Signal Interhemispheric aerosol difference at 0.07, inside the 0.1 boundary

Summary

The global aerosol control variable is the difference in aerosol optical depth between the northern and southern hemispheres, showing how much higher loading is in the north, where most human activity and emissions occur. The current value of 0.07 sits below the 0.1 boundary and within the safe operating space, against a pre-industrial baseline of 0.04 and a high-risk line of 0.25. The gap moved between 0.08 and 0.06 while declining over 2010 to 2025, and despite a slight uptick in the latest data the report expects the post-2010 trend toward safer values to continue. The 0.1 boundary rests on observations of volcanic eruptions and model studies suggesting that a widening hemispheric gap could trigger regional tipping such as monsoon shifts. Such changes could sharply raise flood and drought risks. The global indicator does not capture regional risks, however, and South Asia has exceeded its regional boundary.