The impact of stochastic physics on climate sensitivity in EC-Earth

TL;DR

This study shows that incorporating stochastic physics into the EC-Earth climate model significantly reduces projected global warming by altering cloud feedback mechanisms, highlighting the importance of stochastic schemes in climate projections.

Contribution

First demonstration that stochastic physics schemes can notably influence long-term climate projections and sensitivity in a fully coupled climate model.

Findings

Global warming projection reduced by 10% with SPPT scheme.

SPPT alters cloud feedbacks, reducing positive low cloud cover feedback.

Cloud liquid water increases rapidly due to non-linear interactions with condensation.

Abstract

Stochastic schemes, designed to represent unresolved sub-grid scale variability, are frequently used in short and medium-range weather forecasts, where they are found to improve several aspects of the model. In recent years, the impact of stochastic physics has also been found to be beneficial for the model's long term climate. In this paper, we demonstrate for the first time that the inclusion of a stochastic physics scheme can notably affect a model's projection of global warming, as well as its historical climatological global temperature. Specifically, we find that when including the 'stochastically perturbed parametrisation tendencies' scheme (SPPT) in the fully coupled climate model EC-Earth v3.1, the predicted level of global warming between 1850 and 2100 is reduced by 10% under an RCP8.5 forcing scenario. We link this reduction in climate sensitivity to a change in the cloud feedbacks with SPPT. In particular, the scheme appears to reduce the positive low cloud cover feedback, and increase the negative cloud optical feedback. A key role is played by a robust, rapid increase in cloud liquid water with SPPT, which we speculate is due to the scheme's non-linear interaction with condensation.