Fatal Flaw in Carbon Cycle Could Plunge Earth into Global Freeze

By U. of California - Riverside, Nov 6, 2025
https://scitechdaily.com/fatal-flaw-in-carbon-cycle-could-plunge-earth-into-global-freeze/

A new study reveals that Earth’s climate “thermostat” may sometimes overreact, turning global warming into an unexpected deep freeze. Credit: Shutterstock

Scientists have uncovered a hidden feedback mechanism in Earth’s carbon cycle that could explain past ice ages and reveal how global warming might eventually lead to extreme cooling.

UC Riverside scientists have uncovered a key element missing from earlier explanations of how Earth recycles its carbon. Their findings suggest that global warming could eventually swing in the opposite direction, tipping the planet into an ice age.

For decades, scientists have believed that Earth’s climate is balanced by a slow yet dependable process called rock weathering.

In this process, rain absorbs carbon dioxide from the atmosphere and reacts with exposed rocks on land (especially silicate rocks such as granite), gradually dissolving them. The rainwater then carries the captured CO2 and dissolved minerals into the ocean. There, the carbon combines with calcium to create seashells and limestone reefs, which trap the carbon on the seafloor for millions of years.

“As the planet gets hotter, rocks weather faster and take up more CO2, cooling the planet back down again,” said Andy Ridgwell, UCR geologist and co-author of the paper published in Science.

Yet, geological records reveal that some ancient ice ages were so severe that nearly the entire planet froze over. According to the researchers, such extreme events cannot be explained by a simple, self-regulating system alone.

The missing factor involves the burial of carbon in the ocean. When atmospheric CO2 rises and global temperatures climb, increased rainfall carries more nutrients such as phosphorus into the sea. These nutrients boost the growth of plankton, which absorb carbon dioxide through photosynthesis. After the plankton die, their remains sink to the ocean floor, carrying the carbon with them and altering the planet’s long-term climate balance.
A Runaway Cooling Feedback

However, in a warmer world with more algal activity, oceans lose oxygen, causing phosphorus to get recycled instead of being buried. This creates a feedback loop where more nutrients in the water create more plankton, whose decay removes even more oxygen, and more nutrients get recycled. At the same time, massive amounts of carbon are buried, and the Earth cools.

Computer simulation of Earth’s climate evolving over one million years in response to a sudden release of carbon dioxide to the atmosphere. 

Credit: Andy Ridgwell/UCR

This system doesn’t gently stabilize the climate, but instead overshoots, cooling Earth far below its starting temperature. In the study’s computer model, this could trigger an ice age.

Ridgwell compares all this to a thermostat working overtime to cool a house.

“In summer, you set your thermostat around 78°F. As the air temperature climbs outside during the day, the air conditioning removes the excess heat inside until the room temperature comes down to 78° and then it stops,” Ridgwell said.
 

When Earth’s Thermostat Overreacts

Using his analogy, Earth’s thermostat isn’t broken, but Ridgwell suggests it might not be in the same room as the air conditioning unit, making performance uneven.

In the study, lower atmospheric oxygen in the geological past made the thermostat much more erratic, hence ancient extreme ice ages.

As humans add more CO2 to the atmosphere today, the planet will continue to warm in the short term. The authors’ model predicts a cooling overshoot will occur. However, the next one will likely be milder because there is more oxygen in the atmosphere now than in the distant past, which dampens the nutrient feedback.

“Like placing the thermostat closer to the AC unit,” Ridgwell added. Still, it could be enough to bring forward the start of the next ice age.

“At the end of the day, does it matter much if the start of the next ice age is 50, 100, or 200 thousand years into the future?” Ridgwell wondered. “We need to focus now on limiting ongoing warming. That the Earth will eventually cool back down, in however wobbly a way, is not going to happen fast enough to help us out in this lifetime.”

 

 

 

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