Summary
The study tests previously proposed “safe” aerosol optical depth (AOD) limits for South Asia using climate‑model simulations. Increasing regional AOD from today’s ~0.14 to 0.25 or 0.5 weakens the South Asian summer monsoon, reducing Indian rainfall by 10–20%, enough to trigger drought. Both fast atmospheric adjustments and slower sea‑surface‑temperature‑driven circulation changes contribute. Regardless of aerosol type, higher aerosol loading suppresses monsoon precipitation, confirming that exceeding the AOD 0.25–0.5 boundary poses major hydrological risks.
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.
