South Asian black carbon is threatening the water sustainability of the Asian Water Tower.

Long-range transport of black carbon from South Asia to the Tibetan plateau and its deposition on glaciers directly enhances glacier melt. Here we find South Asian black carbon also has an indirect effect on the plateau's glaciers shrinkage by acting to reduce the water supply over the southern Tibetan plateau. Black carbon enhances vertical convection and cloud condensation, which results in water vapor depletion over the Indian subcontinent that is the main moisture flux source for the southern Tibetan plateau. Increasing concentrations of black carbon causes a decrease in summer precipitation over the southern Tibetan plateau, resulting in 11.0% glacier deficit mass balance on average from 2007 to 2016; this loss rises to 22.1% in the Himalayas. The direct (accelerated melt) and indirect (mass supply decrease) effects of black carbon are driving the glacial mass decline of the so-called "Asian Water Tower". South Asian black carbon's pairing activities (direct and indirect effects) are driving glacial mass decline of the so-called "Water Tower of Asia" with the potential to threaten future water resources for the countries that rely upon its source. [ABSTRACT FROM AUTHOR]