The geological regulation of Earth's climate over timescales of hundreds of thousands of years is a key topic in the natural sciences, with implications for the evolution of life and the long-term impacts of anthropogenic carbon emissions. The prevailing view is that climate stabilization occurs through negative, stabilizing feedback – a planetary climate “thermostat” – where climate-sensitive weathering of silicate rocks on land removes atmospheric carbon dioxide.

Although this mechanism acts as a primary regulation, Dominik Hülse and Andy Ridgwell presented model results that suggest that this stabilization may be surpassed by faster processes. The episodic occurrence of extreme cooling and "Snowball Earth" events during the Precambrian (539 million years ago) suggests that effective regulation may have failed in the past. The historical occurrence of extensive organic carbon deposition suggests the operation of another planetary CO2 removal mechanism and potentially a second thermostat.

The authors focused on the nature of the interaction between weathering and organic carbon deposition, which was previously unknown due to the lack of suitable global carbon cycle models. They used an efficient model that already included a silicate weathering thermostat and added key organic geological processes, such as the release of CO2 from kerogen weathering and its removal by the deposition of organic carbon in marine sediments. They also included phosphorus (P), which regulates nutrient availability to marine plankton and thus organic carbon production and deposition.

The findings show that the organic carbon-based thermostat may become more sensitive and override the silicate weathering thermostat, thereby coming to dominate long-term climate regulation. The model simulated the response to an initial massive release of CO2 and global warming perturbations. In a warmer and more productive ocean, the solubility of oxygen O2 decreases and the rate of organic matter decomposition increases, leading to spread of anoxia on the seabed.

Under these anoxic conditions, phosphorus (P) is recycled back into the ocean more efficiently than being buried with carbon. This redox-sensitive phosphorus recovery process enhances sedimentary burial of organic carbon, leading to even higher productivity and lower O2 levels. As a result, CO2 declines faster than phosphorus, and the global surface climate cools below its initial value.

This rapid feedback creates unexpected climate instability. Over a period of a few hundred thousand years, an "overcooling" occurs. The extent of this overcooling depends on the initial state of the C and P cycles and, at lower atmospheric O2 levels (60 % of present levels), can exceed 6 °C, which is more than the global temperature difference between today and the Last Glacial Maximum. This instability depends on the initial balance between the two thermostats.

Supercooling is strongest in transitional states of ocean and atmospheric oxygenation and offers a causal link between major transitions in oxygenation during the Precambrian and the occurrence of extreme snowball-type cooling events on Earth. The existence of a critical instability in geological climate regulation is a consequence of the presence of microbial lifeResearch by Hülse and Ridgwell (Science, 2025) suggests that a new perspective on how Earth's thermostats work is needed. JRi

Article published in in the journal Science .