Climate sensitivity studies primarily focus on the energy budget at the tropopause or top of the atmosphere, but the surface energy budget also has a profound influence on tropospheric temperatures and the strength of the hydrological cycle under greenhouse gas forcing. Latent heat flux is a key player in this budget because it balances the radiative forcing and also controls the strength of the water vapor feedback. According to the Clausius-Clapeyron equation, one might expect the global mean precipitation and evaporation to increase at a rate of 7% per Kelvin surface warming, but the IPCC AR4 models actually simulate a muted response (~2% K^-1) to the water vapor content increase (~7% K^-1), with a range of 0.6% K^-1 to 2.5% K^-1. This muted response of the hydrological cycle may contribute to the amount and uncertainty of warming predicted.

According to the standard bulk formula, evaporation increases with sea surface temperature (SST) but is also controlled by atmospheric parameters including surface wind speed, surface relative humidity (SRH), and air-sea temperature difference. In the IPCC AR4 simulations these atmospheric surface parameters reduce the response of the hydrological cycle from 7% K^-1 to 2% K^-1, with a 1% increase of SRH during the 21st century contributing 1/3 of this reduction.

In the present study, we use the Community Atmosphere Model 3.0 coupled to a slab ocean model to conduct a set of equilibrium sensitivity tests, in which latent heat flux calculations over the oceans use SRH prescribed from either present day or CO₂ doubling climatology. The results show that a 1% increase in RH contributes 0.49 K to the SST warming, or 23% of the total warming due to CO₂ doubling (2.11 K). This suggests an SRH feedback, in which global warming-induced SRH increase inhibits latent heat release, which in turn warms the ocean. When increased SRH is prescribed, strong surface warming occurs in subtropical stratocumulus regions. This constitutes a positive shortwave radiation feedback, in which SRH-induced SST warming reduces low cloud fraction and increase surface insolation. On the other hand, SST warming and low cloud reduction act to increase surface longwave output and thus nearly balance the enhancement of solar insolation globally, but not locally. In the stratocumulus regions, solar enhancement dominates.