Signal South Asian aerosol optical depth at 0.32 against a 0.25 regional boundary

Summary

The 2026 Planetary Health Check introduces, for the first time, a regional aerosol assessment for South Asia. Aerosol optical depth over the region currently stands at about 0.32, above the proposed regional boundary of 0.25. The report explains that the global control variable, the difference in aerosol loading between the hemispheres, remains within the safe space but misses such regional risks. Climate model runs show that an AOD of 0.25 cuts South Asian monsoon rainfall by about 10 percent, while 0.5 reduces it by 15 to 20 percent and severely weakens the monsoon. Fossil fuel and biomass burning are the main sources, and heavy aerosol loads are linked to longer droughts, lower farm output and more respiratory illness. The trend over the latest year points further away from the safe space.