Characterizing four decades of accelerated glacial mass loss in 1 the West Nyainqentanglha Range of the Tibetan Plateau 2

: 12 Glacier retreat is altering the water regime of the Tibetan Plateau (TP) as the region’s climate changes, but 13 there remain substantial gaps in our knowledge of recent glacier loss in this region due to the difficulty of making- 14 direct high-mountain observations and this limits our ability to predict the future of this important water resource. 15 Here, we assessed 44 years of glacier area and volume changes in the major West Nyainqentanglha Range (WNT) 16 that supplies meltwater to the densely populated Lhasa River basin and Nam Co, the second largest endorheic 17 lake on the TP. Between the two periods 1976-2000 and 2000-2020, we found that the glacier areal retreat rate 18 was more than doubled (from -0.54 ± 0.21 % a -1 to -1.17 ± 0.30 % a -1 ) and surface lowering also accelerated ( from 19 -0.26 ± 0.09 m w.e.a -1 to -0.37 ± 0.15 m w.e.a -1 ) with particularly intense melting after 2014. This acceleration is 20 similar in both timing and magnitude to that observed for Himalayas glaciers farther south. Besides, the areal 21 retreat rate and mass loss rate of most glaciers in WNT were not synchronized. To understand the sensitivity of 22 WNT glaciers to climate forcing, we examined the effects of topography, debris-cover, and the presence of 23 proglacial lakes on our observed changes. We found consistently faster areal retreat but slower thinning rates on 24 steeper slopes and an inconsistent relationship with aspect. We concluded that our observed spatial and temporal 25 patterns of glacier change were dominated by observed local variations temperature and precipitation, the melt- 26 reducing role of supraglacial debris, and the increasing influence of ice-marginal lakes on glacier retreat. 0.23 -1 +0.14 0.08 a -1 the -0.14± 0.14 m w.e. a -1 50 TP Maurer et al. (−0.22 0.13 a 0.14 a −1 DEMs. that TP in be used as a reference for glacier changes in other regions. We have compiled a complete glacier inventory in 94 the years 1976, 2000, 2014 and 2020 with the Landsat and KH-9 images and quantified the geodetic glacier mass 95 balance during 1976–2000 and 2000–2020 with DEMs derived from KH-9, ASTER and SRTM3.0. We report area 96 and mass changes for periods 1976 to 2000 and from 2000 to 2020, and we examine and compare the influence 97 of topographic, climatic and glaciological factors on these changes during 1976-2000 and 2000-2020. debris-covered glacier types. From 2000 to 2020, the ablation rate of debris-covered 290 glaciers was slightly lower than that of regular glaciers at low altitudes, but progressively greater at higher altitudes, 291 leading to a slightly more negative total mass balance for the debris-covered type. In the same period, the thinning 292 of lake-terminating glaciers continued to exceed that of regular glacier. Our results suggest that debris cover in 293 the WNT suppressed glacier thinning to some extent and enabled the debris-covered ice to survive at lower 294 elevations than adjacent clean ice glaciers. In contrast, a glacial lake at the end of a glacier accelerated its retreat, 295 and this behavior was more pronounced at lower elevations.