Elevation changes in Antarctica 1992–1996: Implications for Ice Sheet Mass Balances

Abstract

The uncertainties in the mass balance of the ice sheet of Antarctica of 5×10¹⁴ kg yr^-1 are the largest uncertainties in the causes of the observed rise in sea level. The uncertainty in the mass balance of Antarctica has increased in time as the estimated role of ice shelf bottom melting has grown. The past few years have seen dramatic improvements in the force modelling of altimeter satellites and improved correction of the surface and volume scattering contributions of ice sheet altimeter echoes. In consequence, we have been able to constrain the elevation change 1992–1996 of 60% of the Antarctic ice sheet to 0,5+0,7 cm yr^-1. The possible sources of error from the satellite orbit, the time-variant scattering at the ice sheet surface, and the echo travel-time corrections are all examined, and it is concluded these are too small to be significant to the five-year elevation trend. The relationship between the fluctuations of elevation and mass is examined through numerical modelling of the densification of firn at the surface of the ice sheet and it is concluded that, at a single location, time-variant densification, due to fluctuations in surface mass balance and surface density, may cause the elevation time-series to misrepresent the mass change by as much as +2 cm yr^-1. However, evidence from deep Antarctic cores, and from atmospheric models, point to a spatial correlation length of these surface fluctuations of less than 1.000 km. On this basis, we estimate the elevation time series to track the mass fluctuations to within +0,6 cm yr^-1. We thus conclude that 60% of the sheet has been in balance to within 7% in the period 1992–1996. Extended over Antarctica this is a mass balance uncertainty of 140 Gt yr^-1, a substantial reduction on the previous estimate.