A warming Earth could accidentally trigger a deep freeze

"When the planet warms, rocks weather faster and absorb more CO₂, allowing the Earth to cool down again," explains Dominik Hülse.

Yet, there have been times when the planet froze over completely, covered in ice from pole to pole. The researchers note that this cannot be explained by rock weathering alone, meaning other forces must have been involved in these dramatic cool-downs.

A crucial clue lies in how the ocean stores carbon. As atmospheric CO₂ levels rise and the planet warms, more nutrients such as phosphorus are washed into the sea. These nutrients fuel algae blooms that draw in carbon through photosynthesis. When the algae die, they sink to the ocean floor, carrying that carbon with them.

In a warmer climate, however, rapid algae growth also leads to lower oxygen levels in the water. With less oxygen, phosphorus tends to be recycled rather than buried in sediments. This creates a powerful feedback loop: more nutrients lead to more algae, which consume more oxygen as they decompose, which in turn releases even more nutrients. At the same time, large amounts of carbon become trapped in marine sediments, ultimately cooling the planet.

For years, Hülse and Ridgwell have been developing an advanced computer model of Earth's climate system that includes these complex interactions. "This more complete Earth System model does not always stabilize the climate gradually after a warming phase, rather it can overcompensate and cool the Earth far below its initial temperature -- a process that can still take hundreds of thousands of years, however. In the computer model of the study this can trigger an ice age. With the silicate weathering alone, we were unable to simulate such extreme values," explains Dominik Hülse.

Their results suggest that when oxygen levels in the atmosphere were lower, as they were in Earth's distant past, these nutrient feedbacks became stronger and could have driven the severe ice ages that marked early geological history.

As humans today add more CO₂ into the atmosphere, the planet will continue to warm. But according to the scientists' model, it could lead again to a cooling overshoot in the long run. However, the next event will likely be milder, because today's atmosphere contains more oxygen than in the distant past, which dampens the nutrient feedback.

"At the end of the day, does it really matter much if the start of the next ice age is 50, 100, or 200 thousand years into the future?" asks Ridgwell. "We need to focus now on limiting ongoing warming. That the Earth will naturally cool back down is not going to happen fast enough to help us out."

The research received support from the MARUM-based Cluster of Excellence "The Ocean Floor -- Earth's Uncharted Interface." Hülse now aims to use the model to explore how Earth has sometimes rebounded surprisingly quickly from past climate shifts and how the ocean floor played a role in those recoveries.