Investigating the land-ocean surface warming contrast in simulations of climate change with an idealized GCM

Climate models robustly predict that surface temperatures will increase more rapidly over land than ocean under global warming, an effect known as the “land-ocean warming contrast”. The enhanced warming over land is not solely a transient effect due to the different thermal inertias of the land and ocean, but rather it is a fundamental response of the climate system that persists in the equilibrium response of the system. It has previously been proposed that the warming contrast arises as a result of unequal changes in the lapse rates over land and ocean, related to the difference in surface moisture availability. We evaluate this mechanism over a wide range of climates using simulations with an idealized GCM. The land surface hydrology in the model is a simple bucket model, and we prescribe various zonal and meridional bands of land. The warming contrast is confined to low and middle latitudes in the model. At low latitudes and in cold or moderately warm climates, the warming contrast is well described by a theory based on moist adiabatic lapse rates, although care must be taken to allow for variability in low level humidity over land. In very warm climates, the assumption of moist adiabatic lapse rates becomes inadequate over land. Consequently, a full understanding of the warming contrast is made difficult by the need for a theory of lapse rates in both tropical and extratropical dynamical regimes and in arid and moist environments.