New research suggests that rising global temperatures might paradoxically lead to extreme cooling due to biological and oceanic processes. Scientists have modeled how algae blooms and nutrient cycles could trap carbon and initiate ice ages. While this overshoot is unlikely to aid current climate efforts, it highlights complex Earth system dynamics.
For much of Earth's history, the slow weathering of silicate rocks has been considered the primary mechanism regulating the planet's climate. Rainwater absorbs carbon dioxide from the atmosphere, dissolves rocks, and carries carbon and calcium to oceans, where it forms long-lasting sediments like limestone. 'When the planet warms, rocks weather faster and absorb more CO2, allowing the Earth to cool down again,' explains Dominik Hülse, a researcher at MARUM - Center for Marine Environmental Sciences, University of Bremen.
However, this process alone cannot account for periods when Earth froze completely from pole to pole. A study published in Science reveals that biological and oceanic feedback loops play a critical role. As temperatures rise and atmospheric CO2 increases, more nutrients like phosphorus wash into the seas, spurring algae blooms. These algae absorb carbon through photosynthesis, and upon dying, sink it to the ocean floor.
In warmer conditions, this leads to oxygen depletion as algae decompose, causing phosphorus to recycle rather than bury in sediments. This feedback amplifies: more nutrients fuel more algae, which consume more oxygen and release additional nutrients, trapping vast amounts of carbon and cooling the planet dramatically. Hülse and co-author Andy Ridgwell developed an advanced Earth system model incorporating these 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,' Hülse states. With silicate weathering alone, such extremes were impossible to simulate.
The model indicates that lower atmospheric oxygen in Earth's distant past intensified these feedbacks, driving severe ice ages. Today, higher oxygen levels would temper any future cooling overshoot from human-induced warming. '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, supported by the MARUM Cluster of Excellence, underscores the ocean's role in past climate recoveries.