Cloud-Radiative Feedbacks Driven by Changes in Radiative Balance: A Comparative Study Between Past, Present and Future Climates

Clouds are important for regulating Earth's climate due to their influence on Earth's radiation budget; their reflective shortwave and absorptive longwave properties help regulate Earth's global temperature. Less reflected sunlight from clouds has been proposed as a partial solution to the Faint Young Sun Paradox, helping maintain liquid water on Earth's surface during the Archean eon, when the solar constant was low. Here we focus on quantifying cloud-radiative feedbacks that occur due to CO₂ doubling under different background climate states. In a GCM we vary background solar insolation incrementally from 90% to 105% of the present-day value, and apply an offsetting CO₂ concentration for each case for the purpose of maintain the present global mean surface temperature. We do this to reveal the effects of varying radiative forcing alone, ignoring any changes directly tied to thermodynamic processes. The changes in physical characteristics of clouds (i.e. cloud fraction, optical depth, albedo, etc.) after CO₂ doubling are then contrasted against that of a present-day climate to reveal the character and intensity of any cloud-radiative feedbacks that may arise.