S Asian black carbon aerosols increase glacial mass loss in Tibet: Study

Black carbon aerosols have indirectly affected the mass gain of the Tibetan Plateau glaciers by changing long-range water vapour transport from the South Asian monsoon region, a study has found.

The South Asia region adjacent to the Tibetan Plateau has among the highest levels of black carbon emission in the world, the researchers said.

Black carbon aerosols are produced by the incomplete combustion of fossil fuels and biomass, and are characterised by strong light absorption.

Many studies have emphasised black carbon aerosols from South Asia can be transported across the Himalayas to the inland region of the Tibetan Plateau.

Researchers noted that black carbon deposition in snow reduces the albedo of surfaces -- a measure of how much of Sun's radiations are reflected -- which may accelerate the melting of glaciers and snow cover, thus changing the hydrological process and water resources in the region.

The study, published in Nature Communications, found that since the 21st century, South Asian black carbon aerosols have indirectly affected the mass gain of the Tibetan Plateau glaciers by changing long-range water vapour transport from the South Asian monsoon region.

"Black carbon aerosols in South Asia heat up the middle and upper atmosphere, thus increasing the NorthSouth temperature gradient," said Professor KANG Shichang from Chinese Academy of Sciences (CAS).

"Accordingly, the convective activity in South Asia is enhanced, which causes convergence of water vapour in South Asia. Meanwhile, black carbon also increases the number of cloud condensation nuclei in the atmosphere," Shichang said.

These changes in meteorological conditions caused by black carbon aerosols make more water vapour form precipitation in South Asia, and the northward transport to the Tibetan Plateau was weakened, the researchers said.

As a result, precipitation in the central and the southern Tibetan Plateau decreases during the monsoon, especially in the southern Tibetan Plateau, they said.

The decrease in precipitation further leads to a decrease of mass gain of glaciers. From 2007 to 2016, the reduced mass gain by precipitation decrease accounted for 11 per cent of the average glacier mass loss on the Tibetan Plateau and 22.1 per cent in the Himalayas.

"The transboundary transport and deposition of black carbon aerosols from South Asia accelerate glacier ablation over the Tibetan Plateau.

"Meanwhile, the reduction of summer precipitation over the Tibetan Plateau will reduce the mass gain of plateau glaciers, which will increase the amount of glacier mass deficit," Shichang added.