additional aerosols might, in particular, cause significant depletion in polar regions to persist for decades (Tilmes et al., 2008).

Rasch et al. (2008a) summarize the results of the newest model calculations that have been carried out to evaluate the use of sulfate aerosols for solar radiation management. They find that there are many details that will need to be investigated, ranging from considerations of the effects on particle size distribution of injecting the particles using a million 4-hour aircraft flights per year to issues of the seasonal effects of the aerosols and their lifetimes and distribution in the stratosphere. Calculations by Robock et al. (2008) also indicate that, while the temperature change from the rising concentrations of greenhouse gases can largely be offset, this may not be the case for precipitation.

Much more analysis is needed to clarify these issues, however, not only of the physics governing a relatively uniform aerosol distribution (as covered in Rasch et al., 2008a), but also for other particle distributions (e.g., having latitudinal and/or longitudinal gradients) and time (e.g., seasonal differences) and for greater or lesser offsets than the changing radiative forcing from greenhouse gases. In addition, analyses of the impacts of aerosol injections need to look at more than the average responses of temperature and precipitation, looking, in particular, at changes in the higher moments of atmospheric behavior (i.e., the weather and its variability) and how humans and ecosystems might be affected (Kravitz et al., 2008; Robock, 2008a).

In addition to analyses of the offsets and their impacts, research, analysis, and even prototype demonstrations are needed of the various proposed approaches for augmenting the stratospheric loading of sulfate aerosols. Among the approaches proposed have been aircraft, artillery, balloon-held hoses, and upward mixing and then stratospheric oxidation of tropospherically inert carbonyl sulfide (COS), each of which would introduce scientific and technological aspects meriting research and development.

There have been several suggestions for how to overcome the negative impacts on stratospheric ozone and sky color. For example, Crutzen (2006), presumably recalling the “nuclear winter” studies of the 1980s (Pittock et al., 1986), suggested injecting soot instead of sulfates, which not only would reduce scattering but would warm the stratosphere and help to offset CO2-induced stratospheric cooling and the consequent reduction in ozone. The history of alternative suggestions goes back considerably further, however. Chang and Shih (1991) not only proposed the use of “Welsbach materials and other oxides of metals which have high emissivity (and thus low reflectivities) in the visible and 8-12 micron infrared wavelength regions,” but also patented the idea.

As an alternative approach, Canavan and Teller (1991) and Teller et al. (1997) proposed injecting microscopic particles that were actually small corner reflectors.<sup>9</sup> Such reflectors would efficiently bounce the solar radiation back toward its source without scattering significant radiation. Because most ultraviolet (UV) radiation is absorbed by the ozone in the stratosphere, having the corner reflectors sized to optimally reflect such radiation, which was the wavelength

> 9 Corner reflectors are created by three intersecting, orthogonal planes, just as are the corners in a room. Assuming each of the planes are coated with a reflective material, light incident on a corner reflector strikes the three planes in succession and comes out in exactly the opposite direction, much like a handball hit into a corner.

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