key Takeaways From the Study

Phosphate, an essential nutrient for marine life, acted as a hidden regulator of Earth's carbon cycle for the last 60 million years, although its exact role had not been fully understood.

Sea level influenced how much phosphate reached the open ocean. That controlled marine productivity, the amount of carbon buried in seafloor sediments, and the amount of carbon dioxide left in the atmosphere.

A sea level "sweet spot" roughly 10 to 40 meters above modern levels produced the strongest carbon burial. This process acted as a natural brake on warming for millions of years and helped move Earth toward its present cooler climate.

The research included collaborators at the University of Oxford (Rickaby and Thomas Wood) and the University of Copenhagen (Christian J. Bjerrum). It was supported by two National Science Foundation grants.

The findings add to a larger body of work from Lu's laboratory that uses the iodine-to-calcium method to reconstruct oxygen conditions in ancient oceans.

An earlier study, published in January in Nature Geoscience, used the same technique to show that tropical oceans during the Proterozoic Eon were rich in oxygen. That pattern was the exact reverse of what exists today. The researchers also found that a planetary tipping point hundreds of millions of years ago caused the global oxygen distribution to flip.