Is a Low Cloud Signal in Response to CO2 Forcing Potentially Observable in the Satellite Record?

Abstract:

Low-level clouds in the subsidence regime of the subtropical ocean have a net cooling affect on the climate and are an important factor to consider in regards to climate change. This study looks at the cloud fraction output from 6 different CMIP5 global climate models to understand how low-level clouds may respond to increasing CO2 on different time scales. We examine simulations of 1% CO2 increase per year, simulations of 4xCO2 equilibrium change, and simulations employing historical radiative forcing during 1980-2005. In particular, we are interested to determine whether the century-scale transient cloud response is similar to the equilibrium cloud response, and whether simulated cloud changes occurring since 1980 are similar to the century-scale transient cloud response. Although different models produce different cloud responses to increasing CO2, the transient cloud change for a particular model resembles the equilibrium cloud change for that model, indicating internal consistency. Furthermore, if the simulated cloud change since 1980 resembles the century-scale transient cloud change, this would suggest that observed low-level cloud changes over the subtropical ocean since 1980 may be a response to climate change rather than natural variability.