Scientists have uncovered a missing feedback in Earth’s carbon cycle that could cause global warming to overshoot into an ice age. As the planet warms, nutrient-rich runoff fuels plankton blooms that bury huge amounts of carbon in the ocean. In low-oxygen conditions, this process can spiral out of control, cooling Earth far beyond its original state. While this won’t save us from modern climate change, it may explain Earth’s most extreme ancient ice ages.