Ice Melting Versus Tectonic Uplift: Current Mass Balance in Tibet as seen by GRACE

Uplift of the Tibetan Plateau started ~50 m.y. ago as a result of the collision between the Indian and the Eurasian Plates. It now consists of extensive flat terrain with average elevation of 5 km and the world highest mountain range at its southern rim. GPS surveys in 1990s demonstrated the average uplift rate of ~0.8 cm/yr in the plateau and ~1.6 cm/yr in the southern part (Himalaya) [Xu et al., 2000]. The total increase of mass is comparable to those in Fennoscandia where rapid uplift by postglacial rebound (PGR) makes a clear signal of gravity increase in the GRACE time-variable gravity data. On the other hand, the Tibetan Plateau is known as the third pole with the largest store of ice in the low/middle latitude zone. These mountain glaciers have relatively large seasonal changes in thickness, and their total mass is susceptible to recent global warming [Meier, 1984]. In fact, their current retreat rates suggest that 2/3 of the glaciers may disappear by 2050 [Qiu, 2008]. So the Tibetan ice mass loss should be observable with GRACE as a gravity decrease about a half of that observed in southern Alaska. Trend in gravity in Tibet 2002-2008 by GRACE shows only moderate amount of decrease. This would be a mixture of the two opposite factors, i.e. (1) gravity increase due to rapid tectonic uplift, and (2) gravity decrease due to ice melting. In this study, we try to present a self-consistent model by reconciling the mass gain due to uplift and mass loss caused by the glacial retreat. First we modeled the vertical velocity field in Tibet using GPS data, and calculated the gravity increase due to the uplift. Then we subtracted this gravity increase from the gravity trend observed by GRACE, to isolate the gravity signature of ice melting. The inferred mass loss, about a half of the value in Alaska, and 1.5 times as large as those in the Southern Andes, was fully consistent with the glaciologically expected rate of ice melting in Tibet. In order to assess its robustness to tectonic models, we assumed the vertical motions of Moho beneath Tibet in several different ways, i.e. (1) it uplifts coherently with surface, (2) it remains at the same position, and (3) it subsides to partially achieve isostasy, and evaluated the difference in the estimated ice melting rate.