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Earth may have a built-in climate control system that has kept the planet habitable for tens of millions of years, and scientists now believe they have uncovered how it works. A new study identifies a hidden chain of events linking sea level, ocean nutrients, and carbon dioxide, revealing a natural feedback mechanism that helped regulate Earth’s climate over the last 60 million years.
The international research team found that fluctuations in sea level controlled the amount of phosphate reaching the open ocean. That, in turn, influenced marine life, the burial of carbon in seafloor sediments, and the concentration of carbon dioxide in the atmosphere. The findings, published in Proceedings of the National Academy of Sciences, provide new insight into one of Earth’s longest-running climate stabilizing processes.
The study was co-authored by Zunli Lu of Syracuse University and led by Ros Rickaby of the University of Oxford. The researchers examined geological records spanning 60 million years to understand how sea level and ocean oxygen conditions affected Earth’s carbon cycle.
Their analysis centers on phosphate, an essential nutrient for marine organisms. When sea levels were high, vast continental shelves trapped phosphate in coastal sediments, reducing the amount available in the open ocean. With fewer nutrients, marine productivity declined, less organic carbon became buried on the seafloor, and more carbon dioxide remained in the atmosphere, contributing to a warmer climate.
As sea levels fell, the process reversed. More phosphate entered the ocean, fueling marine life that ultimately transported larger amounts of carbon to the seafloor. The decomposition of this organic matter created low-oxygen conditions, releasing even more phosphate from sediments and reinforcing the cycle. This positive feedback increased long-term carbon burial and reduced atmospheric carbon dioxide.
The researchers identified a “sweet spot” where this feedback was most effective, when sea levels stood roughly 10 to 40 meters above today’s level. Geological evidence, including carbon isotope records and chemical analyses of fossilized marine microorganisms, supported the proposed mechanism.
The findings also help explain why Earth remained exceptionally warm during the Eocene Epoch, between about 56 and 34 million years ago. At that time, extremely high sea levels trapped phosphate on flooded continental shelves, limiting carbon burial and allowing carbon dioxide to accumulate in the atmosphere.
The researchers believe this newly identified feedback acted as Earth’s natural thermostat for millions of years, helping stabilize the planet’s climate over geological timescales. While the process unfolds far too slowly to offset modern human-driven warming, the discovery provides a clearer understanding of how Earth’s climate has remained habitable through dramatic changes in its distant past.
