Science Science 0036-8075 1095-9203 11 30 2012 338 6111 A Reconciled Estimate of Ice-Sheet Mass Balance Andrew Shepherd School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK. Erik R. Ivins Jet Propulsion Laboratory, M/S 300-233, 4800 Oak Grove Drive, Pasadena, CA 91109, USA. Geruo A Department of Physics, University of Colorado, Boulder, CO 80309–0390, USA. Valentina R. Barletta Geodynamics Department, Technical University of Denmark, DTU SPACE, National Space Institute, Elektrovej, Building 327, DK-2800 Kgs. Lyngby, Denmark. Mike J. Bentley Department of Geography, Durham University, South Road, Durham DH1 3LE, UK. Srinivas Bettadpur Center for Space Research, University of Texas at Austin, 3925 West Braker Lane, Suite 200, Austin, TX 78759–5321, USA. Kate H. Briggs School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK. David H. Bromwich Polar Meteorology Group, Byrd Polar Research Center, and Atmospheric Sciences Program, Department of Geography, The Ohio State University, 1090 Carmack Road, Columbus, OH 43210, USA. René Forsberg Geodynamics Department, Technical University of Denmark, DTU SPACE, National Space Institute, Elektrovej, Building 327, DK-2800 Kgs. Lyngby, Denmark. Natalia Galin Centre for Polar Observation and Modelling, Department of Earth Sciences, University College London, London WC1E 6BT, UK. Martin Horwath Institut für Astronomische und Physikalische Geodäsie, Technische Universität München, Arcisstraße 21, 80333 München, Germany. Stan Jacobs Lamont-Doherty Earth Observatory (LDEO), 205 Oceanography, 61 Route 9W - Post Office Box 1000, Palisades, NY 10964, USA. Ian Joughin Polar Science Center, Applied Physics Laboratory, University of Washington, 1013 NE 40th Street, Seattle, WA 98105–6698, USA. Matt A. King School of Civil Engineering and Geosciences, Cassie Building, Newcastle University, Newcastle upon Tyne NE1 7RU, UK. School of Geography and Environmental Studies, University of Tasmania, Hobart 7001, Australia. Jan T. M. Lenaerts Utrecht University, Institute for Marine and Atmospheric Research, Princetonplein 5, Utrecht, Netherlands. Jilu Li Center for Remote Sensing of Ice Sheets, University of Kansas, Nichols Hall, 2335 Irving Hill Road, Lawrence, KS 66045, USA. Stefan R. M. Ligtenberg Utrecht University, Institute for Marine and Atmospheric Research, Princetonplein 5, Utrecht, Netherlands. Adrian Luckman Department of Geography, College of Science, Swansea University, Singleton Park, Swansea SA2 8PP, UK. Scott B. Luthcke National Aeronautical and Space Administration (NASA) Goddard Space Flight Center, Planetary Geodynamics Laboratory, Greenbelt, MD 20771, USA. Malcolm McMillan School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK. Rakia Meister Centre for Polar Observation and Modelling, Department of Earth Sciences, University College London, London WC1E 6BT, UK. Glenn Milne Department of Earth Sciences, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada. Jeremie Mouginot Department of Earth System Science, University of California, 3226 Croul Hall, Irvine, CA 92697–3100, USA. Alan Muir Centre for Polar Observation and Modelling, Department of Earth Sciences, University College London, London WC1E 6BT, UK. Julien P. Nicolas Polar Meteorology Group, Byrd Polar Research Center, and Atmospheric Sciences Program, Department of Geography, The Ohio State University, 1090 Carmack Road, Columbus, OH 43210, USA. John Paden Center for Remote Sensing of Ice Sheets, University of Kansas, Nichols Hall, 2335 Irving Hill Road, Lawrence, KS 66045, USA. Antony J. Payne School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, UK. Hamish Pritchard British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK. Eric Rignot Jet Propulsion Laboratory, M/S 300-233, 4800 Oak Grove Drive, Pasadena, CA 91109, USA. Department of Earth System Science, University of California, 3226 Croul Hall, Irvine, CA 92697–3100, USA. Helmut Rott Institute of Meteorology and Geophysics, University of Innsbruck, Innsbruck, Austria. Louise Sandberg Sørensen Geodynamics Department, Technical University of Denmark, DTU SPACE, National Space Institute, Elektrovej, Building 327, DK-2800 Kgs. Lyngby, Denmark. Ted A. Scambos National Snow and Ice Data Center, University of Colorado, Boulder, CO 80309, USA. Bernd Scheuchl Department of Earth System Science, University of California, 3226 Croul Hall, Irvine, CA 92697–3100, USA. Ernst J. O. Schrama Delft University of Technology, Faculty of Aerospace Engineering, Kluyverweg 1, 2629 HS Delft, Netherlands. Ben Smith Polar Science Center, Applied Physics Laboratory, University of Washington, 1013 NE 40th Street, Seattle, WA 98105–6698, USA. Aud V. Sundal School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK. Jan H. van Angelen Utrecht University, Institute for Marine and Atmospheric Research, Princetonplein 5, Utrecht, Netherlands. Willem J. van de Berg Utrecht University, Institute for Marine and Atmospheric Research, Princetonplein 5, Utrecht, Netherlands. Michiel R. van den Broeke Utrecht University, Institute for Marine and Atmospheric Research, Princetonplein 5, Utrecht, Netherlands. David G. Vaughan British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK. Isabella Velicogna Jet Propulsion Laboratory, M/S 300-233, 4800 Oak Grove Drive, Pasadena, CA 91109, USA. Department of Earth System Science, University of California, 3226 Croul Hall, Irvine, CA 92697–3100, USA. John Wahr Department of Physics, University of Colorado, Boulder, CO 80309–0390, USA. Pippa L. Whitehouse Department of Geography, Durham University, South Road, Durham DH1 3LE, UK. Duncan J. Wingham Centre for Polar Observation and Modelling, Department of Earth Sciences, University College London, London WC1E 6BT, UK. Donghui Yi SGT Incorporated, NASA Goddard Space Flight Center, Cryospheric Sciences Laboratory, Code 615 Greenbelt, MD 20771, USA. Duncan Young Institute for Geophysics, University of Texas, Austin, TX 78759, USA. H. Jay Zwally NASA Goddard Space Flight Center, Cryospheric Sciences Laboratory, Code 615 Greenbelt, MD 20771, USA. Warming and Melting

Mass loss from the ice sheets of Greenland and Antarctica account for a large fraction of global sea-level rise. Part of this loss is because of the effects of warmer air temperatures, and another because of the rising ocean temperatures to which they are being exposed. Joughin et al. (p. 1172 ) review how ocean-ice interactions are impacting ice sheets and discuss the possible ways that exposure of floating ice shelves and grounded ice margins are subject to the influences of warming ocean currents. Estimates of the mass balance of the ice sheets of Greenland and Antarctica have differed greatly—in some cases, not even agreeing about whether there is a net loss or a net gain—making it more difficult to project accurately future sea-level change. Shepherd et al. (p. 1183 ) combined data sets produced by satellite altimetry, interferometry, and gravimetry to construct a more robust ice-sheet mass balance for the period between 1992 and 2011. All major regions of the two ice sheets appear to be losing mass, except for East Antarctica. All told, mass loss from the polar ice sheets is contributing about 0.6 millimeters per year (roughly 20% of the total) to the current rate of global sea-level rise.

The mass balance of the polar ice sheets is estimated by combining the results of existing independent techniques.

We combined an ensemble of satellite altimetry, interferometry, and gravimetry data sets using common geographical regions, time intervals, and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of Earth’s polar ice sheets. We find that there is good agreement between different satellite methods—especially in Greenland and West Antarctica—and that combining satellite data sets leads to greater certainty. Between 1992 and 2011, the ice sheets of Greenland, East Antarctica, West Antarctica, and the Antarctic Peninsula changed in mass by –142 ± 49, +14 ± 43, –65 ± 26, and –20 ± 14 gigatonnes year −1 , respectively. Since 1992, the polar ice sheets have contributed, on average, 0.59 ± 0.20 millimeter year −1 to the rate of global sea-level rise.

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