Researchers at UC Riverside say they have identified a critical gap in how scientists have long understood Earth's carbon recycling system. By filling in this missing piece, they now believe that periods of global warming can swing too far in the opposite direction, potentially setting the stage for an ice age.

For decades, scientists have thought Earth's climate was regulated by a slow but dependable natural process driven by rock weathering. This mechanism was seen as a stabilizing force that kept temperatures from drifting too far in either direction.

How Rock Weathering Helps Regulate Climate

In this process, rain absorbs carbon dioxide from the atmosphere and then falls onto exposed land surfaces. As the water interacts with rocks, especially silicate rocks such as granite, it gradually breaks them down. The dissolved material, along with the captured CO2, is carried into the oceans.

Once there, the carbon combines with calcium released from the rocks to form shells and limestone reefs. These materials settle on the ocean floor, locking carbon away for hundreds of millions of years and slowly reducing the amount of carbon dioxide in the atmosphere.

"As the planet gets hotter, rocks weather faster and take up more CO2, cooling the planet back down again," said Andy Ridgwell, a UC Riverside geologist and co-author of the study published in Science.

Why Ancient Ice Ages Were So Extreme

Geological records, however, tell a more dramatic story. Evidence shows that some of Earth's earliest ice ages were so severe that ice and snow covered nearly the entire planet. According to the researchers, this level of freezing cannot be explained by a climate system that simply fine tunes itself.

That realization led the team to look for an additional process that could push the climate beyond gentle balance and into extremes.

The Role of Oceans, Nutrients, and Plankton

The newly identified factor involves how carbon is buried in the ocean. As atmospheric CO2 rises and temperatures increase, rainfall carries larger amounts of nutrients such as phosphorus into the sea. These nutrients stimulate the growth of plankton, microscopic organisms that absorb carbon dioxide through photosynthesis.

When plankton die, they sink to the ocean floor, taking the carbon they captured with them. This process removes carbon from the atmosphere and stores it in ocean sediments.

In warmer conditions, however, this system changes. Increased plankton growth can reduce oxygen levels in the ocean. With less oxygen available, phosphorus is more likely to be released back into the water instead of being permanently buried. This recycled phosphorus fuels even more plankton growth, whose decay further depletes oxygen and keeps nutrients circulating.