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      Observing and Modeling Ice Sheet Surface Mass Balance

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          Abstract

          Surface mass balance (SMB) provides mass input to the surface of the Antarctic and Greenland Ice Sheets and therefore comprises an important control on ice sheet mass balance and resulting contribution to global sea level change. As ice sheet SMB varies highly across multiple scales of space (meters to hundreds of kilometers) and time (hourly to decadal), it is notoriously challenging to observe and represent in models. In addition, SMB consists of multiple components, all of which depend on complex interactions between the atmosphere and the snow/ice surface, large‐scale atmospheric circulation and ocean conditions, and ice sheet topography. In this review, we present the state‐of‐the‐art knowledge and recent advances in ice sheet SMB observations and models, highlight current shortcomings, and propose future directions. Novel observational methods allow mapping SMB across larger areas, longer time periods, and/or at very high (subdaily) temporal frequency. As a recent observational breakthrough, cosmic ray counters provide direct estimates of SMB, circumventing the need for accurate snow density observations upon which many other techniques rely. Regional atmospheric climate models have drastically improved their simulation of ice sheet SMB in the last decade, thanks to the inclusion or improved representation of essential processes (e.g., clouds, blowing snow, and snow albedo), and by enhancing horizontal resolution (5–30 km). Future modeling efforts are required in improving Earth system models to match regional atmospheric climate model performance in simulating ice sheet SMB, and in reinforcing the efforts in developing statistical and dynamic downscaling to represent smaller‐scale SMB processes.

          Key Points

          • Emerging (remote) observational techniques provide enhanced insights in spatial and temporal variability of ice sheet surface mass balance (SMB)

          • Regional climate models can be used to assess ice sheet SMB, although deficiencies remain in representing subgrid processes

          • In the near future, Earth System Models can be used to assess internal variability, forced change, and positive feedbacks on ice sheet SMB

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          Most cited references366

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          A reconciled estimate of ice-sheet mass balance.

          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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            Antarctic ice-sheet loss driven by basal melting of ice shelves.

            Accurate prediction of global sea-level rise requires that we understand the cause of recent, widespread and intensifying glacier acceleration along Antarctic ice-sheet coastal margins. Atmospheric and oceanic forcing have the potential to reduce the thickness and extent of floating ice shelves, potentially limiting their ability to buttress the flow of grounded tributary glaciers. Indeed, recent ice-shelf collapse led to retreat and acceleration of several glaciers on the Antarctic Peninsula. But the extent and magnitude of ice-shelf thickness change, the underlying causes of such change, and its link to glacier flow rate are so poorly understood that its future impact on the ice sheets cannot yet be predicted. Here we use satellite laser altimetry and modelling of the surface firn layer to reveal the circum-Antarctic pattern of ice-shelf thinning through increased basal melt. We deduce that this increased melt is the primary control of Antarctic ice-sheet loss, through a reduction in buttressing of the adjacent ice sheet leading to accelerated glacier flow. The highest thinning rates occur where warm water at depth can access thick ice shelves via submarine troughs crossing the continental shelf. Wind forcing could explain the dominant patterns of both basal melting and the surface melting and collapse of Antarctic ice shelves, through ocean upwelling in the Amundsen and Bellingshausen seas, and atmospheric warming on the Antarctic Peninsula. This implies that climate forcing through changing winds influences Antarctic ice-sheet mass balance, and hence global sea level, on annual to decadal timescales.
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              Time variability of the Earth's gravity field: Hydrological and oceanic effects and their possible detection using GRACE

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                Author and article information

                Contributors
                jan.lenaerts@colorado.edu
                Journal
                Rev Geophys
                Rev Geophys
                10.1002/(ISSN)1944-9208
                ROG
                Reviews of Geophysics (Washington, D.C. : 1985)
                John Wiley and Sons Inc. (Hoboken )
                8755-1209
                13 June 2019
                June 2019
                : 57
                : 2 ( doiID: 10.1002/rog.v57.2 )
                : 376-420
                Affiliations
                [ 1 ] Department of Atmospheric and Oceanic Sciences University of Colorado Boulder Boulder CO USA
                [ 2 ] Cryospheric Sciences Laboratory NASA GSFC Goddard MD USA
                [ 3 ] Institute for Marine and Atmospheric Research Utrecht University Utrecht The Netherlands
                [ 4 ] Faculty of Civil Engineering and Geosciences Delft University of Technology Delft The Netherlands
                Author notes
                [*] [* ] Correspondence to: J. T. M. Lenaerts,

                jan.lenaerts@ 123456colorado.edu

                Author information
                https://orcid.org/0000-0003-4309-4011
                https://orcid.org/0000-0002-9838-3665
                https://orcid.org/0000-0003-4662-7565
                https://orcid.org/0000-0002-1086-2435
                Article
                ROG20187 2018RG000622
                10.1029/2018RG000622
                6774314
                31598609
                80819b4d-dc85-4d92-8c48-4e94cf0cd2f5
                ©2019. The Authors.

                This is an open access article under the terms of the http://creativecommons.org/licenses/by-nc-nd/4.0/ License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non‐commercial and no modifications or adaptations are made.

                History
                : 13 November 2018
                : 17 March 2019
                : 19 March 2019
                Page count
                Figures: 16, Tables: 0, Pages: 45, Words: 21124
                Funding
                Funded by: National Aeronautics and Space Administration (NASA)
                Award ID: 80NSSC18K0201
                Award ID: 80NSSC17K0565
                Award ID: ICESat‐2 Project Science Office
                Funded by: Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)
                Award ID: 016.Vidi.171.065
                Categories
                Grand Challenges in the Earth and Space Sciences
                Cryosphere
                Ice Sheets
                Mass Balance
                Instruments and Techniques
                Modeling
                Geodesy and Gravity
                Mass Balance
                Ocean/Earth/atmosphere/hydrosphere/cryosphere interactions
                Informatics
                Modeling
                Natural Hazards
                Physical Modeling
                Feature Article
                Feature Article
                Custom metadata
                2.0
                rog20187
                June 2019
                Converter:WILEY_ML3GV2_TO_NLMPMC version:5.6.9 mode:remove_FC converted:01.10.2019

                ice sheets,observations,climate modeling,surface mass balance,antarctica,greenland

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