Earth's natural thermostat: How sea level and phosphate controlled climate for 60 million years

A new study from Syracuse University and the University of Oxford reveals a natural climate regulation system that has kept Earth within a habitable temperature range for over 60 million years. The mechanism links sea level changes to phosphate availability in the ocean, which in turn controls how much organic carbon gets buried in marine sediments. When sea levels fell within a specific range, more carbon was trapped on the seafloor, reducing atmospheric CO2 and cooling the planet. Researchers found that the strongest carbon burial occurred when sea level stood about 10 to 40 meters above its present height. In this sweet spot, low-oxygen zones in the ocean aligned with organic-rich shelf sediments, creating a feedback loop that removed carbon from the atmosphere for millions of years. The study used a new iodine-to-calcium proxy to measure ancient ocean oxygen levels, providing direct geological evidence for this process. This research matters for understanding Earth's long-term carbon cycle and how natural systems have buffered climate change over geological timescales. It also highlights the sensitivity of ocean nutrient cycles to sea level changes, which has implications for how we think about carbon storage in marine environments today.