Research conducted by scientists from the MARUM Center for Marine Environmental Sciences at the University of Bremen shows that global warming could unexpectedly lead to glaciation.
The Earth's climate is maintained in balance not only through the slow weathering of silicate rocks, which act as a carbon reservoir, but also through the interaction of biological and oceanic cycles. In particular, the role of algae, oxygen, and phosphorus turns out to be much more significant than previously thought.
For a long time, scientists believed that the primary mechanism regulating the climate was the weathering of rocks. During this process, rain absorbs carbon dioxide from the atmosphere, which then dissolves exposed rocks. The released carbon and calcium enter the ocean and serve as the basis for the formation of shells and limestone reefs, which store carbon in marine sediments for millions of years.
“The warming of the planet accelerates the weathering of rocks, which in turn leads to increased CO₂ absorption and cooling of the Earth,” explains researcher Dominik Hülse.
However, throughout Earth's geological history, there have been events when the planet completely froze. Researchers emphasize that this cannot be explained solely by weathering, and that other factors exist for such drastic temperature fluctuations.
A key point is how carbon is stored in the oceans. With increasing CO₂ concentration in the atmosphere and warming waters, more nutrients, such as phosphorus, enter the sea. These substances promote the growth of algae, which absorb carbon during photosynthesis. When algae die, they settle to the ocean floor, carrying carbon with them.
However, under conditions of elevated water temperatures, the active growth of algae can lead to oxygen depletion. In conditions of oxygen scarcity, phosphorus is recycled rather than sequestered, thus creating a closed cycle: an excess of nutrients causes an increase in algae, which, when decomposing, consume even more oxygen, further enhancing nutrient release. As a result, carbon remains in marine sediments, contributing to the cooling of the planet.
Hülse and his colleague Andy Ridgwell are working on creating a complex computer model of the climate system that accounts for all these interactions.
“Our model shows that the climate does not always stabilize after warming. On the contrary, it can cool to values significantly below the initial ones, although this process may take hundreds of thousands of years. Within our model, this could lead to the onset of an ice age. Using only silicate weathering, we were unable to model such extreme changes,” adds Hülse.
The results of the study indicate that during periods of low atmospheric oxygen, as was the case in ancient times, such cycles could lead to severe ice ages that shaped the early geological history of the Earth.
With increasing CO₂ emissions into the modern atmosphere, the planet will continue to warm. However, according to the scientists' model, this could ultimately lead to a new cooling in the long term, although it is likely to be less abrupt since today's oxygen levels in the atmosphere are higher than in the past, which weakens the described cycles.
“Is it important whether the next ice age begins in 50, 100, or 200 thousand years?” Ridgwell questions. “Right now, we should focus on limiting current warming. Natural cooling will not happen quickly enough to help us.”
The research is supported by the "Ocean Floor - An Unexplored Interface of the Earth" cluster of excellence based at MARUM. Hülse intends to use his model to study how the Earth may have recovered after past climate changes and what role the ocean floor played in this.
