Cloud Optical Depth Feedbacks and Climate Modelling

Recent general circulation model studies1–3 performed to assess the equilibrium climate response to doubling atmospheric CO2 suggested a global mean surface warming of 3.5–4.2 °C. Part of this warming was attributed to a change in cloud cover1. But all of these studies neglected changes of cloud optical properties which were shown to provide a substantial negative feedback in radiative-convective models if the cloud liquid water content was assumed to increase with increasing temperature4,5. This hypothesis is examined in a climate model where clouds are simulated interactively with dynamics, radiation and hydrological cycle6. The thermal forcing is introduced by a 2% increase of the solar constant which is equivalent to a doubling of CO21. The results show the anticipated increase of cloud liquid water and cloud optical depth. A feedback analysis of the simulated climate change supports earlier suggestions of the importance of cloud optical depth feedbacks4,5. The net effect of clouds is to provide a negative feedback on surface temperature, rather than the positive feedback found in earlier general circulation model studies without considering cloud optical depth feedbacks1.