Water vapour feedback amplifies high latitude warming

When the climate system is forced by a doubling of the atmospheric CO₂ content, a number of feedback processes are induced, such as changes of water vapour, clouds and surface albedo. Here the CO₂ forcing and concomitant feedbacks are studied individually using a general circulation model coupled to an aqua-planet mixed-layer ocean. A technique for fixing the radiative effects of moisture and clouds by re-using these variables from 1×CO₂ and 2×CO₂ equilibrium climates in the model's radiation code allows for a detailed decomposition of the problem in terms of forcings, feedbacks and their responses. The cloud feedback is in this model found to give only a small global average effect and surface-albedo feedbacks have been eliminated. As in previous studies, the water-vapour radiative feedback is found to approximately double the climate sensitivity, but while its radiative effect is strongly amplified at low latitudes, the resulting climate response displays about the same degree of polar amplification as the full all-feedbacks experiment. In fact, the atmospheric energy transports are found to change in such a way as to yield the same meridional pattern of response as when the water-vapour feedback is turned off. The analysis further allows for a study of the vertical profiles of responses associated with the individual forcings and feedbacks. It is found that although CO₂ forcing and water-vapour radiative feedback are not surface processes, their associated high-latitude temperature changes are greatest near the surface. This implies that a surface-amplified warming is not necessarily due to surface-based feedbacks.