Sulfate geoengineering: a review of the factors controlling the needed injection of sulfur dioxide

Sulfate geoengineering has been proposed as an affordable and climate-effective means to temporarily offset the warming produced by the increase of well-mixed greenhouse gases (WMGHGs). This technique would likely have to be applied while and after global intergovernmental measures on emissions of WMGHGs are implemented in order to achieve surface temperature stabilization. The direct radiative effects of sulfur injection in the tropical lower stratosphere can be summarized as increasing shortwave scattering with consequent tropospheric cooling and increasing long wave absorption with stratospheric warming. Indirect radiative effects are related to induced changes in the ozone distribution; stratospheric water vapor abundance,; formation and size of upper-tropospheric cirrus ice particles; and lifetime of long-lived species, namely CH4 in connection with OH changes through several photochemical mechanisms. Direct and indirect effects of sulfate geoengineering both concur to determine the atmospheric response. A review of previous studies on these effects is presented here, with an outline of the important factors that control the amount of sulfur dioxide to be injected in an eventual realization of the experiment. However, we need to take into account that atmospheric models used for these studies have shown a wide range of climate sensitivity and differences in the response to stratospheric volcanic aerosols. In addition, large uncertainties exist in the estimate of some of these aerosol effects.