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Giant mirrors in space. Chemicals injected into the atmosphere. Roofs painted white the world over. Some proposed geoengineering technologies seem far-fetched and highly technical. Many, however, are being given serious consideration.
Although a wide spectrum of ideas are on the table, two major approaches are being considered. The first set of strategies are based on solar radiation management (SRM), which proposes to decrease warming by 'reducing the incidence and subsequent absorption of incoming solar radiation' or to put it simply, to cut down the amount of sunlight reaching the earth. The second set of techniques involve removal of carbon dioxide from the earth’s atmosphere (for more on this, see next section).
Nature itself provided an example of how solar radiation management works. During the Mount Pinatubo eruption in 1991, an ash stream was projected 35 kilometers into the air, injecting 20 million tons of sulfur dioxide into the atmosphere. As a result, much sunlight was reflected back into space and the global average temperature dropped by 0.5 C.
Solar or SRM technologies aspire to produce a similar effect by putting installations in space to deflect solar radiation. For example, cloud albedo may be increased by injecting seawater or sulfur aerosols into the atmosphere, or surface albedo can be increased by painting roofs white or planting crops with shiny leaves.
By reducing the amount of sunlight that reaches the earth’s surface, solar radiation absorbed by the earth is diminished and less heat is generated. This may help compensate for the increased warming brought about by greenhouse gases that prevent the earth’s heat from escaping back into space. The earth's climate is affected by the balance between incoming radiation (energy) from the sun and outgoing thermal radiation (heat) which cools the earth but may be blocked by greenhouse gases. SRM works by reducing incoming radiation rather than reducing greenhouse gases. In other words, SRM does not treat the primary cause of global warming - greenhouse gases. As a result, if SRM is used on a large scale but no efforts are made to control greenhouse gases, then large and rapid increases in global temperatures could occur if SRM were suddenly stopped.
Compared to other geoengineering technologies, SRM could rapidly modify the climate. However, a lot of uncertainty remains about its potential impacts. The CBD draft report on Impacts of Climate Related Geo-engineering on Biological Diversity questions whether the impacts from SRM might not be worse than the problem it is supposed to resolve.
The combination of changes – more diffuse light, unpredictably, altered precipitation patterns, potentially high CO2 concentrations – would be unlike any known combination that extant species and ecosystems have experienced in their evolutionary history. However, it is not clear whether the novel environment of the SRM world would be more or less challenging for today's species than that caused by the climate change that it would be seeking to counter.1
Another potential impact of injecting sulfur dioxides into the upper atmosphere could be a further thinning of the ozone layer. Arctic communities have already experienced the impact of a weakened ozone layer due to chlorofluorocarbons (CFCs). In a 2007 article, Inuit activist Sheila Watt-Cloutier explains: “Because one of the major (ozone) "holes" sits over the North Pole, our people are bombarded with the sun's damaging ultraviolet radiation at much higher levels than the rest of the world... more of the sun's heat and radiation is also being locked into our atmosphere by another set of pollutants: greenhouse gases. For Inuit, these problems are very much connected, as the rapidly increasing temperatures around our Arctic combine with the heightened UV radiation, affecting our ability to hunt, travel and maintain our traditional subsistence culture.”2 If SRM, by adding a third chemical to this atmospheric mix - sulfur dioxide - that may exacerbate ozone depletion while, according to some, locally intensifying greenhouse warming over the Arctic. For this reason, it’s perhaps understandable that some Arctic peoples remain skeptical about the benefits of solar geoengineering.
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1 See page 4 for the executive summary of the CBD draft report, which is also open for a second round of peer review comments (http://www.cbd.int/climate/geoengineering/review/). Section 4.1.3 goes into further detail.
2 Watt-Cloutier, Sheila (2007). Ozone treaty offers insurance against climate change. The Globe and Mail. Published Thursday, Sep. 06, 2007. Available at: http://www.theglobeandmail.com/news/opinions/article780681.ece