SRM techniques are inherently less safe than CDR methods in that, while they provide a correction to the global radiation imbalance introduced by the greenhouse gases, they do not return the atmosphere to its natural state. They do nothing to reduce other effects of high CO2 concentrations, such as ocean acidification. They also place the climate in an unnatural “High CO2 Low Sun” state under which atmospheric winds and circulations may be quite different and could result in unforeseen impacts on, for example, regional precipitation patterns. SRM schemes have the advantage of being generally less expensive. They would also be easier than CDR methods to implement (or reverse) swiftly. If, however, they were introduced with a view to long term mitigation of global warming, then humankind would be committed to maintaining them into the indefinite future. Any sudden cessation of the SRM would plunge the world very fast into the much warmer state associated with higher CO2 concentrations (3).
While the scientific and technical issues posed in the development of geoengineering methods are challenging, possibly an even greater problem would come in addressing ethical and political issues (4).
Figure 2. A cost-benefit diagram for geoengineering proposals, along with conventional climate change mitigation. The risks associated with these proposals are indicated as low (green), medium (yellow) and high (red). The safest alternative to conventional mitigation is CO2 air capture, but stratospheric aerosol injection has the best benefit-to-cost ratio. Reprinted with permission from (4)