This category of geoengineering strategies involves reflecting away some portion of incoming solar radiation in order to cool the planet. In general, these strategies address the effects of global warming, but do nothing to mitigate the root cause, which is increased greenhouse gases in the atmosphere. They are usually thought of as quicker and more effective than most CDR strategies. The different SRM options being considered vary greatly.
One of the more practical SRM techniques involves whitening surfaces like roofs, croplands and the oceans in order to increase the Earth's albedo and reflect greater amounts of radiation back into space. While this method would be comparatively cheap, it is generally agreed that there are not enough of these surfaces to change in order to make a huge net difference on global temperatures. Therefore, these techniques are not widely regarded within the geoengineering field (1).
Another technique that is on the opposite spectrum of practicality involves using giant mirrors or satellites to reflect solar radiation away from Earth. In theory, this would work remarkably well to decrease global temperatures, with fewer possible negative environmental effects. However, this strategy is so expensive that it is not really being considered as an option, at least not within this century (1).
The most widely known SRM strategy involves the use of stratospheric aerosols. In this, small reflective particles are injected into the atmosphere in order to scatter a percentage of incoming sunlight back into space. In theory, this method follows the same premise as large volcanic explosions. In the past, large eruptions have ejected enough sulfur dioxide into space to almost immediately start cooling the planet. This effect was seen most recently in the 1991 Mt. Pinatubo eruption. Global average temperatures cooled by 0.5 degrees Celsius for about two years (2). This SRM strategy builds off this idea, and hopes to develop some sort of sulfur or aluminum oxide particle that could remain in the atmosphere for longer (3). This strategy is thought of as more cost-effective than most CDR techniques, and it is also praised because it could work to reduce global temperatures very quickly (1). However, because it involves manipulation of the climate, a very high degree of uncertainty is involved. It is unknown if the change in temperatures will affect all regions equally, or if some areas will see more severe side effects. There is also concern that the use of stratospheric aerosols could negatively impact precipitation and evaporation (2).
Sources: (1) Caldeira, K. W., Keith, D. (2010). The Need for Climate Engineering Research. Issues In Science & Technology, 27(1), 57.
(3) Ricke, K., et. al. (2008, May). Unilateral Geoengineering. Briefing Notes for a Workshop at the Council on Foreign Relations. Retrieved from www.cfr.org/content/thinktank/GeoEng_041209.pdf