Response to Comment by A. Richter, M. Horwath, R. Dietrich (2016) on 'Mass gains of the Antarctic ice sheet exceed losses' by H. J. Zwally and others (2015)

Abstract

Richter, Howarth and Dietrich (2016) and their Russian colleagues have made very precise geodetic measurements on Vostok Subglacial Lake using GPS (and GLONASS) in situ measurements. They use those measurements to estimate the rate of change of the height of the ice-surface (dHsurface/dt, or dhGPS/dt in our notation) over the Lake, concluding that the surface height has been stable at a rate close to zero (e.g. dhGPS/dt = +0.3 ±4.9 mm a-1 in Richter and others (2008) and dhGPS/dt= +1 ±5 mm a-1 in Richter and others (2014)). That conclusion differs from our findings of an average rate of +20 mm a-1 over the Lake, as derived from two independent satellite altimetry measurements (ERS-1/ERS-2 and ICESat) over 17 a. The objective of geodetic measurements with markers in the firn is to estimate a relatively small rate (e.g. 10 mm a-1) of surface motion to an accuracy of a few mm a-1 as the difference of two larger rates (e.g. -62.1 mm a-1 marker velocity and 72 mm a-1 new snow growth), where the uncertainty in snow growth is large for reasons we described. Our finding of a variation of dh/dt from <0 to >30 mm a-1 over the Lake from ICESat data indicates how important taking account of the spatial variability is to achieve valid comparisons with the satellite data. We also believe that the vertical strain rate in the compressing firn and the emplacement of the geodetic markers requires more attention. The lack of a discernable systematic reduction in dh/dt at the transitions from the grounded ice to the ice over the Lake or in the central area (certainly not in a ratio of 8%) raises fundamental questions about the concept of hydrostatic equilibrium as applied to the analysis of 'floating' of ice on Lake Vostok. Overall, we believe that the uncertainties in the geodetic measurements are sufficiently large that the in situ measurements and our altimetry measurements are not necessarily inconsistent.