A temporary, moderate and responsive scenario for solar geoengineering

201 One cannot meaningfully evaluate solar geoengineering without a scenario for its implementation. It is now common, for example, to assert that more scientific research is needed to assess the balance between the risks and benefits of solar geoengineering, hereafter called solar radiation management (SRM). Yet the balance between risks and benefits depends at least as strongly on how SRM is deployed (for example on technology choice, timing and magnitude of the induced radiative forcing) as it depends on the climate’s response to a specified SRM scenario. Clear language is an essential tool for analysing this messy topic. We use SRM to denote a technology used to deliberately alter radiative forcing at sufficient scale to measurably alter the global climate. Any technology for producing radiative forcing will have a set of technology-specific impacts, such as ozone loss arising from the introduction of aerosol particles in the stratosphere. However the radiative forcing is produced, the efficacy of SRM is inherently limited by the fact that a change in solar radiative forcing cannot perfectly compensate for the radiative forcing caused by increasing greenhouse gases. SRM has been variously framed as a substitute for cutting emissions (mitigation), as an emergency measure to be used if climate risks are higher than expected, or as a means to restoring surface temperatures to pre-industrial. Explicit or implicit, such scenarios shape any assessment of risk and efficacy of SRM. Ocean acidification has been listed as a risk of SRM1, yet acidification depends almost solely on cumulative CO2 emissions and is unaffected by SRM. Ocean acidification is a risk of SRM only if SRM is used as a substitute for emissions mitigation; and in this case, the risk derives from the increase in emissions not from SRM. Reduced precipitation is another frequently cited risk of SRM (see Supplementary Information for examples). It is true that if the SRM radiative forcing is large enough to offset all of the change in global mean temperature due to anthropogenic CO2 — a common assumption — then precipitation will indeed be reduced in most locations2. Simple physical arguments demonstrate that it takes a smaller SRM forcing to stop the rise in precipitation as CO2 concentrations increase than is required to stop the rise in temperature3. Reduction in precipitation is, however, a product of the magnitude of SRM used in the scenario. If the SRM radiative forcing was adjusted A temporary, moderate and responsive scenario for solar geoengineering