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Effective Radiative Forcing in a GCM With Fixed Surface Temperatures

Timothy Andrews, Christopher J. Smith, Gunnar Myhre, Piers M. Forster, Robin Chadwick, Duncan Ackerley

Effective radiative forcing (ERF) is evaluated in the ACCESS1.0 General Circulation Model (GCM) with fixed land and sea-surface-temperatures (SST) as well as sea-ice. The 4xCO2 ERF is 8.0 W m−2. In contrast, a typical ERF experiment with only fixed SST and sea-ice gives rise to an ERF of only 7.0 W m−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 W m−2) and troposphere (−0.90 W m−2), (ii) reduced land snow-cover and albedo (+0.17 W m−2), (iii) increased water-vapor (+0.49 W m−2), and (iv) a cloud adjustment (−0.26 W m−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 W m−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.

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