How rock weathering sets Earth’s thermostat

Since its formation 4.5 billion years ago our planet has liquid water at its surface. If all water were frozen as on the colder Mars, or if all water had evaporated into space as on the hot Venus, there would be no life on Earth.  We owe Earth’s habitability to a fascinating geologic climate regulation. Every year, processes of “weathering”, i.e. the reaction between CO₂ dissolved in rain with rocks, withdraw roughly the same amount of CO₂ that is added to the atmosphere by volcanic emissions. As a result, thanks to the greenhouse effect of CO₂, Earth’s surface temperature is kept “just right”, similar to a thermostat.

Earth-historical reconstructions show that this delicate thermostatic balance has shifted in the last 15 million years. The stable isotopes of oxygen (¹⁸O/¹⁶O) show that Earth became colder, eventually entering a state in which it has a glaciated northern hemisphere. The isotopes of boron (¹¹B/¹⁰B) reveal the reason for this cooling: atmospheric CO₂ decreased, leading to a reduced greenhouse effect. For years, Earth scientists have debated on the cause of decreasing CO₂. One hypothesis is that the rise of large mountain belts like the Himalayas or the Andes would have led to increased chemical weathering because more rocks got to the surface and got eroded – withdrawing higher amounts of CO₂ from the atmosphere. However, this hypothesis is challenged by measurements of the ratio of the cosmogenic isotope ¹⁰Be to the stable isotope ⁹Be released from rocks by weathering (¹⁰Be/⁹Be), which instead suggests constant weathering over the same period. In this case, the land surface would have gotten more “reactive” to chemical weathering, such that the same amount of CO₂ is withdrawn with time even with decreasing atmospheric CO₂ concentration, an alternative confirmed by the stable isotopes of lithium (⁷Li/⁶Li). Over its geological history, Earth has experienced many shifts in climate, each time adjusting progressively its thermostat. Today a dramatic shift is imminent, because humans are massively interfering with this delicate equilibrium through industrial CO₂ emissions.