Open Issues

Whether radiative flux perturbations exhibit the same additivity as practiced for traditional radiative forcings remains unknown. The relationships between flux perturbations and traditional forcings for greenhouse gases and aerosols need to be quantified across the ensemble of climate models used for IPCC.

To calculate forcing (as precisely defined), GCM simulations using a double-call to the radiation scheme need to run typically for only one year in order to sample the full seasonal cycle. In contrast, simulations to calculate the flux perturbation must last considerably longer, typically from 5-10 years. This is because the meteorology usually differs between control and perturbed simulations in calculating the flux perturbation, and thus one has to run the simulations for a number of years to obtain a good signal-to-noise ratio.

One difficulty with the flux perturbation is that usually only SSTs and sea-ice extents are prescribed. This means that although feedbacks via surface temperature over ocean areas are eliminated, those via changes in land-surface temperature are not; neither are other land-surface responses, such as changes in soil moisture or snow cover. Ideally, land-surface temperatures would also be prescribed, but the complex formulation of land-surface parameterization schemes in current GCMs makes this difficult and may not be possible for all models. Thus, we suggest the flux perturbation in the form of "fixed-SST forcing."

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