Earth’s geological carbon cycle is central to both climate and life. It is generally considered to act as a “thermostat,” regulating climate and preventing global mean temperatures from fluctuating wildly. The textbook model of this regulation involves variations in solid Earth degassing rates, leading to changes in atmospheric CO₂ concentrations, surface temperature, and precipitation; in turn, these modulate the rate of alkalinity production via continental silicate weathering, which changes the rate of carbonate mineral formation, thereby rebalancing the carbon cycle. The articles in this issue highlight many alternative or additional mechanisms that may be equally or more important in regulating Earth’s carbon cycle. This issue highlights advances in our understanding of the regulation of the long-term carbon cycle, but also emphasizes the large uncertainties that still remain for both the modern day and in the past: what is the rate of carbon degassing? What are the relative roles of continental and seafloor weathering in regulating the carbon cycle? What fraction of the alkalinity added to the ocean plays a role in the carbon cycle? What role does organic carbon play in the carbon cycle? We are still far from having a complete understanding of the fundamental controls of Earth’s life-support system, but we clearly need to move beyond the textbook model.

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