Spatiotemporal Assessment of Snow Cover Dynamics, Black Carbon Radiative Forcing, and Albedo Decline in Northern Pakistan (2001–2024) Using MODIS Cloud-Gap-Filled Time Series and Google Earth Engine

Snow cover across the Hindu Kush–Karakoram–Himalaya (HKH) ranges in Northern Pakistan forms a vital water tower sustaining over 220 million downstream residents in the Indus Basin. The primary goal of this study is to quantify multi-decadal cryospheric dynamics and assess how atmospheric aerosols and climatic warming co-modulate snow retreat. We investigate the hypothesis that while macro-scale snow cover area (SCA) may exhibit apparent stability, localized hypsometric upward shifts in snowline elevation occur concurrently with aerosol-driven snow albedo degradation. Addressing key gaps in multi-parameter cryospheric monitoring, this study couples 24 years (2001–2024) of cloud-gap-filled MODIS (MOD10A1/MYD10A1) daily time series processed via the Terra–Aqua Combination (TAC) algorithm in Google Earth Engine (cloud coverage reduced from 38% to 11%) with Black Carbon (BC) aerosol deposition and radiative forcing models across district, tehsil, and 500 m elevation zones. Snow mapping validation against Sentinel-2 (10 m) imagery demonstrated high accuracy (Overall Accuracy: 90.61%, Kappa: 0.81). Trend analyses using Mann–Kendall and Sen’s slope estimators reveal that regional annual SCA remained relatively stable (+0.01% yr⁻¹; p > 0.05), yet snowline elevation underwent a statistically significant upward migration of +12 m yr⁻¹ (p < 0.05). Mean BC concentration was 4.30 × 10⁻⁴ kg m⁻², contributing to localized radiative forcing between +5 and +56 W m⁻². BC exhibited significant negative correlations with snow albedo (r = -0.24) and SCA (r = -0.29), while temperature was inversely associated with SCA (r = -0.73, p < 0.001). The novel contribution of this work lies in integrating fine-scale administrative and hypsometric cryospheric monitoring with continuous aerosol radiative impact metrics, providing critical empirical evidence for downstream water security and hazard mitigation.