The Radiative Flux Perturbation as an Alternative

Standard radiative forcing is computed as the change in radiation fluxes at the tropopause attibutable to an external perturbation, with tropospheric temperature and humidity profiles held fixed, but allowing for fast (order of months) temperature adjustment in the stratosphere. In contrast, radiative flux perturbation allows for rapidly responding tropospheric meteorological fields to adjust as well. The radiative flux perturbation concept does amount to "fixed-SST forcing" (Shine et al. 2003; Hansen et al. 2005; Forster et al. 2007) or "quasi-forcing" (Rotstayn and Penner 2001).

The radiative flux perturbation is likely to yield a more consistent scaling with climate change than forcing. It is also likely to convey more intermodel uncertainty than forcing, thus including some information about the level of scientific uncertainty.

Recent work suggests that one of the consequences of adopting a flux perturbation approach for the assessment of greenhouse gas forcing is that the flux perturbation may include a significant cloud response component (Gregory and Webb 2008; Andrews and Forster 2008). This work further suggests that the cloud-climate feedback, when strictly defined as being the response of clouds to increases in surface temperature, may actually be quite small. Thus, uncertainty in the proportionality between radiative flux perturbation and global mean change in surface temperature would probably be less than for the proportionality between forcing and global mean temperature change. Of course, this inclusion of response to forcing in the radiative flux perturbation does not reduce overall uncertainty in, for example, expected warming at the end of the 21st century.

The proposal to adopt the radiative flux perturbation concept for aerosol radiative impacts implies the convolution of direct and indirect effects, rapid cloud responses, and other rapid climate responses in a single measure. This convolution is similar to the mixing of forcing and response in the actual climate system. Observational estimates of flux perturbations should be based on total derivatives of regional and global planetary albedo with respect to measurable extrinsic aerosol properties, such as optical depth. Since the flux perturbation is based upon fixed SSTs, it might be useful to composite oceanic observations with similar spatial patterns of SST but varying measures of aerosol loading. Since the flux perturbation includes the radiative adjustment of the troposphere to anthropogenic aerosol perturbations, it may be necessary to combine observations over the appropriate radiative dynamic timescales.

Aerosol indirect effects other than the Twomey effect are difficult to define. The distinction between first, second, and semi-direct effects is obsolete, and many more cloud-aerosol interaction "effects" may be defined. Thus, the replacement of various ill-defined aerosol indirect effects by just the combined radiative flux perturbation quantification appears to be an advantage of this concept.

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