eclipse the Sun, causing sunlight at the surface to flicker. To avoid the flickering problem, Kahle and Deirmendjian (1973), expanding on an idea appearing in Hoyle (1957), had much earlier proposed creation of an orbiting cloud of particles, although this would be difficult to maintain given the effects of the solar wind, which would tend to push the particles out of orbit. Given the relatively significant problems of tackling global warming from near-Earth orbit, such approaches have received little attention and are not discussed further in this article.

**_Stratosphere and above:_** Enhancing the global loading of sulfate aerosol particles in the stratosphere, as proposed several decades ago by Budyko (1974) and more recently by Crutzen (2006), has received considerable attention. An important advantage in evaluating this approach is that the diminution in solar radiation would be roughly equivalent to the influence of an annual to biennial series of major volcanic eruptions similar to what has been experienced in the past, although the geoengineering activity would need to persist indefinitely. One advantage of stratospheric aerosols, as compared to augmenting the sulfate aerosol loading of the troposphere that presently offsets a substantial fraction of the radiative forcing from increased greenhouse gas concentrations, is that the lifetime of stratospheric sulfate particles is typically a few years as opposed to a few days. Another advantage of the sulfate-injection approach is that it can be tried in an incremental manner and, in the event of adverse consequences, be quickly halted.

Although enhancing stratospheric aerosol loading would lead to very colorful sunrises and sunsets, an important disadvantage of sulfate aerosol particles is that they forward scatter roughly five to ten times as much radiation as they reflect, whitening the skies and seriously diminishing the direct solar beam (Izrael, 2008). Such scattering would reduce the effectiveness of the technologies that generate energy using the direct component of the solar beam, such as the solar power tower.<sup>8</sup> In addition, the biosphere would be affected; for example, following the Pinatubo eruption, Gu et al. (2003) found that the greater amount of diffuse radiation was able to better penetrate the vegetation canopy, enhancing primary productivity and carbon uptake.

In addition, because the aerosols reduce the amount of solar radiation reaching the surface, they tend to reduce the amount of energy available to drive the hydrologic cycle. Model simulations by Bala et al. (2008) indicate that, for similar changes in the net change in the energy balance at the tropopause, stratospheric aerosols and greenhouse gases have similar effects on surface temperature, but that the aerosols result in a larger proportional reduction in the hydrologic cycle than would a decrease in the greenhouse gas concentration.

Another important side effect is that the injected sulfate aerosols, like volcanic aerosols, have the potential to exacerbate loss of stratospheric ozone or, at the very least, slow recovery from the depletion caused by injections of chlorofluorocarbons and other halocarbons. With the increasing concentration of CO2 tending to cool the lower stratosphere, having a large permanent loading of

> 8 Following the El Chichón eruption in 1982, engineers from the Sandia National Laboratory in Livermore CA asked atmospheric scientist at the nearby Lawrence Livermore National Laboratory how their observations at the Barstow, California prototype solar power tower installation, which was roughly in the same latitudinal band as the early volcanic cloud, could be showing that total solar radiation dropped ~2% whereas power production had dropped ~25%. They were encouraged to upgrade their instrumentation to measure the reduction in the direct beam radiation, and found that it had, like their power production, dropped about 25%. Once the volcanic aerosol loading decreased, the system worked as designed.

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