A model-based assessment of the regional safe aerosol boundary for the South Asian monsoon region

Abstract Recent papers on planetary and safe Earth System boundaries have proposed a limit of 0.25 for regional aerosol optical depth (AOD) in South Asia (SA) with a zone of uncertainty of 0.25–0.50 to avoid major disruptions to regional hydrology. These values are based on expert judgment, and a rigorous model-based confirmation of these boundary values is lacking in literature. In this paper, we address this important research gap using idealized climate model simulations. We analyze the response of the South Asian summer monsoon precipitation when the regional mean AOD is increased from the modeled present-day value of 0.14 to the proposed planetary boundary values of 0.25 and 0.5. Our simulations confirm that a regional AOD of 0.25 could indeed lead to drought conditions (>10% mean precipitation reduction) in India, while an AOD of 0.5 reduces Indian summer monsoon precipitation by about 19%. The reduction in the summer monsoon precipitation is driven by both fast adjustments (rapid adjustment of the atmosphere to aerosol radiative forcing) and slow responses (responses to changes in sea surface temperature). The rapid adjustment to anthropogenic aerosols, predominantly sulfates, involves enhanced atmospheric stability, subsidence, and suppressed cloud formation and precipitation. The slow response involves zonal surface temperature gradients between the North Indian Ocean and Western Pacific Ocean leading to changes in the Walker circulation, anomalous subsidence over SA, and a decrease in monsoon precipitation. Enhanced aerosol loading over SA reduces monsoon precipitation, regardless of aerosol composition, though the magnitude of reduction depends on whether the aerosols are primarily reflective or absorbing. Our findings confirm the risk of major disruptions to regional hydrology in SA when regional aerosol loading exceeds the boundary values. Future studies should assess the robustness of our results using other climate models and for other monsoon regions.