Editing IPCC:AR6/WGI/TS (section)

== Box TS.8 | Earth System Response to Solar Radiation Modification ==

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'''Since AR5, further modelling work has been conducted on aerosol-based solar radiation modification (SRM) options such as stratospheric aerosol injection, marine cloud brightening, and cirrus cloud thinning<sup>[[#footnote-000|21]]</sup> and their climate and biogeochemical effects. These investigations have consistently shown that SRM could offset some of the effects of increasing greenhouse gases on global and regional climate, including the carbon and water cycles (''high confidence''). However, there would be substantial residual or overcompensating climate change at the regional scales and seasonal time scales (''high confidence''), and large uncertainties associated with aerosol–cloud–radiation interactions persist. The cooling caused by SRM would increase the global land and ocean CO <sub>2</sub> sinks (''medium confidence''), but this would not stop CO <sub>2</sub> from increasing in the atmosphere or affect the resulting ocean acidification under continued anthropogenic emissions (''high confidence''). It is ''likely'' that abrupt water cycle changes will occur if SRM techniques are implemented rapidly. A sudden and sustained termination of SRM in a high CO <sub>2</sub> emissions scenario would cause rapid climate change (''high confidence''). However, a gradual phase-out of SRM combined with emissions reduction and carbon dioxide removal (CDR) would avoid these termination effects (''medium confidence''). Links to chapters 4.6.3, 5.6.3. 6.4.6, 8.6.3 .'''

Solar radiation modification (SRM) refers to deliberate, large-scale climate intervention options that are studied as potential supplements to deep mitigation, for example, in scenarios that overshoot climate stabilization goals. SRM options aim to offset some of the warming effects of GHG emissions by modification of Earth’s shortwave radiation budget. Following SR1.5, the SRM assessed in this Report also includes some options, such as cirrus cloud thinning, that alter the longwave radiation budget.

SRM contrasts with climate change mitigation activities, such as emissions reductions and CDR, as it introduces a ‘mask’ to the climate change problem by altering Earth’s radiation budget, rather than attempting to address the root cause of the problem, which is the increase in GHGs in the atmosphere. By masking only the climate effects of GHG emissions, SRM does not address other issues related to atmospheric CO <sub>2</sub> increase, such as ocean acidification. This Report assesses physical understanding of the Earth system response to proposed SRM, and the assessment is based primarily on idealized climate model simulations. There are other important considerations, such as risk to human and natural systems, perceptions, ethics, cost, governance, and trans-boundary issues and their relationship to the United Nations Sustainable Development Goals – issues that the WGII (Chapter 16) and WGIII (Chapter 14) Reports address. Links to chapters 4.6.3

SRM options include those that increase surface albedo, brighten marine clouds by increasing the amount of cloud condensation nuclei, or reduce the optical depth of cirrus clouds by seeding them with ice nucleating particles. However, the most commonly studied approaches attempt to mimic the cooling effects of major volcanic eruptions by injecting reflective aerosols (e.g., sulphate aerosols) or their precursors (e.g., sulphur dioxide) into the stratosphere. Links to chapters 4.6.3, 5.6.3, 6.4.6

SRM could offset some effects of greenhouse gas-induced warming on global and regional climate, but there would be substantial residual and overcompensating climate change at the regional scale and seasonal time scales (''high confidence''). Since AR5, more modelling work has been conducted with more sophisticated treatment of aerosol-based SRM approaches, but the uncertainties in cloud–aerosol–radiation interactions are still large (''high confidence''). Modelling studies suggest that it is possible to stabilize multiple large-scale temperature indicators simultaneously by tailoring the deployment strategy of SRM options (''medium confidence'') but with large residual or overcompensating regional and seasonal climate changes. Links to chapters 4.6.3

SRM approaches targeting shortwave radiation are ''likely'' to reduce global mean precipitation, relative to future CO <sub>2</sub> emissions scenarios, if all global mean warming is offset. In contrast, cirrus cloud thinning, targeting longwave radiation, is expected to cause an increase in global mean precipitation (''medium confidence''). If shortwave approaches are used to offset global mean warming, the magnitude of reduction in regional precipitation minus evapotranspiration (P–E) (Box TS.5), which is more relevant to freshwater availability, is smaller than precipitation decrease because of simultaneous reductions in both precipitation and evapotranspiration (''medium confidence''). Links to chapters 4.6.3, 8.2.1, 8.6.3 .

If SRM is used to cool the planet, it would cause a reduction in plant and soil respiration and slow the reduction of ocean carbon uptake due to warming (''medium confidence''). The result would be an enhancement of the global land and ocean CO <sub>2</sub> sinks (''medium confidence'') and a slight reduction in atmospheric CO <sub>2</sub> concentration relative to unmitigated climate change. However, SRM would not stop CO <sub>2</sub> from increasing in the atmosphere or affect the resulting ocean acidification under continued anthropogenic emissions (''high confidence''). Links to chapters 5.6.3

The effect of stratospheric aerosol injection on global temperature and precipitation is projected by models to be detectable after one to two decades, which is similar to the time scale for the emergence of the benefits of emissions reductions. A sudden and sustained termination of SRM in a high GHG emissions scenario would cause rapid climate change and a reversal of the SRM effects on the carbon sinks (''high confidence''). It is also ''likely'' that a termination of strong SRM would drive abrupt changes in the water cycle globally and regionally, especially in the tropical regions by shifting the Inter-tropical Convergence Zone and Hadley cells. At the regional scale, non-linear responses cannot be excluded, due to changes in evapotranspiration. However, a gradual phase-out of SRM combined with emissions reductions and CDR would avoid larger rates of changes (''medium confidence''). Links to chapters 4.6.3, 5.6.3, 8.6.3 .
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