Abstract
Effective radiative forcing (ERF) is evaluated in the ACCESS1.0 General
Circulation Model (GCM) with fixed land and sea-surface-temperatures as
well as sea-ice. The 4xCO2 ERF is 8.0 Wm-2. In contrast, a typical ERF
experiment with only fixed sea-surface-temperatures (SST) and sea-ice
gives rise to an ERF of only 7.0 Wm-2. This difference arises due to the
influence of land warming in the commonly used fixed-SST ERF
experimental design, which results in: (i) increased emission of
longwave radiation to space from the land surface (-0.45 Wm-2) and
troposphere (-0.90 Wm-2), (ii) reduced land snow-cover and albedo (+0.17
Wm-2), (iii) increased water-vapour (+0.49 Wm-2), and (iv) a cloud
adjustment (-0.26 Wm-2) due to reduced stability and cloudiness over
land (positive ERF) counteracted by increased lower tropospheric
stability and marine cloudiness over oceans (negative ERF) . The sum of
these radiative adjustments to land warming is to reduce the 4xCO2 ERF
in fixed-SST experiments by ~1.0 Wm-2. CO2 stomatal
effects are quantified and found to contribute just over half of the
land warming effect and adjustments in the fixed-SST ERF experimental
design in this model. The basic physical mechanisms in response to land
warming are confirmed in a solar ERF experiment. We test various methods
that have been proposed to account for land warming in fixed-SST ERFs
against our GCM results and discuss their strengths and weaknesses.