Robust Land-Ocean Contrasts in Energy and Water Cycle Feedbacks

Simulations from the CMIP3 archive forced by increasing greenhouse gases reveal a disproportionate influence of land regions on the changes in the planetary energy budget that is robust across models and is enabled by stronger warming over land versus oceans, as well as decreases in relative humidity and cloud amount. During warming, land temperatures increase faster than those of the ocean and this contrast is sustained through equilibrium. However, aridity on land increases because precipitation and the hydrological cycle are governed primarily by transports of moisture onto land that are largely determined by ocean surface temperatures that fail to keep pace with terrestrial warming. A disproportionate increase in outgoing longwave radiation (OLR), particularly at low latitudes, hastens planetary equilibration. As a result of the coincidence between strong warming and a muted net greenhouse feedback, related to clouds and moisture, a negative net top-of-atmosphere (TOA) radiative perturbation emerges over land as the simulations approach and attain equilibrium. In contrast over oceans, a positive radiative imbalance continues and the increased water vapor and other greenhouse gases does not allow local TOA equilibration to occur. The contrast in feedbacks results in an increase in the transport of energy from ocean to land that is accompanied by lasting increases in both outgoing longwave and absorbed shortwave fluxes globally. A conceptual model to describe the simulated variability is proposed that involves: 1) the differing albedos and lower tropospheric lapse rates of land and ocean, 2) the non-linearity of the saturated lapse rate in a warming environment, and 3) the disproportionate response in temperature, moisture, clouds, and radiation over land versus ocean. While the land-ocean contrast plays a key role in achieving overall radiative equilibrium, it does not preclude large temperature changes over land and instead relies explicitly on them.