Summary
The study investigates how stratospheric aerosol geoengineering (SAG) alters tropical monsoon precipitation by analysing 15 climate model simulations with varying aerosol amounts and hemispheric distributions. It finds that monsoon rainfall changes can be explained primarily by two factors: global mean aerosol optical depth (GMAOD) and interhemispheric AOD difference (IHAODD). Increasing GMAOD reduces precipitation across all monsoon regions, while IHAODD shifts the ITCZ, redistributing rainfall between hemispheres. A simple two‑parameter framework accurately reproduces modelled monsoon precipitation responses.
Abstract
Stratospheric aerosol geoengineering (SAG) is one of the several solar geoengineering options that have been proposed to counteract climate change. In the case of SAG, reflective aerosols injected into the stratosphere would reflect more sunlight and cool the planet. When assessing the potential efficacy and risks of SAG, the sensitivity of tropical monsoon precipitation changes should be also considered. Using a climate model, we perform several stylized simulations with different meridional distributions and amounts of volcanic sulfate aerosols in the stratosphere. Because tropical monsoon precipitation responds to global mean and interhemispheric difference in radiative forcing or temperature, we quantify the sensitivity of tropical monsoon precipitation to SAG in terms of two parameters: global mean aerosol optical depth (GMAOD) and interhemispheric AOD difference (IHAODD). For instance, we find that the simulated northern hemisphere monsoon precipitation has a sensitivity of −1.33 ± 0.95% per 0.1 increase in GMAOD and −7.62 ± 0.27% per 0.1 increase in IHAODD. Our estimated precipitation changes in terms of the two sensitivity parameters for the global mean precipitation and for the indices of tropical, northern hemisphere, southern hemisphere and Indian summer monsoon precipitation are in good agreement with the model simulated precipitation changes. Similar sensitivity estimates are also made for unit changes in global mean and interhemispheric differences in effective radiative forcing and surface temperature. Our study based on planetary energetics provides a simpler framework for understanding the tropical monsoon precipitation response to external forcing agents.
