Forcing and response in simulated 20th and 21st century surface energy and precipitation trends

A simple methodology is applied to a transient integration of the Met Office Hadley Centre Global Environmental Model version1 (UKMO-HadGEM1) fully coupled atmosphere-ocean general circulation model in order to separate forcing from climate response in simulated 20th century and future global mean surface energy and precipitation trends. Forcings include any fast responses that are caused by the forcing agent and that are independent of global temperature change. Results reveal that surface radiative forcing is dominated by shortwave forcing over the 20th and 21st centuries, which is strongly negative. However, when fast responses of surface turbulent heat fluxes are separated from climate feedbacks, and included in the forcing, net surface forcing becomes positive. The nonradiative forcings are the result of rapid surface and tropospheric adjustments and impact 20th century, as well as future, evaporation and precipitation trends. A comparison of energy balance changes in eight different climate models finds that all models exhibit a positive surface energy imbalance by the late 20th century. However, there is considerable disagreement in how this imbalance is partitioned between the longwave, shortwave, latent heat and sensible heat fluxes. In particular, all models show reductions in shortwave radiation absorbed at the surface by the late 20th century compared to the pre-industrial control state, but the spread of this reduction leads to differences in the sign of their latent heat flux changes and thus in the sign of their hydrological responses.