Yet Earth's history shows extreme phases, when ice sheets covered the entire globe, that cannot be explained by silicate weathering alone. Researchers argue that additional processes in the oceans must have driven these dramatic cooling events.
The missing link lies in carbon burial at the seafloor. When atmospheric CO2 rises and global temperatures climb, nutrient runoff such as phosphorus increases. This fuels algae blooms, which take in carbon through photosynthesis. As algae die and sink, carbon is carried into marine sediments.
But oxygen loss in warmer oceans destabilizes this process. Rather than locking phosphorus into sediments, low oxygen recycles it into the water. This creates a powerful feedback: more nutrients drive more algae growth, more decomposition depletes oxygen, and more nutrients are recycled. Large quantities of carbon become buried in seafloor sediments, driving global cooling.
Hulse and colleague Andy Ridgwell integrated these nutrient-carbon-oxygen feedbacks into advanced Earth System models. The simulations revealed that climate recovery after warming does not always stabilize gradually. Instead, the system can overshoot, cooling far below its starting state and triggering ice ages over hundreds of thousands of years. "With the silicate weathering alone, we were unable to simulate such extreme values," says Hulse.
The model also suggests that low atmospheric oxygen levels in early Earth history amplified these nutrient cycles, explaining why ancient "Snowball Earth" events occurred.
Although modern CO2 emissions will continue to warm the planet, the long-term model indicates a possible cooling overshoot in the distant future. However, today's oxygen-rich atmosphere likely weakens the nutrient feedback, making any future ice age milder than past extremes.
"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 work was supported by the MARUM-based Cluster of Excellence "The Ocean Floor - Earth's Uncharted Interface." Hulse now plans to apply the model to investigate how Earth sometimes rebounded quickly from past climate disruptions and the role of marine sediments in that recovery.
Research Report:Instability in the geological regulation of Earth's Climate
Related Links
MARUM - Center for Marine Environmental Sciences, University of Bremen
Beyond the Ice Age
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