Global Ice Mass Balance and its Contribution to Early Twenty-first Century Sea Level Rise

Sea level change is an important consequence of anthropogenic climate change, both for societies and for the environment. For the past several decades, analysis of tide gauge measurements show an average rise in sea level of 1.7+/-0.3 mm/yr from 1950 to 2009, while the analysis of satellite altimetry data indicates a rise of 3.3+/-0.3 mm/yr from 1993 to 2009. The discrepancy of the sea level rise estimate could be due to an accelerated sea level rise over the last three decades, or that the satellite estimate is contaminated by interannual or longer oceanic signals. Mass imbalance of the polar ice sheets and major mountain glacier systems is one of the main contributors to present-day global mean sea level rise. Recent advances in the determination of the global ice mass balance show that polar ice sheets and most of the major glacier systems are losing mass. However, large discrepancy exists among these contemporary observations.;The Gravity Recovery and Climate Experiment (GRACE) satellite gravimetry mission launched in March 2002 provides a means of unprecedented accuracy and temporal and spatial resolutions to quantify global ice mass changes. In this study, we estimate ice mass balance and the associated uncertainty for the world's ice sheets, major glacier and ice cap systems by using GRACE monthly gravity fields. Our result shows that the ice mass change rate over Greenland, from Jan 2003 to Dec 2012, is -267+/-10 Gt/yr, which is equivalent to sea level rise of 0.74+/-0.03 mm/yr. Evident accelerated ice mass loss in Greenland was found occurring during 2010, 2011 and 2012. The mass loss rates are about twice for the years before 2010. For Antarctica, we report ice mass changes of -118+/-30 Gt/yr (0.33+/-0.08 mm/yr sea level rise) and -194+/-86 Gt/yr (0.54+/-0.24 mm/yr sea level rise) using two different Glacial isostatic Adjustment (GIA) models, the Whitehouse and the Paulson GIA models, respectively, to correct for subglacial topography uplifts due to GIA process. The contribution of mass loss from global mountain glacier and ice caps from 5 major systems including Alaska, Iceland, Canadian Arctic, High Mountain Asia (HMA) and Patagonia is considerable. We report mass changes of --171+/-9 Gt/yr (mass loss, and an estimate of contribution to sea level rise of 0.48+/-0.02mm/yr, assuming the melt water immediately reaches the oceans).;In this study, we geometrically corrected the effects of spurious jumps and artificial trends in GRACE de-aliasing products due to the known errors in the ECMWF operational atmosphere model. However, the effect of these errors on the actual GRACE monthly gravity field inversion requires further studies. We found that the errors in the long-wavelength geopotential coefficients (C20 and C40) in GRACE determined monthly gravity fields are more significant than previously thought for ice sheet mass balance estimates using GRACE. More accurate estimates of HMA glacier mass balance are only possible by improving the current knowledge of hydrologic fluxes or hydrologic model fidelity over the Qinghai-Tibetan Plateau. Here we estimated the uncertainties of GRACE-derived mass changes based an elaborate error budget. Final estimates of global ice mass balance are calculated by applying the appropriate error propagation resulting from the estimated uncertainties. We reduced uncertainties in the estimates of GRACE-derived global mass balance by taking into account of corrections or errors of corrections not considered in previous studies. Finally, the contribution of global ice mass balance to the present-day sea level rise is estimated to be 1.55+/-0.10 mm/yr from January 2003 to December 2012 using GRACE monthly gravity fields.