Accelerated West Antarctic ice mass loss continues to outpace East Antarctic gains

https://doi.org/10.1016/j.epsl.2015.01.029Get rights and content

Highlights

  • A decade of time-variable gravity observations details Antarctic ice melting.

  • Slepian-function analysis reduces footprint of satellite-gravity based approaches.

  • Localized estimation approach dramatically reduces number of degrees of freedom.

  • We robustly resolve detailed patterns of ice melting using GRACE data products.

  • Glacio-isostatic adjustment models affect trends, not patterns or accelerations.

Abstract

While multiple data sources have confirmed that Antarctica is losing ice at an accelerating rate, different measurement techniques estimate the details of its geographically highly variable mass balance with different levels of accuracy, spatio-temporal resolution, and coverage. Some scope remains for methodological improvements using a single data type. In this study we report our progress in increasing the accuracy and spatial resolution of time-variable gravimetry from the Gravity Recovery and Climate Experiment (GRACE). We determine the geographic pattern of ice mass change in Antarctica between January 2003 and June 2014, accounting for glacio-isostatic adjustment (GIA) using the IJ05_R2 model. Expressing the unknown signal in a sparse Slepian basis constructed by optimization to prevent leakage out of the regions of interest, we use robust signal processing and statistical estimation methods. Applying those to the latest time series of monthly GRACE solutions we map Antarctica's mass loss in space and time as well as can be recovered from satellite gravity alone. Ignoring GIA model uncertainty, over the period 2003–2014, West Antarctica has been losing ice mass at a rate of 121±8Gt/yr and has experienced large acceleration of ice mass losses along the Amundsen Sea coast of 18±5Gt/yr2, doubling the mass loss rate in the past six years. The Antarctic Peninsula shows slightly accelerating ice mass loss, with larger accelerated losses in the southern half of the Peninsula. Ice mass gains due to snowfall in Dronning Maud Land have continued to add about half the amount of West Antarctica's loss back onto the continent over the last decade. We estimate the overall mass losses from Antarctica since January 2003 at 92±10Gt/yr.

Introduction

Knowing where and how mass currently changes in polar ice sheets is of great importance (Oppenheimer, 1998, Mitrovica et al., 2011, Stocker et al., 2013). Observations indicate that the Antarctic Ice Sheet is very sensitive to climate change (Raymo and Mitrovica, 2012, Bromwich et al., 2013, Cook et al., 2013, Kopp et al., 2013), and knowledge of individual glaciers and ice streams is important to understand the process. Ultimately, the continental ice sheet response to global change is the sum of the behaviors within individual drainage basins, which are subject to the combined effects of surface mass balance, calving and basal melting, and influenced by their geographic location and topography. The contemporary record of ice sheet mass balance has solidified significantly partly owing to the data gathered since 2002 by GRACE, the Gravity Recovery and Climate Experiment (Chen et al., 2006, Chen et al., 2009; Velicogna and Wahr, 2006). The continent-wide, decadally averaged mass balances of Antarctica, estimated by a variety of techniques, all show that Antarctica is losing mass (Shepherd et al., 2012, Hanna et al., 2013) at an accelerated rate (Luthcke et al., 2013, Williams et al., 2014).

While the large-scale spatial and long-term temporal signal trends during the 1990s and 2000s have been well determined, we focus here on the smaller scales recoverable by satellite gravity, and quantify their uncertainty. In Antarctica, improvements in modeling the ongoing glacio-isostatic adjustment from the Last Glacial Maximum deglaciation (for an accessible review, see King, 2013) have increased the precision of gravimetric mass balance estimates. As a result, the detailed pattern of mass change has been the focus of recent GRACE studies (Sasgen et al., 2010, Sasgen et al., 2013; Harig and Simons, 2012, Horwath et al., 2012, King et al., 2012, Lee et al., 2012, Luthcke et al., 2013, Velicogna and Wahr, 2013, Bouman et al., 2014). To inform our estimates of sea level change for the coming century it is imperative that we continue to build and improve the detailed record of changes in ice mass (Overpeck et al., 2006; Little et al., 2013a, Little et al., 2013b). In this paper we show when and where Antarctica has been losing mass over the last decade, using a method of spherical Slepian functions.

Section snippets

Motivation

As GRACE processing of intersatellite range-rates (Rowlands et al., 2005, Luthcke et al., 2006, Bettadpur and the CSR Level-2 Team, 2012) and global statistical estimation techniques (Schmidt et al., 2006, Han et al., 2008, Baur et al., 2009, Rowlands et al., 2010) have improved in recent years, the opportunity for contemporary gravimetric studies is to produce a better-resolved ice mass history and to understand its error structure at the same time. Knowledge of ice mass balance at a fine

Methods

In this study we use time-variable gravimetry to determine the mass change in Antarctica since 2003. We closely follow the methods of Harig and Simons (2012) and analyze GRACE Level 2 data using scalar spherical Slepian functions (Simons et al., 2006). GRACE data are released as coefficients to spherical harmonic functions, which spread their energy over the entire globe. In order to examine geophysical signals in specific regions, most authors (ourselves included) project global gravity data

Data and models

We used 129 monthly GRACE Release 5 geopotential fields from the Center for Space Research (CSR RL-05 in our labeling), University of Texas at Austin, covering the longest GRACE time span to date, from January  2003 to June 2014, including nine months with data gaps. These solutions include the October 2014 reprocessing which corrected errors in the atmosphere–ocean model (AOD1B) used to create GRACE Level 2 data, affecting monthly solutions from June 2013 onward. Degree-two order-zero

Continent-wide ice mass loss

Fig. 1, the map of the total ice mass change modeled from the CSR solutions between January 2003 and June 2014, shows the overall pattern of mass change over the whole of Antarctica. Our map was produced with a Slepian basis localized over the (buffered) portion of Antarctica that includes grounded ice, and the patterns of mass loss retrieved suggested subregions for closer inspection. Antarctica's surface area warrants the use of just 100 Slepian basis functions, a favorable reduction of the

Conclusions

While data from multiple sources have confirmed that Antarctica is losing ice at an accelerating rate, models made from observations have shown much variability in the details of its geographically highly variable mass balance (Rignot et al., 2008a, Pritchard et al., 2009, Lenaerts et al., 2012). Approaches for combining heterogeneous data types to constrain ice mass loss in space and time are invariably better at approximating averages than at capturing individual details. We have applied a

Acknowledgments

We thank the Editor Yanick Ricard, Matt King, and an anonymous reviewer for their attention to detail with which they reviewed our manuscript, which was substantially improved in the process. This work was supported by the U.S. National Science Foundation, via grants NSF-1245788 jointly funded by the Antarctic Glaciology and Geophysics Programs to F.J.S. and C.H., and EAR-1014606 to F.J.S. Figures were plotted using the Generic Mapping Tools (Wessel and Smith, 1998). Our computer codes are

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