www.sciencemag.org/cgi/content/full/science.aaa0940/DC1 Supplementary Material for Volume loss from Antarctic ice shelves is accelerating Fernando S. Paolo,* Helen A. Fricker, Laurie Padman *Corresponding author. E-mail: [email protected]Published 26 March 2015 on Science Express DOI: 10.1126/science.aaa0940 This PDF file includes: Materials and Methods Figs. S1 to S4 Table S1 and S2 Full Reference List Other Supplementary Material for this manuscript includes the following: (available at www.sciencemag.org/content/science.aaa0940/DC1) Movie S1
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Published 26 March 2015 on Science Express DOI: 10.1126/science.aaa0940
This PDF file includes: Materials and Methods Figs. S1 to S4 Table S1 and S2 Full Reference List Other Supplementary Material for this manuscript includes the following: (available at www.sciencemag.org/content/science.aaa0940/DC1)
Movie S1
Supplementary Materials:
Materials and Methods
Figures S1-S4
Table S1-S2
Movie S1
References (30-46)
Materials and Methods
Antarctic ice-shelf mask
To precisely define the ice-shelf boundaries (e.g., grounding lines, ice fronts) we used a 1-km-
resolution Antarctic mask (15), constructed using a composite of InSAR (30), ICESat (31,32),
MOA (33) and ASAID (34) products.
Raw radar altimeter data editing
Our satellite radar altimeter data are from NASA/GSFC’s Version 4 Level 2 Ice Data Records
(http://icesat4.gsfc.nasa.gov/radar_data/data_products.php). We rejected altimeter height
estimates in the following cases: i) The return altimeter waveform had no leading edge, meaning
that the part of the return corresponding to the surface was not captured; ii) The waveform had a
shape indicating specular reflection, which could indicate the presence of ponded surface water
(35); iii) One or more geophysical corrections were missing; iv) The point was located within
3 km of any ice-shelf boundary (grounding line or ice front); this removed across-boundary
measurements and minimized the impact of changes in ice-shelf perimeter.
Altimeter data corrections
All satellite radar altimeter data were retracked with the 5-parameter β-retracker algorithm (36).
We corrected for surface scattering variations for each satellite mission independently, similar to
Zwally et al. (13), Davis et al. (16), and Wingham et al. (17). We found that over the ice shelves
those methods provided more consistent results (i.e., similar performance over a wide range of
surface conditions) than accounting for short-time variability of surface properties as done by
Khvorostovsky (37). We corrected for tides using the Circum-Antarctic Tidal Simulation
CATS2008a (updated from 38), and load tide based on the TPXO7.2 ocean tide model (39). We
corrected for trends in atmospheric pressure (inverse barometer) using values from the ERA-
Interim (global atmospheric reanalysis) to estimate (mbar/year) for 1994-2012 (40). We
corrected for regional sea-level trends using the AVISO SSALTO/DUACS multi-mission
altimeter product (41), propagating and smoothing values for the unsurveyed regions underneath
ice shelves and persistent sea ice. Many of these corrections are small compared to the
magnitude of long-term thickness-change signals.
Radar-altimeter observations are relatively insensitive to fluctuations in the firn column due to
radar signal penetration into the firn layer (unlike laser pulses from laser altimeters that reflect
from the surface). We did not attempt to make a firn correction based on RACMO (42) as has
been adopted by some other researchers (e.g., 5), as we found no significant correlation between
the modeled firn-height trends and our observed height-change trends. Furthermore, we know
from work on the Amery Ice Shelf (43) that the radar extinction coefficient derived from the
return waveform (which is inversely proportional to penetration depth) is higher on the ice shelf
than it is on the drier parts of the ice-sheet interior (i.e., penetration is lower for the ice shelves
than for the ice-sheet interior). Additionally, in the vast majority of grid cells, densification of the
surface (a “competing” effect with penetration bias, i.e., opposite sign) is by far the dominant
effect, which we minimized by performing a backscatter correction.
In summary, our estimates of time-dependent ice-shelf height account for the lag of the satellite’s
Table S2. Comparison of our estimated thickness-change rates (m/year) with previous studies.
Table comparing our estimates (Paolo et al.) with Pritchard et al. (5), Shepherd et al. 1 (46) and
Shepherd et al. 2 (4). Missing values correspond to either different ice-shelf boundary definition
or ice shelf not reported. When required, we converted all the estimates to thickness change (in
m/year) and rounded values to facilitate the comparison. Values not significantly different from
zero were set to 0.0. See text for explanation on potential differences.
Ice shelf Paolo et al.
18 years
(1994-2012)
Pritchard et al.
5 years
(2003-2008)
Shepherd et al. 1
9 years
(1992-2001)
Shepherd et al. 2
14 years
(1994-2008)
Sulzberger 0.0 0.3
Nickerson 0.0 0.0
Getz -1.6 -1.7 -1.6 -1.8
Dotson -2.6 -5.2 -3.3
Crosson -3.1 -3.3 -4.5
Thwaites -2.8 -5.6 -5.5 -8.3
Pine Island -2.3 -4.9 -3.9 -6.0
Cosgrove -0.2 -0.6 -0.7
Abbot -0.2 0.4 -0.6
Venable -3.6 -2.5 -16.0
Stange -0.8 -0.6
Bach -0.9 -0.7 8.8
Wilkins -0.6 -0.6
George VI -1.1 -0.9 -0.8
Larsen B -0.4 -2.3
Larsen C -0.5 -0.9 -0.8
Larsen D -0.2 0.4
Brunt 0.3 0.3 0.6
Riiser 0.1 0.3
Fimbul 0.3 0.0 -0.5
Lazarev 0.0 -0.6
Amery 0.2 -0.6 0.9
West 0.0 -1.1
Shackleton 0.0 -1.1
Totten 0.0 -3.8
Moscow 0.0 -1.0 5.4
Holmes 0.0 -2.8
Dibble -1.0 -2.2
Mertz 0.0 0.3
Cook 0.0 1.1
Rennick -0.5 -1.2
Mariner 0.0 0.2
Drygalski 0.0 -0.3
Ross -0.2 0.1 0.2
Filchner-Ronne 0.0 0.2 0.5
Movie S1. Animation of cumulative thinning for the West Antarctic ice shelves (1994 to 2012).
Each time step corresponds to a three-month average thickness centered at the midpoint of the
time interval. Each time step represents thickness loss with respect to 1994.
References and Notes 1. A. Shepherd, E. R. Ivins, G. A, V. R. Barletta, M. J. Bentley, S. Bettadpur, K. H. Briggs, D. H.
Bromwich, R. Forsberg, N. Galin, M. Horwath, S. Jacobs, I. Joughin, M. A. King, J. T. Lenaerts, J. Li, S. R. Ligtenberg, A. Luckman, S. B. Luthcke, M. McMillan, R. Meister, G. Milne, J. Mouginot, A. Muir, J. P. Nicolas, J. Paden, A. J. Payne, H. Pritchard, E. Rignot, H. Rott, L. S. Sørensen, T. A. Scambos, B. Scheuchl, E. J. Schrama, B. Smith, A. V. Sundal, J. H. van Angelen, W. J. van de Berg, M. R. van den Broeke, D. G. Vaughan, I. Velicogna, J. Wahr, P. L. Whitehouse, D. J. Wingham, D. Yi, D. Young, H. J. Zwally, A reconciled estimate of ice-sheet mass balance. Science 338, 1183–1189 (2012). Medline doi:10.1126/science.1228102
2. T. C. Sutterley, I. Velicogna, E. Rignot, J. Mouginot, T. Flament, M. R. van den Broeke, J. M.van Wessem, C. H. Reijmer, Mass loss of the Amundsen Sea embayment of West Antarctica from four independent techniques. Geophys. Res. Lett. 41, 8421–8428 (2014). doi:10.1002/2014GL061940
3. I. Joughin, R. B. Alley, Stability of the West Antarctic ice sheet in a warming world. Nat.Geosci. 4, 506–513 (2011). doi:10.1038/ngeo1194
4. A. Shepherd, D. Wingham, D. Wallis, K. Giles, S. Laxon, A. V. Sundal, Recent loss offloating ice and the consequent sea level contribution. Geophys. Res. Lett. 37, L13503 (2010). doi:10.1029/2010GL042496
5. H. D. Pritchard, S. R. Ligtenberg, H. A. Fricker, D. G. Vaughan, M. R. van den Broeke, L.Padman, Antarctic ice-sheet loss driven by basal melting of ice shelves. Nature 484, 502–505 (2012). Medline doi:10.1038/nature10968
6. T. A. Scambos, J. A. Bohlander, C. A. Shuman, P. Skvarca, Glacier acceleration and thinningafter ice shelf collapse in the Larsen B embayment, Antarctica. Geophys. Res. Lett. 31, L18402 (2004). doi:10.1029/2004GL020670
7. C. Schoof, Ice sheet grounding line dynamics: Steady states, stability, and hysteresis. J.Geophys. Res. 112 (F3), F03S28 (2007). doi:10.1029/2006JF000664
8. D. Goldberg, D. M. Holland, C. Schoof, Grounding line movement and ice shelf buttressing inmarine ice sheets. J. Geophys. Res. 114 (F4), F04026 (2009). doi:10.1029/2008JF001227
9. L. Favier, G. Durand, S. L. Cornford, G. H. Gudmundsson, O. Gagliardini, F. Gillet-Chaulet,T. Zwinger, A. J. Payne, A. M. Le Brocq, Retreat of Pine Island Glacier controlled by marine ice-sheet instability. Nat. Clim. Change 4, 117–121 (2014). doi:10.1038/nclimate2094
10. I. Joughin, B. E. Smith, B. Medley, Marine ice sheet collapse potentially under way for theThwaites Glacier Basin, West Antarctica. Science 344, 735–738 (2014). Medline doi:10.1126/science.1249055
11. T. Scambos, C. Hulbe, M. Fahnestock, in Antarctic Peninsula Climate Variability: Historicaland Paleoenvironmental Perspectives, E. Domack et al., Eds. (American Geophysical Union, Washington, D. C., 2003), vol. 79, pp. 79-92.
12. P. Dutrieux, J. De Rydt, A. Jenkins, P. R. Holland, H. K. Ha, S. H. Lee, E. J. Steig, Q. Ding, E. P. Abrahamsen, M. Schröder, Strong sensitivity of Pine Island ice-shelf melting to climatic variability. Science 343, 174–178 (2014). Medline doi:10.1126/science.1244341
13. H. J. Zwally, M. B. Giovinetto, J. Li, H. G. Cornejo, M. A. Beckley, A. C. Brenner, J. L. Saba, D. Yi, Mass changes of the Greenland and Antarctic ice sheets and shelves and contributions to sea-level rise: 1992-2002. J. Glaciol. 51, 509–527 (2005). doi:10.3189/172756505781829007
14. E. Rignot, S. Jacobs, J. Mouginot, B. Scheuchl, Ice-shelf melting around Antarctica. Science 341, 266–270 (2013). Medline doi:10.1126/science.1235798
15. M. A. Depoorter, J. L. Bamber, J. A. Griggs, J. T. Lenaerts, S. R. Ligtenberg, M. R. van den Broeke, G. Moholdt, Calving fluxes and basal melt rates of Antarctic ice shelves. Nature 502, 89–92 (2013). Medline doi:10.1038/nature12567
16. C. H. Davis, A. C. Ferguson, Elevation change of the Antarctic ice sheet, 1995-2000, from ERS-2 satellite radar altimetry. IEEE Trans. Geosci. Rem. Sens. 42, 2437–2445 (2004). doi:10.1109/TGRS.2004.836789
17. D. J. Wingham, D. W. Wallis, A. Shepherd, Spatial and temporal evolution of Pine Island Glacier thinning, 1995-2006. Geophys. Res. Lett. 36, L17501 (2009). doi:10.1029/2009GL039126
18. Information on materials and methods is available at the Science Web site.
19. B. Efron, R. J. Tibshirani, An Introduction to the Bootstrap, vol. 57 of Monographs on Statistics and Applied Probability (Chapman and Hall, New York, 1993).
20. Corrections include lag of the satellite’s leading-edge tracker (retracking), surface scattering variations, surface slope, dry atmospheric mass, water vapor, the ionosphere, solid Earth tide, ocean tide and loading, atmospheric pressure and regional sea-level variation (see Supplementary Materials, 18).
21. R. Tibshirani, Regression shrinkage and selection via the lasso. J. R. Stat. Soc., B 58, 267–288 (1996).
22. H. A. Fricker, L. Padman, Thirty years of elevation change on Antarctic Peninsula ice shelves from multi-mission satellite radar altimetry. J. Geophys. Res. 117 (C2), C02026 (2012). doi:10.1029/2011JC007126
23. S. S. Jacobs, A. Jenkins, C. F. Giulivi, P. Dutrieux, Stronger ocean circulation and increased melting under Pine Island Glacier ice shelf. Nat. Geosci. 4, 519–523 (2011). doi:10.1038/ngeo1188
24. M. Thoma, A. Jenkins, D. Holland, S. Jacobs, Modelling circumpolar deep water intrusions on the Amundsen Sea continental shelf, Antarctica. Geophys. Res. Lett. 35, L18602 (2008). doi:10.1029/2008GL034939
25. E. Rignot, J. Mouginot, M. Morlighem, H. Seroussi, B. Scheuchl, Widespread, rapid grounding line retreat of Pine Island, Thwaites, Smith, and Kohler glaciers, West Antarctica, from 1992 to 2011. Geophys. Res. Lett. 41, 3502–3509 (2014). doi:10.1002/2014GL060140
26. J. Weertman, Stability of the junction of an ice sheet and an ice shelf. J. Glaciol. 13, 3–11 (1974).
27. A. J. Cook, D. G. Vaughan, Overview of areal changes of the ice shelves on the Antarctic Peninsula over the past 50 years. Cryosphere 4, 77–98 (2010). doi:10.5194/tc-4-77-2010
28. P. R. Holland, A. Brisbourne, H. F. J. Corr, D. McGrath, K. Purdon, J. Paden, H. A. Fricker, F. S. Paolo, A. H. Fleming, Atmospheric and oceanic forcing of Larsen C Ice Shelf thinning. Cryosphere Discuss. 9, 251–299 (2015). doi:10.5194/tcd-9-251-2015
29. C. P. Borstad, E. Rignot, J. Mouginot, M. P. Schodlok, Creep deformation and buttressing capacity of damaged ice shelves: Theory and application to Larsen C Ice Shelf. Cryosphere 7, 1931–1947 (2013). doi:10.5194/tc-7-1931-2013
30. E. Rignot, J. Mouginot, B. Scheuchl, Antarctic grounding line mapping from differential satellite radar interferometry. Geophys. Res. Lett. 38, L10504 (2011). doi:10.1029/2011GL047109
31. H. A. Fricker, L. Padman, Ice shelf grounding zone structure from ICESat laser altimetry. Geophys. Res. Lett. 33, L15502 (2006). doi:10.1029/2006GL026907
32. K. M. Brunt, H. A. Fricker, L. Padman, T. A. Scambos, S. O'Neel, Mapping the grounding zone of the Ross Ice Shelf, Antarctica, using ICESat laser altimetry. Ann. Glaciol. 51, 71–79 (2010). doi:10.3189/172756410791392790
33. T. A. Scambos, T. M. Haran, M. A. Fahnestock, T. H. Painter, J. Bohlander, MODIS-based mosaic of Antarctica (MOA) data sets: Continent-wide surface morphology and snow grain size. Remote Sens. Environ. 111, 242–257 (2007). doi:10.1016/j.rse.2006.12.020
34. R. Bindschadler, H. Choi, A. Wichlacz, R. Bingham, J. Bohlander, K. Brunt, H. Corr, R. Drews, H. Fricker, M. Hall, R. Hindmarsh, J. Kohler, L. Padman, W. Rack, G. Rotschky, S. Urbini, P. Vornberger, N. Young, Getting around Antarctica: New high-resolution mappings of the grounded and freely-floating boundaries of the Antarctic ice sheet created for the International Polar Year. Cryosphere 5, 569–588 (2011). doi:10.5194/tc-5-569-2011
35. H. A. Phillips, Surface meltstreams on the Amery Ice Shelf, East Antarctica. Ann. Glaciol. 27, 177–181 (1998).
36. A. C. Brenner, R. A. Blndschadler, R. H. Thomas, H. J. Zwally, Slope-induced errors in radar altimetry over continental ice sheets. J. Geophys. Res. 88 (C3), 1617–1623 (1983). doi:10.1029/JC088iC03p01617
37. K. S. Khvorostovsky, Merging and analysis of elevation time series over Greenland Ice Sheet from satellite radar altimetry. IEEE Trans. Geosci. Rem. Sens. 50, 23–36 (2011). doi:10.1109/TGRS.2011.2160071
38. L. Padman, H. A. Fricker, R. Coleman, S. Howard, S. Y. Erofeeva, A new tidal model for the Antarctic ice shelves and seas. Ann. Glaciol. 34, 247–254 (2002). doi:10.3189/172756402781817752
39. G. D. Egbert, S. Y. Erofeeva, Efficient inverse modeling of barotropic ocean tides. J. Atmos. Ocean. Technol. 19, 183–204 (2002). doi:10.1175/1520-0426(2002)019<0183:EIMOBO>2.0.CO;2
40. L. Padman, M. King, D. Goring, H. Corr, R. Coleman, Ice shelf elevation changes due to atmospheric pressure variations. J. Glaciol. 49, 521–526 (2003). doi:10.3189/172756503781830386
41. P. Y. Le Traon, F. Nadal, N. Ducet, An improved mapping method of multisatellite altimeter data. J. Atmos. Ocean. Technol. 15, 522–534 (1998). doi:10.1175/1520-0426(1998)015<0522:AIMMOM>2.0.CO;2
42. E. van Meijgaard et al., “The KNMI regional atmospheric model RACMO version 2.1” (Tech. Rep. 302, Royal Netherlands Meteorological Institute, 2008).
43. H. A. Phillips, thesis, Institute of Antarctic and Southern Ocean Studies, University of Tasmania (1999).
44. Y. Li, C. H. Davis, Improved methods for analysis of decadal elevation-change time series over Antarctica. IEEE Trans. Geosci. Rem. Sens. 44, 2687–2697 (2006). doi:10.1109/TGRS.2006.871894
45. P. Fretwell, H. D. Pritchard, D. G. Vaughan, J. L. Bamber, N. E. Barrand, R. Bell, C. Bianchi, R. G. Bingham, D. D. Blankenship, G. Casassa, G. Catania, D. Callens, H. Conway, A. J. Cook, H. F. J. Corr, D. Damaske, V. Damm, F. Ferraccioli, R. Forsberg, S. Fujita, Y. Gim, P. Gogineni, J. A. Griggs, R. C. A. Hindmarsh, P. Holmlund, J. W. Holt, R. W. Jacobel, A. Jenkins, W. Jokat, T. Jordan, E. C. King, J. Kohler, W. Krabill, M. Riger-Kusk, K. A. Langley, G. Leitchenkov, C. Leuschen, B. P. Luyendyk, K. Matsuoka, J. Mouginot, F. O. Nitsche, Y. Nogi, O. A. Nost, S. V. Popov, E. Rignot, D. M. Rippin, A. Rivera, J. Roberts, N. Ross, M. J. Siegert, A. M. Smith, D. Steinhage, M. Studinger, B. Sun, B. K. Tinto, B. C. Welch, D. Wilson, D. A. Young, C. Xiangbin, A. Zirizzotti, Bedmap2: Improved ice bed, surface and thickness datasets for Antarctica. Cryosphere 7, 375–393 (2013). doi:10.5194/tc-7-375-2013
46. A. Shepherd, D. Wingham, E. Rignot, Warm ocean is eroding West Antarctic Ice Sheet. Geophys. Res. Lett. 31, L23402 (2004). doi:10.1029/2004GL021106