High concentrations of atmospheric CO2, about three times the current levels, could trigger the break-up of stratocumulus cloud decks, according to a modelling study published this week in Nature Geoscience. The break-up of these cloud decks could lead to increased global warming of up to 8 °C, in addition to the warming from rising CO2levels.

Stratocumulus cloud decks cover around 20% of Earth’s low-latitude oceans and are important components of the Earth’s energy balance. Unlike other clouds, they are sustained by cooling at the cloud top, rather than heating at the Earth’s surface, leaving them potentially vulnerable to rising levels of greenhouse gases in the Earth’s atmosphere.

Tapio Schneider and colleagues present high-resolution simulations that resolve the most energetic cloud-scale processes of stratocumulus clouds in conditions of rising atmospheric CO2 levels. These cloud-scale processes are often too small to be resolved in most climate models. In the authors’ simulations, cloud decks break up when CO2 concentrations rise above 1,200 parts per million, with severe consequences for global mean surface temperature. Moreover, once broken up, the cloud decks re-form only at much lower levels of greenhouse gases.

The authors suggest that the break-up of stratocumulus cloud decks may have contributed to past hothouse climates, such as the Eocene, 50 million years ago.