The near-ultraviolet and near-infrared spectral bands contain roughly half of the total insolation in energetic terms. These wavelengths may be largely superfluous
(or actually deleterious, in the case of the shorter wavelength ultraviolet) for bio-spheric purposes, and thus portions of these spectra may be attractive candidates for being scattered back into space by an engineered scattering system (which can be designed to have considerable spectral selectivity). For example, the use of Rayleigh scattering to preferentially scatter back into space an appropriate fraction of the deeper ultraviolet portion of insolation appears to be a relatively appealing approach, since a usefully large portion of total insolation is available for attenuation and this solar spectral band's radiation appears to be net damaging to the biosphere: single photons of UV-B and UV-C insolation are deleterious to both plants and animals, primarily due to photodamage of their DNA. Indeed, the World Health Organization estimates approximately 60 000 human deaths occur annually due to sunlight-engendered skin cancer, which is generally believed to be due rather exclusively to the UV-B spectral component of insolation (WHO 2006). Associated direct economic losses may significantly exceed $10 thousand million per year, and economic impacts of crop damage may be of comparable scale. Thus, there is potential for geoengineering to diminish risks to both climate and cancer, as well as to avoid substantial direct economic costs in sectors ranging from agriculture to public health. The sky may become discernibly bluer.
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