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

The Tibetan Plateau, known as the Asian Water Tower, sustains major Asian river systems and billions of people. Its water balance depends largely on external water vapor transported by the South Asian monsoon from the Arabian Sea and Bay of Bengal. Summer (June–September) precipitation accounts for over 60% of annual precipitation on the Plateau and exhibits contrasting trends: increasing in the north and declining in the south since the 1990s, contributing to accelerated glacier shrinkage. South Asia, upstream of moisture transport to the Plateau, has seen increased monsoon precipitation since 2002. While aerosols—particularly black carbon (BC)—are known to influence the South Asian monsoon by absorbing radiation and altering convection, it remains unclear: (a) to what extent South Asian BC has influenced long-range moisture transport to the Tibetan Plateau; (b) whether South Asian BC has affected the observed decrease in summer precipitation over the southern Tibetan Plateau (STP); and (c) the overall influence of BC on the Plateau’s water sustainability. This study addresses these questions using integrated observations, reanalyses, numerical simulations, and statistical analyses.

Prior studies identify South Asia as a global hotspot for air pollution and document aerosol effects on the South Asian monsoon, highlighting BC’s radiative absorption that warms the atmosphere, enhances instability, and modulates convection and precipitation. However, findings on BC’s impact on monsoon rainfall are mixed, with reports of both increases and decreases depending on region and period. Existing work emphasizes BC’s ability to induce moisture convergence and convection locally in monsoon regions, but the implications for long-range moisture transport to the Tibetan Plateau and its precipitation and water balance have been insufficiently quantified. This study builds on these findings by explicitly linking South Asian BC to STP precipitation and water supply.

Study domain: Tibetan Plateau and South Asia, with the Plateau divided into northern (NTP), southern (STP), and a transition region. Data: Observed daily precipitation from 86 CMDSC stations (1961–2016); CRU monthly gridded precipitation (0.5°); APHRODITE daily gridded precipitation (0.25°); ERA-Interim moisture and winds (0.25°); Peking University monthly anthropogenic BC emissions (0.1°). Analyses: Accumulative anomalies to detect turning points; linear trend estimates (least squares) and Mann–Kendall test; Pearson correlations and two-tailed t-tests between STP precipitation and South Asian BC emissions; moisture budget via vertical integrals of moisture flux and divergence from ERA-Interim. Modeling: WRF-Chem regional simulations at 25 km resolution for summers (May–September; May as spin-up) during 2007–2016. Control runs used default (INTEX-B/MOZART-driven) emissions and BC initial/boundary conditions; sensitivity runs set initial concentrations and all emissions of BC to zero over South Asia at each time step. Differences (control minus sensitivity) diagnose BC effects. Model configurations included Kain–Fritsch cumulus, CBMZ gas-phase chemistry, and MOSAIC aerosol; performance evaluated against in-situ and reanalysis datasets, with resolution and cumulus-scheme sensitivity assessed. Water supply: InVEST (3.7.0) water yield model used to translate simulated precipitation changes into water supply changes using a Budyko-based evapotranspiration formulation (Fu/Zhang) with PET from Global-PET and ω determined by climatic-soil properties. Glaciers: Plateau-wide glacier volume change (2007–2016) estimated using DEM time series following Brun et al.; monthly-scale glacier mass balance modeled by elevation bands following Radić & Hock for 1979–2014, attributing mass balance components (ablation, accumulation, refreezing). Event analysis: Heavy rain days (>100 mm/day; 2007–2015, APHRODITE) assessed to examine co-variability between South Asia and STP and WRF-Chem-simulated BC impacts on convective vs large-scale precipitation. Statistical methods and formulas detailed in Supplementary materials.

- A turning point in 2004 marks a reversal in summer precipitation trends: South Asia increased by 9.9 mm/a (2004–2016) while the STP decreased by 4.4 mm/a, contrasting with earlier periods (1961–2003) where South Asia decreased by 1.4 mm/a and STP showed insignificant increase. - ERA-Interim shows since 2004 reduced specific humidity and weakened southerly flow toward the STP, with enhanced upward motion and moisture convergence over South Asia, indicating reduced long-range moisture transport to the Plateau. - From 2004 onward, STP summer precipitation is significantly negatively correlated with South Asian BC emissions (using CRU precipitation and Peking University emissions), whereas correlations were weak/insignificant before. - WRF-Chem sensitivity simulations (2007–2016 summers) attribute to South Asian BC: increased precipitation over South Asia (up to ~+200 mm, especially eastern India) and decreased precipitation over the STP (up to ~−100 mm). The reduction over STP is dominated by decreases in large-scale precipitation; convective precipitation changes over STP are minor. - BC-induced dynamical changes include enhanced cyclonic circulation over the eastern Indian subcontinent/Bay of Bengal, strengthened vertical moisture transport, reduced low-tropospheric moisture and increased mid-tropospheric moisture, weakened northward moisture transport into the STP, enhanced moisture convergence over South Asia, and increased moisture divergence over the STP. - Thermodynamic and microphysical effects: BC increases meridional temperature gradients, enhancing vertical wind shear and CAPE over eastern India; CCN concentrations (at 1% supersaturation) increase markedly over South Asia, facilitating localized precipitation where moisture and convection are abundant. - Heavy rain day analysis (2007–2015; 228 events accounting for 39.6% of summer rainfall) shows South Asian heavy rain days coincide with comparatively low precipitation over the STP; WRF-Chem indicates BC enhances convective and total precipitation over South Asia during these events, while reductions over the STP are mainly in large-scale precipitation. - InVEST-based assessment translates precipitation changes to water supply: overall decrease over the Plateau, with substantial reductions over the southern and central Plateau and up to ~200 mm/yr decrease in the Himalayas. - Glacier impacts: South Asian BC–induced reduction in water (mass) supply accounts for 11.0% of the glacier deficit mass balance over the STP (2007–2016) and 22.1% in the Himalayas; observed glacier mass loss is largest over the southern Plateau (e.g., −0.53 m w.e. yr⁻¹ regionally). Deposition of BC on glacier surfaces further enhances melt (~15%) and shortens snow cover duration. - Combined direct (albedo reduction and enhanced melt via deposition) and indirect (reduced precipitation/water supply) effects of South Asian BC are driving glacial mass decline in the Asian Water Tower.

Findings demonstrate that since the early 21st century, rising South Asian BC emissions have significantly altered atmospheric dynamics and microphysics to enhance convection and precipitation over South Asia, deplete moisture available for downwind advection, and reduce summer precipitation over the STP. This mechanistic chain—enhanced vertical heating and CAPE, increased CCN, strengthened regional cyclonic circulation, and moisture convergence south of the Plateau—reduces long-range moisture transport into the STP and shifts precipitation regimes. The results reconcile previous mixed conclusions about BC impacts on the monsoon by emphasizing temporal changes in emissions and region-specific responses. The documented precipitation declines contribute to reduced water supply and glacier mass accumulation, compounding direct BC deposition effects and regional warming, thereby threatening the Plateau’s role as Asia’s Water Tower and downstream water security. Mitigating South Asian BC emissions emerges as a key actionable lever to help stabilize the Plateau’s water balance and reduce associated geohazards (e.g., glacial lake outburst floods).

This study integrates observations, reanalyses, and WRF-Chem simulations to establish that increasing South Asian black carbon has, since 2004, reduced summer precipitation over the southern Tibetan Plateau by weakening long-range moisture transport while enhancing localized convection and rainfall over South Asia. The resulting decrease in water supply—particularly pronounced in the Himalayas—has contributed substantially to glacier mass deficits (11% over the STP and 22.1% in the Himalayas for 2007–2016), in addition to BC’s direct albedo-driven melt effects. These combined pathways endanger the Asian Water Tower’s water sustainability and downstream resources. Policy-relevant implications include the need to mitigate BC emissions in South Asia; future work should refine attribution with higher-resolution coupled chemistry–climate modeling, further constrain aerosol–cloud–precipitation interactions, and improve glacier–hydrology coupling to assess cascading impacts on regional water resources and hazards.

While model performance was evaluated and generally satisfactory, uncertainties remain due to model configuration choices and resolution. The WRF-Chem simulations showed low bias in BC concentrations at sites with complex terrain, and sensitivity to spatial resolution and cumulus parameterization was noted. Observational and reanalysis datasets carry inherent uncertainties. The attribution relies on regional modeling experiments and statistical associations, though lag-correlation analyses and sensitivity simulations strengthen causal inference.