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

The Tibetan Plateau, also known as the "Asian Water Tower", is a crucial water resource for Asia, impacting the lives of billions. A significant amount of water flows from the plateau, and its water balance is largely maintained by external water vapor sources, primarily from the Arabian Sea and Bay of Bengal, transported by the South Asian monsoon. These sources regulate precipitation variability over the southern Tibetan Plateau, making precipitation trends key indicators of the plateau's water sustainability. Since the 1990s, precipitation has been predominantly declining, accelerating glacial shrinkage and affecting the plateau's "reservoir" function and future water resources for Asia. Summer precipitation (June-September) accounts for over 60% of the total annual precipitation on the Tibetan Plateau, exhibiting complex spatiotemporal variations and regional trends—increasing in the north and decreasing in the south. This decline in southern precipitation, particularly in the Himalayas, contributes significantly to glacial shrinkage. Conversely, South Asia has seen increased summer monsoon precipitation since 2002. The study investigates the extent to which South Asian black carbon influences long-range moisture transport to the Tibetan plateau, its effect on the observed precipitation decrease over the southern Tibetan plateau, and its overall influence on the plateau's water sustainability.

South Asia is known for severe air pollution, with many studies highlighting aerosols' impact on South Asian monsoon precipitation, primarily due to black carbon's radiation absorption potential. While these studies show varying consequences, they generally agree that black carbon causes warming in the South Asian monsoon region, inducing tropospheric instability and triggering convection and precipitation. However, the extent of South Asian black carbon's influence on long-range moisture transport to the Tibetan Plateau, its role in the observed precipitation decrease over the southern Tibetan Plateau, and its overall impact on the plateau's water sustainability remain unclear. This study addresses these gaps by integrating in-situ observations, reanalysis datasets, numerical simulations, and statistical analyses.

The study domain encompasses the Tibetan Plateau and South Asia. Data sources include daily precipitation data from 86 national observational stations in China (1961–2016), the Climatic Research Unit (CRU) dataset (0.5° resolution), the Asian Precipitation–Highly Resolved Observational Data Integration Towards Evaluation of Water Resources (APHRODITE) dataset (0.25° × 0.25° resolution), ERA-Interim reanalysis data (0.25° resolution), and Peking University's global black carbon emission inventories (0.1° × 0.1° resolution). The Weather Research and Forecasting model with Chemistry (WRF-Chem) was used for simulations at 25 km horizontal resolution. The model was configured using the Kain-Fristch cumulus scheme, CBMZ gas-phase chemical mechanism, and MOSAIC aerosol module, after evaluating its performance against in-situ observations and reanalysis datasets. Two sets of WRF-Chem simulations were conducted: control simulations (with black carbon) and sensitivity simulations (without black carbon from South Asia). The difference between these simulations highlights the impact of South Asian black carbon. The Integrated Valuation of Ecosystem Services and Tradeoffs (InVEST) model (3.7.0) was used to estimate the impact of black carbon on the Tibetan Plateau's water supply, using a Budyko curve approach. A monthly-scale glacier mass balance model was used to assess glacier mass changes (1979–2014). Statistical analyses included linear trend analysis, Mann-Kendall trend tests, Pearson's correlation coefficients, and Student's t-tests.

Analysis of precipitation data revealed a decreasing trend in summer precipitation over the southern Tibetan Plateau since 2004, contrasting with an increasing trend in South Asia. A significant negative correlation was found between summer precipitation over the southern Tibetan Plateau and summer black carbon emissions from South Asia after 2004. WRF-Chem simulations showed that South Asian black carbon increased summer precipitation in South Asia but decreased it over the southern Tibetan Plateau. The model also indicated a reduction in incoming moisture from the southern boundary of the Tibetan Plateau and a strengthening of cyclonic circulation in the eastern Indian subcontinent. Black carbon increased cloud condensation nuclei (CCN) concentrations over the Indian subcontinent, which, combined with its direct radiative effects, enhanced convection and localized precipitation in South Asia, reducing moisture for advection to the Tibetan Plateau. Event analysis confirmed that days with heavy rain in South Asia were associated with lower precipitation over the southern Tibetan Plateau. InVEST modeling showed a substantial decrease in water supply over the southern and central Tibetan Plateau, particularly in the Himalayas (up to 200 mm/annum) due to South Asian black carbon. Glacier mass balance modeling indicated that the reduction in water supply caused by black carbon accounted for 11.0% of the glacier deficit mass balance over the southern Tibetan Plateau (2007-2016), rising to 22.1% in the Himalayas. The direct effect of black carbon deposition on glaciers, reducing albedo, and the indirect effect of decreased precipitation and higher temperatures combined to accelerate glacier deterioration.

The study's findings demonstrate that increasing South Asian black carbon emissions since the beginning of the 21st century have significantly altered summer precipitation patterns over the southern Tibetan Plateau. The inconsistencies in previous modeling studies on black carbon's impact on the South Asian summer monsoon are likely due to variations in black carbon emissions over time. The study confirms that the increasing black carbon emissions have reached a level to significantly affect local convection, augmenting localized convection and precipitation while reducing moisture for advection to the Tibetan Plateau. The combined direct and indirect effects of black carbon lead to reduced precipitation and accelerated glacier melt in the southern Tibetan Plateau. The decadal shift in precipitation and glacier mass accumulation further emphasizes the ongoing impact of black carbon. These findings have profound implications for water resources and the potential for glacial lake outburst floods in the region.

This study highlights the significant and multifaceted impact of South Asian black carbon on the water sustainability of the Tibetan Plateau. Both direct (enhanced glacier melt) and indirect (decreased precipitation) effects are driving glacial mass decline, with consequences for water security in the region. Mitigation of black carbon emissions is crucial to maintain the water balance of the Tibetan Plateau and to avoid future water scarcity and geohazards. Future research should focus on refining the understanding of black carbon's interactions with clouds and precipitation under varying atmospheric conditions and further investigating the feedback mechanisms between black carbon, glacial melt, and regional hydrological processes.

While the study uses advanced modeling techniques and comprehensive datasets, certain limitations exist. The WRF-Chem model, despite its high resolution and rigorous evaluation, has inherent uncertainties, particularly in representing complex terrain effects. The InVEST model's reliance on the Budyko curve might not fully capture the intricate interactions within the water balance. Further research could explore alternative modeling approaches and incorporate additional data to enhance the accuracy and robustness of the findings. The study primarily focuses on black carbon; however, other aerosol components and their combined effects could significantly influence the water cycle in the region.