An Assessment of Radiative Feedbacks in Coupled Ocean-Atmosphere Models

We compare the climate feedbacks in coupled ocean-atmosphere models using a coordinated set of 21st century climate change experiments. Water vapor is found to provide the largest positive feedback in all models and its strength is consistent with that expected from constant relative humidity changes in water vapor mixing ratio. The feedbacks from clouds and surface albedo are also found to be positive in all models, while the only stabilizing (negative) feedback comes from the temperature response. Large intermodel differences in the lapse-rate feedback are observed and shown to be associated with differing regional patterns of surface warming. Consistent with previous studies, we find the vertical changes in temperature and water vapor to be tightly coupled in all models and, importantly, demonstrate that intermodel differences in the sum of lapse-rate and water vapor feedbacks are small. On the other hand, intermodel differences in cloud feedback are found to be a factor of 4 larger than any other radiative feedback process and provide the largest source of uncertainty in current predictions of climate sensitivity.