A missing feedback in earth’s carbon cycle could cause global warming to overshoot into an ice age, say researchers at University of California – Riverside. 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.

For decades, scientists thought earth’s climate was regulated by a slow but dependable natural process driven by rock weathering. This mechanism was seen as a stabilising force that kept temperatures from drifting too far in either direction. According to the study, lower levels of oxygen in earth’s ancient atmosphere made this climate control far less stable, which helps explain the severity of early ice ages. Today, atmospheric oxygen levels are much higher.

As human activity continues to add CO2 to the atmosphere, the planet is expected to keep warming in the near term. The researchers’ model suggests that a cooling rebound will eventually follow. However, this future cooling is likely to be less extreme because higher oxygen levels reduce the strength of the nutrient feedback in the oceans.