It is helpful to frame the Jacobson results on white roofs in terms of direct and indirect effects. The unambiguous effect of white roofs is to increase the direct radiative forcing of any aerosols above. To be precise, brighter surfaces increase absorption by overlying aerosols and thus increase net positive, or reduce net negative, direct radiative forcing by tropospheric aerosols.
This alteration of the direct forcing of overlying aerosols is in addition to, and of the opposite sign as, the direct albedo forcing by the brighter surface. Brighter surfaces (including snow) always have this two-sided radiative effect. Regarding Oliver's question about whether people have looked at this in terms of how dark aerosols above brightened clouds may degrade the cooling effects of the clouds: All studies of cloud-brightening that include interactive black carbon (BC) aerosol have already accounted for the compensating effects of BC heating. The direct heating effect of BC (i.e., warming by BC itself), and its semi-direct effect on clouds (i.e., changes in cloud existence/properties/lifetime due to BC) are automatically accounted for by most climate models. This includes the studies from Phil Rasch's group. These days it is actually more difficult to perform an experiment that neglects cloud/aerosol direct and semi-direct effects. BC aerosol tends to be trapped in the boundary layer where it is emitted, and very little BC lofts above clouds (this is why atmospheric BC has smaller forcing efficacy than most other aerosols and gases). Hence BC has negligible impact on the cooling from cloud-brightening. This is doubly true because sensible cloud-brightening strategies, like Salter/Latham, aim for pristine clouds. These are, almost by definition, predominantly in regions anti-correlated with BC. Two more points worth making about the Jacobson results: 1. It is not a corallary to the Jacobson result that putting snow beneath dark aerosols will warm climate. Snow, like roofs, is bright, so it is understandable why one might think this. However snow, unlike roofs, melts when you warm the overlying atmosphere. So BC unambiguously and in all models (including Jacobson's) destroys snow. 2. That the _net effect_ of the brighter roofs is warmer climate is a model-dependent response to the direct, semi-direct, and indirect radiative forcing of BC. Different models will have different responses to these effects. Crop-brightening experiments show that brighter fields cool climate (in the presence of current distributions of BC, which are not strongly correlated with agriculture). I'm not familiar with any studies of roof-brightening besides Jacobson's. His result is plausible. Given the wide variety of equally plausible aerosol indirect effects, it is not surprising to me that some models might find a non-intuitive warming due to brighter roofs. Jacobson's results show that geoengineering research is simply climate research by another name. The same numerical experiment needs to be performed in other models before the result can be considered robust. I think the jury will remain out on this longer than it will on Conrad Murray. Sitting in an airport with nothing better to do, Charlie -- You received this message because you are subscribed to the Google Groups "geoengineering" group. To post to this group, send email to geoengineering@googlegroups.com. To unsubscribe from this group, send email to geoengineering+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/geoengineering?hl=en.
