Management of trade-offs in geoengineering through optimal choice of non-uniform radiative forcing

Solar radiation management could be used to offset some or all anthropogenic radiative forcing, with the goal of reducing some of the associated climatic change1,2. However, the degree of compensation will vary, with residual climate changes larger in some regions than others. Similarly, the insolation reduction that best compensates climate changes in one region may not be the same as for another, leading to concerns about equity3. Here we show that optimizing the latitudinal and seasonal distribution of solar reduction can improve the fidelity with which solar radiation management offsets anthropogenic climate change. Using the HadCM3L general circulation model, we explore several trade-offs. First, residual temperature and precipitation changes in the worstoff region can be reduced by 30% relative to uniform solar reduction, with only a modest impact on global root-meansquare changes; this has implications for moderating regional inequalities. Second, the same root-mean-square residual climate changes can be obtained with up to 30% less insolation reduction, implying that it may be possible to reduce solar radiation management side-effects and risks (for example, ozone depletion if stratospheric sulphate aerosols are used). Finally, allowing spatial and temporal variability increases the range of trade-offs to be considered, raising the question of how to weight different objectives. Multiple studies have explored the climate impacts of geoengineering4–7, with several focusing in particular on regional disparities3,8,9. In a high-CO2 world, using solar radiation management (SRM) can lead to a climate with temperature and precipitation that are closer in most regions to the baseline (current or pre-industrial) climate than not using SRM. However, the pattern of climate changes due to SRM does not fully match those due to increased CO2, both because the different mechanisms of radiative forcing have different relative impacts on temperature versus precipitation10, and the spatial and temporal distribution of radiative forcing obtained by uniform solar reduction does not match that due to increasedCO2 (ref. 4). As a result, the level of solar insolation reduction that best compensates climate change varies regionally9; similarly, for a given choice of insolation reduction, the degree of climate change compensation varies regionally. The analysis in ref. 9 is extended in ref. 3 to consider the optimal level of insolation reduction that minimizes either the area-weighted, population-weighted, or economy-weighted difference between the high-CO2 world and the desired climate, exploring the question of whether different goals lead to different results.