Scattering light

All matter scatters electromagnetic radiation. Small particles appear to be the most effective form for climate engineering. The goal is to maximize matterradiation interaction favouring forms of the greatest electromagnetic cross section for sunlight. Thus, the particles of greatest interest would be those with dimensions of the order of the wavelength of the optical radiation to be scattered, as such particles tend to scatter radiation with the highest specific efficiency and minimal mass usage. The characteristics of the scattering are specified by the microscopic physics that determines the (complex) polarizability of the material for the wave frequency of interest, and by the material's geometry that deploys this augmented polarizability over that of the underlying vacuum over some portion of three space (Landau & Lifshitz 1984). The choice of particle parameters, primarily size, shape and composition, and the location of their emplacement would likely be based on environmental, economic and aesthetic considerations. Emplacement of (sub-) microscopic particles in the stratosphere, for example in the polar stratosphere, has

reduction in top-of-atmosphere solar flux (%)

Figure 13.5 Change in global annual mean temperature as a function of percentage of reduction in the top-of-atmosphere insolation. Despite large differences in the spatial extent of the insolation reduction, the global mean temperature response is similar.

reduction in top-of-atmosphere solar flux (%)

Figure 13.5 Change in global annual mean temperature as a function of percentage of reduction in the top-of-atmosphere insolation. Despite large differences in the spatial extent of the insolation reduction, the global mean temperature response is similar.

the practical advantage of residence times exceeding a year - long enough to allow economically efficient climate engineering while short enough to provide some reversibility should unintended consequences prove greater than anticipated.

In terms of mass, quasi-resonant scattering materials are markedly superior to metals, which in turn are greatly advantaged over dielectrics as substrates for engineered scatterers (Teller et al. 1997). However, the relative (photo)chemical inertness of selected dielectric materials, repeatedly demonstrated at scale in major volcanic eruptions involving extensive particulate mass insertion into the stratosphere, motivates their initial selection in order to minimize first-time risks of unwanted side effects (Teller et al. 1997, 1999, 2004; Hyde et al. 2003), including significant interactions with stratospheric ozone. The use of materials with large ratios of real to imaginary components of complex polarizability over pertinent spectral bands to constitute engineered scatterers is generally preferable, as scattering of light by the engineered particulates is usually preferred to photon absorption (and consequent local stratospheric heating).

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