DLS – Colin Goldblatt

Earth’s long term climate stabilized by clouds
 
The Sun was dimmer earlier in Earth history, but glaciation was rare in the Precambrian: this is the “Faint Young Sun Problem”. Most solutions rely on changes to the chemical composition of the atmosphere to compensate via a stronger greenhouse effect, whilst physical feedbacks have received less attention. 
We perform Global Climate Model experiments, using two versions of the Community Atmosphere Model, in which a reduced solar constant is offset by higher ce{CO2}.  Model runs corresponding to past climate show a substantial decrease in low clouds and hence planetary albedo compared to present, which contributes 40% of the required forcing to offset the faint Sun. Through time, the climatically important stratocumulus decks have grown in response to a brightening Sun and decreasing greenhouse effect, driven by stronger cloud-top radiative cooling (which drives low-cloud formation) and a stronger inversion (which sustains clouds against dry air entrainment from above). 
We find that systematic changes to low clouds have had a major role in stabilizing climate through Earth’s history, which demonstrates the importance of physical feedbacks on long-term climate stabilization, and a smaller role for geochemical feedbacks.