Earth's climate hides regulatory mechanisms that could hold surprises. Researchers have just discovered a little-known process in the carbon cycle capable of turning global warming into extreme cooling.
Rock weathering constitutes one of the main natural regulators of climate. When rain falls on silicate rocks like granite, it gradually dissolves their surface and captures carbon dioxide present in the atmosphere. This water, loaded with CO₂ and minerals, then flows towards the oceans, where the carbon participates in the formation of shells and coral reefs. These structures trap carbon on the seafloor for millions of years, thus helping to cool the planet.
Scientists at
UC Riverside have identified a crucial missing element in this traditional view. Their work shows that significant warming alters ocean behavior: increased precipitation brings more nutrients like phosphorus, which stimulate plankton growth. These microorganisms then massively absorb CO₂ through photosynthesis, and after their death, their remains sediment to the ocean floor, taking the carbon with them.
This process triggers a self-amplifying feedback loop. This proliferation accelerates deoxygenation and carbon burial, creating a cumulative effect that can tip the climate toward extreme cooling.
Computer models reveal that this system doesn't just gently stabilize temperatures, but tends to amplify the phenomenon in the opposite direction. Andy Ridgwell, a geologist and co-author of the study published in
Science, compares this phenomenon to a thermostat that works too efficiently, cooling the house well below the desired temperature. This overactivity would explain the extreme glaciations of the geological past, where the planet was almost entirely covered in ice.
The current situation, however, differs from ancient conditions. The higher concentration of oxygen in the atmosphere partially attenuates this feedback loop, as if the thermostat were better positioned relative to the air conditioning system. Although future cooling is expected, it will likely be less abrupt than during past ice ages. The researchers emphasize that this natural mechanism will not protect us from the immediate consequences of current global warming.
Computer simulation showing the evolution of Earth's climate over one million years following a sudden release of carbon dioxide into the atmosphere.
Credit: Andy Ridgwell/UCR Silicate rock weathering
This slow but constant geological process plays a fundamental role in regulating Earth's climate over millennial timescales. Silicate rocks like granite or basalt chemically react with slightly acidic rainwater, which has previously absorbed atmospheric carbon dioxide.
This reaction produces bicarbonate and silicate ions that are transported by rivers to the oceans. Once in the marine environment, these compounds participate in the formation of calcium carbonates that make up the skeletons of marine organisms like corals, mollusks, and calcifying plankton.
The efficiency of this process depends on several environmental factors. Temperature directly influences the speed of chemical reactions: a warmer climate accelerates weathering, while vegetation cover and soil biological activity can either protect the rocks or, conversely, promote their degradation through the production of organic acids.
This natural mechanism has helped maintain relatively stable climatic conditions for millions of years, partially counterbalancing the increase in the greenhouse effect due to volcanic activity and other natural sources of CO₂.
The ocean biological pump
The oceans constitute the most important active carbon reservoir on our planet, thanks to the combined action of plant plankton and ocean currents. Phytoplankton, composed of microscopic algae, absorbs carbon dioxide dissolved in surface water through the process of photosynthesis, similar to that of land plants.
When these organisms die or are consumed by zooplankton, part of the organic matter produced sediments toward the ocean depths. This 'marine snow' transports carbon to the deep layers where it can remain isolated from the atmosphere for centuries, even millennia.
The efficiency of this biological pump closely depends on the availability of nutrients like nitrogen, phosphorus, and iron. These essential elements, brought by rivers or upwelling deep water, often limit phytoplankton growth in vast ocean regions.
Current climate changes are disrupting this delicate system. The warming of surface waters reduces their mixing with nutrient-rich deep waters, while ocean acidification affects the ability of some organisms to build their calcareous shells.