Monitoring glacier thinning rate in Rongbuk Catchment on the northern slope of Mt. Qomolangma from 1974 to 2021

Glacier melting in the Mt. Himalayas provides indispensable water resources for downstream inhabitants. Glacier mass balance (MB) studies have important implications for hazard management of glacial lake outburst floods (GLOFs). However, Himalayan glacier MB results vary considerably among different studies with large uncertainties in satellite-based geodetic measurements, highlighting the necessity for further independent investigations and validation from multiple satellite platforms. Based on InSAR, photogrammetry, and laser altimetry techniques, we generate nine data sets of surface elevation differences (Dh) to study glacier geodetic MB in Rongbuk Catchment at the northern slope of Mt. Qomolangma. The data sets include five consecutive time periods over the last 48 years, i.e., 1974-2000, 2000-2006, 2006-2012, 2012-2015, and 2015-2021, and four overlapping periods of 1974-2012, 1974-2015, 2000-2015, and 1974-2021. The main results are summarized as follows. 1) Based on common reference DEM, Geodetic MB was -0.21 & PLUSMN; 0.11 m w.e.a(-1) in 1974-2000, -0.22 & PLUSMN; 0.05 m w.e. a(-1) in 1974-2015, and -0.20 & PLUSMN; 0.03 m w.e. a(-1) in 1974-2021. Geodetic MB presented negative twice since 2000 as measured by ordinary, consecutive short-term analysis, which was -0.41 & PLUSMN; 0.11 m w.e.a(-1) in 2000-2006, -0.50 & PLUSMN; 0.54 m w.e.a(-1) in 2006-2012, and -0.45 & PLUSMN; 0.11 m w.e.a(-1) in 2015-2021. 2) Data validation by in-situ stake-observations at 5350, 5450, and 5500 m a.s.l in 1959-1960 and the 2000s, our results demonstrated convincing altitude effects on glacier change. Between 5150 m a.s.l. and 5800 m a.s.l., Dh presented a more downwasting rate with increasing altitude. While above 5800 m a.s.l., Dh became less negative with increasing altitude, and glaciers presented thickening trend above 6200 m a.s.l. 3) With the difference of Dh by 10 m altitude intervals (DDh), we located a sharp surface lowering zone on the debris-covered glacier tongue at 5240 & PLUSMN; 20 m in the 1970s. We postulated that it could be the glacier terminus under debris cover, which moved upward by 190 & PLUSMN; 20 m in the past five decades. With DDh, we also defined the turning point altitude at 5800 m & PLUSMN; 20 m, where Dh began to increase to be less negative with altitude. Moreover, summer temperature change and the number of melting days clearly explained the Dh change with altitude. 4) Glacier contribution to runoff was estimated by 30 & PLUSMN; 16 % from 1974 to 2021, however, our data also showed that approximately 70 % of runoff was from melting glaciers in 2015-2021, implying an amplifying effect of recent global warming on glacier contrubutions. In summary, our findings proved that DDh was an effective indicator for exploring mountain glacier change. The varying results revealed here indicates that more studies would be helpful to define and predict glacier, climatic and hydrological changes in the Mt. Himalayas with multiple techniques and observations based on a common reference over long term.