That our planet has had liquid water on its surface for more than than 3.5 billion years is testament to the existence of some type of feedback that keeps the Earth's climate in a narrow range that is habitable for life. For decades, this feedback has been attributed to silicate weathering, whereby silicate minerals chemically weather in the presence of CO2, creating bicarbonate, which is eventually buried as calcium carbonate on the seafloor. In theory, if CO2 becomes too high, this feedback will remove excess CO2, cooling the Earth or, if CO2 becomes too loo, silicate weathering will slow down and remove less CO2, until the planet warms again. However, there is little direct evidence for the existence of this feedback.
With Kate Maher, Kimberly Lau, and Adam Jost, I have been using carbon cycle data from the past 65 million years to determine if a negative feedback exists. Remarkably, our results (Caves et al., 2014; 2015) indicate that the differences between the input and removal fluxes of CO2 have never varied by more than approximately 5% over the past 65 million years, demonstrating the existence of a strong negative feedback. To explain the long-term decline in CO2 levels over the Cenozoic requires only a 0.25-0.5% sustained increase in organic carbon burial or silicate weathering. Instead, the strength of the silicate weathering feedback has increased over the Cenozoic. This increase in strength results in a more sensitive feedback that can more quickly remove excess carbon from the surface of the Earth; at steady-state, this same process acts to lower the atmospheric CO2 level.
This work is published in Earth and Planetary Science Letters.