Climate-driven glacier retreat in the Sair Uul massif, Mongolian Altai (1977-2025): remote sensing, climate analysis, and field validation.

High-mountain glaciers of the Mongolian Altai are among the most sensitive cryospheric indicators of continental climate change, yet many small glacier systems remain critically under-monitored. The Sair Uul massif - the site of the highest specific ablation rate in the Mongolian Altai - has received no prior comprehensive glaciological assessment. This study quantifies long-term glacier retreat in Sair Uul using multi-temporal Landsat (MSS, TM, ETM + , OLI, OLI-2) and Sentinel-2 imagery spanning 1977 to 2025, combined with NDSI and NDPCSI spectral indices and extended climate data from the Tolbo meteorological station (2000-2024). Field validation was provided through three annual campaigns (2022-2024) conducted via a youth-participatory mentoring framework involving university researchers, eco-club students, and WWF Mongolia. Regional climate analysis of the Tolbo station (2000-2024) documents an overall warming of + 2.2 °C over 25 years, with winter temperatures rising disproportionately from - 21.73 °C (2000-2009) to - 16.70 °C (2020-2024), a seasonal amplification of + 5.0 °C consistent with Arctic amplification processes. Annual precipitation shows a slight increasing trend (2002-2024), with 50-90% falling during the summer ablation season; however, episodic precipitation gains were insufficient to offset temperature-driven mass loss. The glacier area shrank from 12.8 km2 in 1977 to 3.71 km2 in 2025 - a 71% loss over 48 years. Regression analysis confirms a strong negative correlation between glacier area and air temperature (r = - 0.7832; R2 = 0.6134). Citizen-science-supported field validation by 18 eco-club students reduced mean satellite boundary offset errors from 14 to 5 m, improving confidence in glacier boundary delineation for shaded and debris-covered surfaces.