Earth has a built-in climate regulator that recycles carbon dioxide, but new research from the University of California, Riverside shows this system may sometimes work too well.
Instead of gently balancing the planet’s temperature, it can overshoot, pushing Earth into extreme ice ages.
Normally, scientists believe Earth’s climate stability is thanks to a process called rock weathering.
When rain falls, it captures carbon dioxide from the air and washes over rocks, particularly silicate rocks like granite.
This chemical reaction dissolves the rocks, releasing calcium that flows into the oceans.
There, the calcium and carbon dioxide form seashells and reefs, which eventually settle on the seafloor as limestone, locking away carbon for millions of years.
This “slow carbon cycle” acts like a thermostat. When the Earth warms, rocks weather faster, pulling more CO₂ out of the atmosphere and cooling things down again.
But this gentle picture doesn’t explain why Earth’s past ice ages were so severe—at times covering the entire planet in snow and ice.
The missing factor, researchers say, is how carbon is buried in the oceans. In warmer conditions with more CO₂, extra nutrients like phosphorus wash into the sea, fueling huge blooms of plankton.
These microscopic organisms absorb carbon dioxide as they photosynthesize, and when they die, they sink to the bottom, carrying carbon with them.
Here’s where the problem begins. A surge in plankton growth reduces oxygen in the oceans. Low oxygen causes phosphorus to recycle back into the water instead of being buried.
This fuels even more plankton, creating a powerful feedback loop: more nutrients, more plankton, more oxygen loss, and more carbon buried. The end result is a dramatic cooling effect that can tip the planet into an ice age.
Geologist Andy Ridgwell, a co-author of the study published in Science, likens it to a thermostat that works too hard.
“It’s like your air conditioner in summer. You set it at 78°F, and it cools your house until it reaches that point and then stops. But imagine if the thermostat wasn’t in the same room as the AC—it might overdo it, cooling far more than intended.”
In Earth’s distant past, when oxygen levels in the atmosphere were lower, this system was even more unstable, leading to the “Snowball Earth” ice ages.
Today, with higher oxygen, the feedback loop is weaker, but the model still predicts that as humans add more CO₂, the eventual cooling overshoot could happen sooner, potentially bringing forward the start of the next ice age.
Still, Ridgwell stresses that this won’t happen on human timescales. “Whether the next ice age begins in 50,000 or 200,000 years, it doesn’t change our present challenge,” he said.
“The urgent task now is slowing the warming we are causing. The planet will eventually cool, but not in time to save us from the climate crisis we face today.”
