Researchers have identified previously overlooked mechanisms in Earth’s natural climate regulation system that has maintained planetary habitability for more than 100 million years. The new study, published in Proceedings of the National Academy of Sciences, reveals how changes in global temperature affected polar ice sheets and sea levels, which in turn influenced the availability of phosphate in the ocean and the amount of carbon dioxide remaining in the atmosphere.
Phosphate, an essential nutrient for marine organisms, played a central but previously underappreciated role in this climate feedback system. When sea levels were elevated, phosphate became trapped in coastal sediments, reducing nutrient availability in open ocean waters. This led to decreased marine productivity, less carbon burial on the seafloor, and increased atmospheric carbon dioxide, resulting in a warmer planet. Conversely, when sea levels dropped, more phosphate entered the water, stimulating marine growth and carbon burial in sediments, which reduced atmospheric carbon dioxide and cooled the planet.
The research found that this carbon burial feedback operated most effectively when sea levels stood approximately 10 to 40 meters above modern levels. At this “sweet spot,” low-oxygen water zones overlapped with organic-rich continental shelf sediments, enabling unusually large amounts of carbon to be sequestered for millions of years. Scientists tested this hypothesis by analyzing 60 million years of geological evidence, including carbon isotope records and measurements using a newly refined iodine-to-calcium method to reconstruct ancient ocean oxygen levels.
The Eocene epoch, lasting roughly 56 to 34 million years ago, demonstrated what occurred when this feedback mechanism was largely inactive. Extremely high sea levels flooded broad continental shelves, trapping phosphate and leaving the open ocean nutrient-poor. With marine productivity suppressed and carbon burial reduced, carbon dioxide accumulated in the atmosphere and Earth remained warm. The researchers propose that over geological time, the zones where carbon burial occurs have gradually shifted, potentially stabilizing both atmospheric oxygen and carbon dioxide levels and making Earth’s climate system more resistant to disruption.