Robust Land-Ocean Contrasts in Energy and Water Cycle Feedbacks

Building on recent observational evidence showing disproportionate increases in temperature and aridity over land in a warming climate, this study examines simulated land-ocean contrasts in fully coupled projections from the Third Coupled Model Intercomparison Project (CMIP3) archive. In addition to the projection of disproportionate changes in temperature and moisture over land, the analysis reveals contrasts in clouds and radiative fluxes that play a key role in the eventual equilibration of the planetary energy budget in response to forcing. Despite differences in magnitude, the nature of the feedbacks governing the land-ocean contrast are largely robust across models, notwithstanding the large intermodel differences in cloud parameterizations, and suggest the involvement of fundamental constraints. The model responses are consistent with previously proposed ideas maintaining that relative humidity (RH) over land decreases with warming because precipitation and the hydrological cycle are governed primarily by transports of moisture from the oceans, where increases in lower-tropospheric temperature and saturated humidity fail to keep pace with those over land. Here, it is argued additionally that constraints on RH imply systematic changes in the cloud distribution and radiative feedbacks over land, as decreased RH raises the lifting condensation level, even as moist instability increases, and suppresses convective clouds. This effect is shown to be particularly strong at low latitudes where the dynamical influence of competing sources of maritime deep convection may further suppress convection. It is found that as a result of the coincidence between strong warming and a muted net greenhouse feedback associated with decreases in RH and clouds, the mean increase in outgoing longwave radiation (OLR) over land (1.0 W m(-2) K-1) in transient simulations at 2200 is almost double that over the ocean (0.6 W m(-2) K-1), and a strong negative net top-of-atmosphere (TOA) radiative perturbation emerges as the simulations approach and attain equilibrium. However, over the oceans a positive radiative imbalance persists and the increase in water vapor and other greenhouse gases does not allow a local TOA equilibration to occur. The contrast results in an increase in the transport of energy from ocean to land relative to the twentieth century that is accompanied by lasting increases in both OLR and absorbed shortwave radiation globally. A conceptual model to describe the simulated variability is proposed that involves the following: 1) the differing albedos and lower-tropospheric lapse rates over land and ocean, 2) the nonlinearity of the saturated lapse rate in a warming environment, and 3) the disproportionate response in temperature, moisture, clouds, and radiation over land versus ocean. It is noted that while the land-ocean contrast plays a key role in achieving global radiative equilibrium, it entails disproportionate increases in temperature and aridity over land and therefore is likely to be associated with substantial environmental impacts.