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Accelerated Sea-Level Rise from West Antarctica

Accelerated Sea-Level Rise from West Antarctica R EPORTS 11. A. T. Bell, Science 299, 1688 (2003). comparable reaction conditions for high-surface-area Explanations for the exceptional activity of 12. N. Lopez et al., J. Catal. 223, 232 (2004). supported Au (35). Au nanostructures often include a direct role 13. M.-S. Chen, A. K. Santra, D. W. Goodman, Phys. Rev. 28. G. C. Bond, D. T. Thompson, Catal. Rev. Sci. Eng. 41, played by the support, such as at the perimeter B 69, 155404 (2004). 319 (1999). 14. Materials and methods are available as supporting 29. F. Cosandey, T. E. Madey, Surf. Rev. Lett. 8,73 of the Au-support interface (7, 9, 28, 29). Our material on Science Online. (2001). data show that the TiO support is crucial as a 15. M.-S. Chen, D. W. Goodman, in preparation. 30. J. A. Rodriguez et al., J. Am. Chem. Soc. 124, 5242 dispersant and a promoter of the Au overlayer, 16. Annealing multilayer Ti O or TiO on a Mo(112)- (2002). 2 3 2 but that TiO itself cannot be directly c(22)-[SiO ] surface induces an interface restructur- 31. A. R. Lupini et al., in preparation. ing of the [SiO ] surface followed by decomposition or 32. Z.-P. Liu, X.-Q. Gong, J. Kohanoff, C. Sanchez, P. Hu, involved in the bonding of O or CO, because desorption at 1200 K. Phys. Rev. Lett. 91, 266102 (2003). in the Au-(11) and -(13) structures, the Au 17. Q. Guo, W. S. Oh, D. W. Goodman, Surf. Sci. 437,49 33. L. M. Molina, M. D. Rasmussen, B. Hammer, J. Chem. overlayer precludes access to the Ti cation (1999). Phys. 120, 7673 (2004). 18. The Mo-O phonon feature is at 78 meV, as deduced 34. M.-S. Chen, D. W. Goodman, unpublished data. sites by the reactants. That an optimum from the Mo(112) surface (Fig. 3A, curve a); the 35. T. V. Choudhary et al., J. Catal. 207, 247 (2002). reactivity is seen for the (13) Au structure comparable feature for Ti O is at 88 meV (17). A 36. We thank Y. Kuroda (Okayama University) for his 2 3 2þ strongly suggests that a combination of the single phonon at 70 meV, corresponding to Ti -O- valuable assistance as a visiting scientist and W. T. &þ Pd , was observed on a Pd-TiO complex (34) (fig. Wallace, B. K. Min, K. Gath, E. Ozensoy, C.-W. Yi, first- and second-layer Au sites is necessary to 3þ S7). This feature for a Ti -O-Mo species should Z. Yan, and K. Luo for fruitful discussions, com- promote reaction between CO and O .The appear between 78 and 88 meV. ments, and the supporting data displayed online. 3þ interaction of the first-layer Au with Ti of 19. C. Ruggiero, P. Hollins, Surf. Sci. 377–379, 583 Supported by the U.S. Department of Energy, &– (1997). Office of Basic Energy Sciences, Division of the support, yielding Au , likely is crucial for 20. F. Boccuzzi et al., J. Catal. 202, 256 (2001). Chemical Sciences; the Robert A. Welch Founda- activation of O (3, 4, 21–23). CO, however, 21. E. Wahlstro ¨m et al., Phys. Rev. Lett. 90, 026101 tion; and the Texas Advanced Technology Program has been shown to adsorb strongly on the Au (2003). under grant no. 010366-0022-2001. 22. A. Vijay, G. Mills, H. Metiu, J. Chem. Phys. 118, 6536 bilayer structure (5, 6). For the more general (2003). Supporting Online Material case of Au/TiO , mono- and bilayer Au 23. N. Lopez et al., J. Catal. 223, 232 (2004). www.sciencemag.org/cgi/content/full/1102420/DC1 islands in the G4-nm diameter range form as 24. P. D. Holmes, B. E. Koel, J. Vac. Sci. Technol. A 8, Materials and Methods a result of Au atoms nucleating initially at 2585 (1990). Figs. S1 to S7 3þ 25. D. A. Outka, R. J. Madix, Surf. Sci. 179, 351 (1987). Ti defect sites (2), then with the addition of 26. M. Haruta et al., J. Catal. 144, 175 (1993). 7 July 2004; accepted 16 August 2004 more Au, evolve into Au bilayer islands 27. The factor of 45 is based on a reaction rate for the Published online 26 August 2004; j5 -1 -2 stabilized by the bonding between the inter- Au-(13) structure of 1.82  10 molIs Im 10.1126/science.1102420 j7 -1 -2 3þ compared to a rate of 4.0  10 molIs Im at Include this information when citing this paper. facial Au atoms and Ti defects that accumulate at the Au-TiO interface (21–23, 30). The mono- and bilayer Au islands seen Accelerated Sea-Level Rise from for Au on TiO (110) (2) have also been observed for Au supported on high-surface- area TiO by Lupini and co-workers (31). 2 West Antarctica These mono- and bilayer Au structures appear 1,2 2,3 2 4 R. Thomas, E. Rignot, G. Casassa, P. Kanagaratnam, to be truncated analogs of the extended Au- 2 4 5 1 4 (11) and Au-(13) structures described C. Acun ˜ a, T. Akins, H. Brecher, E. Frederick, P. Gogineni, 6 1 4 2,7 here. In the arrangement of the (13) surface, W. Krabill, S. Manizade, H. Ramamoorthy, A. Rivera, 1 1 1 1 6 all the first-layer atoms of the bilayer Au R. Russell, J. Sonntag, R. Swift, J. Yungel, J. Zwally structure are accessible to the reactants, a morphology that may very well contribute to Recent aircraft and satellite laser altimeter surveys of the Amundsen Sea its exceptional catalytic activity. sector of West Antarctica show that local glaciers are discharging about 250 Recent density functional theoretical cal- cubic kilometers of ice per year to the ocean, almost 60% more than is culations have shown that O adsorbs pref- accumulated within their catchment basins. This discharge is sufficient to erentially and readily dissociates at the raise sea level by more than 0.2 millimeters per year. Glacier thinning rates Au-TiO interface (32). However, as we noted near the coast during 2002–2003 are much larger than those observed during earlier, for the Au-(13) structure, access by the 1990s. Most of these glaciers flow into floating ice shelves over bedrock O to the Au and Ti interface is precluded. In up to hundreds of meters deeper than previous estimates, providing exit any case, if O activation is promoted by the routes for ice from further inland if ice-sheet collapse is under way. Au-(11) sites and CO adsorbs on the Au- (13) sites, theory predicts a relatively small Perhaps half the present increase in global 1990s, nonpolar glaciers accounted for an barrier for the CO-O reaction (33). sea level of È1.8 mm/year is caused by estimated 0.4 mm/year (2)and Greenland melting of terrestrial ice (1). During the for È0.15 mm/year (3). Although data from References and Notes Antarctica are still sparse, they suggest a 1. M. Haruta, N. Yamada, T. Kobayashi, S. Ijima, J. 1 net loss from West Antarctica equivalent to EG&G Inc., NASA Goddard Space Flight Center Catal. 115, 301 (1989). È0.2 mm/year and approximate balance in (GSFC)/Wallops Flight Facility (WFF), Building N- 2. M. Valden, X. Lai, D. W. Goodman, Science 281, 1647 159, Wallops Island, VA 23337, USA. Centro de East Antarctica, where uncertainty remains (1998). Estudios Cientıficos (CECS), Avenida Arturo Prat 514, 3. D. W. Goodman, J. Catal. 216, 213 (2003). large (4). Substantial grounding line retreat Casilla 1469, Valdivia, Chile. Jet Propulsion Labora- 4. C. Chusuei, X. Lai, K. Luo, Q. Guo, D. W. Goodman, (5, 6), thinning (7), and acceleration (8) Top. Catal. 14, 71 (2001). tory (JPL), 4800 Oak Grove Drive, Pasadena, CA have been observed on glaciers flowing 5. D. C. Meier, D. W. Goodman, J. Am. Chem. Soc. 126, 91109, USA. Radar Systems and Remote Sensing 1892 (2004). Laboratory, University of Kansas, Lawrence, KS into the Amundsen Sea, with small ice 6. V. A. Bondzie, S. C. Parker, C. T. Campbell, Catal. Lett. 66045, USA. Byrd Polar Research Center, Ohio State shelves now but larger ones in the past 63, 143 (1999). University, Columbus, OH 43210, USA. Code 972, (9). These glaciers flow into ice shelves 7. M. Haruta, CATTECH 6, 102 (2002). NASA-GSFC, Greenbelt, MD 20771, USA. Departa- 8. J. J. Pietron, R. M. Stroud, D. R. Rolison, Nano Lett. 2, over beds well below sea level, and mento de Geografı ´a, Universidad de Chile, Casilla 545 (2002). 3387, Santiago, Chile. sustained thinning would allow them to 9. J. Guzman, B. C. Gates, J. Am. Chem. Soc. 126, 2672 float free from bedrock, potentially easing *To whom correspondence should be addressed. (2004). 10. M. M. Schubert et al., J. Catal. 197, 113 (2001). E-mail: [email protected] resistive forces acting on upstream ice www.sciencemag.org SCIENCE VOL 306 8 OCTOBER 2004 255 Downloaded from Downloaded from Downloaded from Downloaded from www.sciencemag.org www.sciencemag.org www.sciencemag.org www.sciencemag.org on May 4, 2015 on May 4, 2015 on May 4, 2015 on May 4, 2015 R EP ORTS and thereby leading to further glacier sheets are susceptible to near-coastal pertur- 16 km of ice over a catchment area of acceleration. bations are correct (10–12). Using our ice- 393,000 km (21). The extent to which ice shelves affect the thickness measurements, together with ve- Satellite radar altimetry data show all dynamics of tributary glaciers remains an locity estimates for 1996 (SMI, HAY, KOH, surveyed glaciers to have thinned rapidly unresolved controversy within glaciology. POP) and 2000 (PIG, THW) derived from during the 1990s (5, 22), with thinning rates Early suggestions that ice-shelf weakening interferometric synthetic aperture radar decreasing from È2, 3, and 5 m/year near would result in increased discharge from (InSAR), we find that the entire ice-sheet PIG, THW, and SMI grounding lines, the ice sheet (10–12) require ice-shelf Bback sector Eearlier estim ates ( 4– 6) only respectively, to È0.1 m/year hundreds of forces[ to affect glacier dynamics over long addressed individual glaciers^ bounded by kilometers inland. Comparison of our mea- distances. If correct, this implies that and including PIG and KOH discharged surements with surface elevations derived Bmarine ice sheets[ with beds deep below 253 T 5km /year of ice at the time of the from satellite laser altimeter data acquired sea level may be vulnerable to rapid velocity measurements, compared to a total by NASA_s ICESat (22, 23) in late 2003 and collapse if their deep beds extend to the annual snow accumulation equivalent of 160 T early 2004 shows thinning for each of the coast and if buttressing ice shelves are removed. But if glacier behavior is deter- mined mainly by local conditions, it is almost immune to distant perturbations (13, 14). The behavior of the Amundsen Sea glaciers, recent acceleration of tributary glaciers soon after ice-shelf breakup along the east side of the Antarctic Peninsula (15), and rapid acceleration of Greenland_s fastest glacier, Jakobshavn Isbrae, after thinning and breakup of its floating tongue (16, 17) may help to resolve this issue, which is particularly important because it could imply far more rapid ice discharge than currently predicted (1) from Antarctica in a warmer climate. Understanding these observations and predicting future glacier behavior requires detailed measurements of surface elevation and ice thickness, but the remoteness of the Amundsen Sea glaciers limited comprehen- sive measurements until late 2002, when surveys were made from Punta Arenas by CECS aboard a Chilean Navy P-3 aircraft equipped with NASA sensors, including a conically scanning laser altimeter (18) and ice-sounding radar (19). Four flights yielded measurements of surface elevations (to T0.4 m) at a dense array of 2-m laser footprints within a swath È500 m wide, and ice thickness to T20 m, along a total flight track of 3500 km. Surveys included Pine Island (PIG), Thwaites (THW), Haynes (HAY), Pope (POP), Smith (SMI), and Kohler (KOH) glaciers (Fig. 1), where our measurements show much deeper bedrock near the coast than had been estimated earlier (20). For the flight running mainly close to the coast between PIG and KOH, the bedrock was on average 400 m deeper than previous estimates (Fig. 2), reaching more than 1 km deeper for the SMI and KOH glacier troughs where no data had previously been obtained, with beds up to 2 km below sea level that may connect to the Byrd Subglacial Basin (BSB). Flights over Fig. 1. Part of West Antarctica, showing ice velocities (on a logarithmic scale) derived from 1996 PIG show its northern side to be shallower European Remote Sensing Satellites 1 and 2 (ERS-1/2) interferometric radar data (21), overlaid on than earlier estimates, but its main trunk and a radar image from Radarsat (28). The four CECS/NASA flight lines over Pine Island (PIG), Thwaites particularly its southern tributaries are (THW), Haynes (HAY), Pope (POP), Smith (SMI), and Kohler (KOH) glaciers, and Crosson (CRO) deeper further inland, suggesting another and Dotson (DOT) ice shelves, are shown in white, brown, yellow, and green lines. Boundaries of link to the BSB and the potential for ice- catchment basins are marked as thin broken black lines. Grounding line positions in 1996 inferred sheet collapse if suggestions that marine ice from ERS-1/2 are shown as thin continuous black lines. Inset shows location in Antarctica. 256 8 OCTOBER 2004 VOL 306 SCIENCE www.sciencemag.org R EPORTS four flight lines (Fig. 3), with average values effect of large radar footprints (several our conclusion that these measurements ranging from 0.4 m/year (for the flight kilometers, versus laser footprints of È2m show a real increase in thinning rates. primarily over PIG tributaries) to 1.8 m/ for aircraft and 60 m for ICESat), which Earlier observations (25) showed the year (for the flight crossing the seaward results in underestimation of high thinning seaward 25 km of PIG to be a grounded ends of THW, HAY, POP, SMI, and KOH), rates along narrow channels occupied by Bice plain[ with surface elevation less than and a mean value of 1.0 m/year for all some outlet glaciers, and partly because of a 30 m above flotation elevation. Our 2002 flights. Although the short time interval real increase in thinning. Our results from measurements include resurvey of a 1998 implies that elevation changes less than a 2002–2003 and 2004 show thinning along airborne survey (25), with estimated surface few tens of centimeters per year may simply the entire main trunk of PIG (Fig. 4A), elevation errors of T0.7 m, and show surface reflect natural fluctuations in snow accumu- averaging È1.2 m/year between 100 and lowering by 20 to 30 m across the ice plain lation rates, our results show many areas 300 km inland from the grounding line, or during the 5-year interim. Similar high with changes greater than 1 m/year and a double the value from satellite radar altim- thinning rates are also shown by comparison pattern of rapid thinning over fast-moving etry (24) for the period 1992–1999 in an of our measurements with those from ICE- parts of surveyed glaciers. If the 1 m/year area of smoother near-horizontal ice where Sat along an orbit track that followed our average thinning typifies conditions within the radar measurements should give reliable flight line across the ice plain (Fig. 4C). If the È60,000 km encompassed by our estimates. Further west, flights over THW, this continues, most of the ice plain should survey, this alone represents a volume loss SMI, and KOH show rapid thinning within float free from its bed within the next 5 of È60 km /year from only 15% of the total 50 km of the coast (Fig. 4B), decreasing years. catchment area. Although this estimate is with increasing distance from the sea. This Flotation of parts of the ice plain may approximate, it is consistent with losses follows a similar pattern to that inferred have been responsible for a 9% PIG velocity inferred from mass-budget calculations if from satellite radar altimetry (7), but with increase (8) between 1996 and 2001 (26), average thinning over the rest of the catch- higher thinning rates within 50 km of representing almost half the 22% increase ment area is 0.1 m/year, as inferred from grounding lines. Further inland, within the between 1974 and 2000 (27). Here, we com- satellite radar altimetry data (22). THW catchment basin, our results agree pared 24-day repeat data collected by Thinning rates near the grounding lines with those from radar altimetry data (7) RADARSAT-1 in January–February 2003 of all surveyed glaciers reach local maxima indicating little change in thinning rates. and in April 2001, using a speckle-tracking exceeding 5 m/year, but with high spatial This region has surface characteristics very technique, to show that PIG velocity in- variability resulting from rapid forward similar to those of inland regions of PIG, creased another 3.5 T 0.5% during that time motion of the undulating surface. These where our results show thinning rates period. This new observation of continued values are much higher than the earlier approximately double those from earlier velocity increase is consistent with the estimates, partly because of the smoothing radar altimetry data, lending confidence to increase in thinning rates inferred from our Fig. 2 (left). Bed topography, H (20), of Amundsen Sea glaciers, omitting the location of the Byrd Subglacial Basin. Fig. 3 (right). Thinning rates, elevations above sea level (color scale at lower left), and differences, dH/dt, interpolated from ERS-1/2 radar altimetry comparisons (7)over dH, between this and our measurements along flight tracks (color scale Amundsen Sea glaciers, with overlaid values obtained by comparing at lower right). Sections of floating ice along flight tracks are shown in CECS/NASA airborne and ICESat satellite laser altimetry data. Thin black red. Grounding line positions in 1996 are shown in black. BSB marks lines mark locations of profiles shown in Fig. 4. www.sciencemag.org SCIENCE VOL 306 8 OCTOBER 2004 257 R EP ORTS A C PIG THW HAY POP SMI KOH PIG -2000 -4000 -6000 0 100 200 300 400 0 50 100 150 200 250 300 020 40 60 Distance (km) Fig. 4. Surface and bed elevations, shown on the y axis in meters (black starting from the north. Thinning is largest along the channels of ice lines), and rates of elevation change in millimeters per year [red for discharge occupied by glaciers, as indicated by the ice velocity shown in earlier estimates from ERS-1/2 radar altimetry data (7); blue for our yellow in meters per year on the y axis. High spatial variability in our results], plotted against distance along sections of flight tracks shown in estimated thinning rates is apparent near the PIG grounding line [(A) and Fig. 3 for measurements (A) along PIG from the ice shelf to the interior; (C)], where down-glacier motion of high-amplitude surface undulations (B) along the coast crossing THW, HAY, POP, SMI, and KOH from east to causes excessive local thickening at some locations and thinning at west; and (C) across the main trunk of PIG near the grounding line others nearby. 2. M. Dyurgerov, Polar Geogr. 25, 241 (2001). largely canceled. Other error sources contribute elevation measurements on PIG. Flotation of 3. W. Krabill et al., Science 289, 428 (2000). about T0.2 m. Accuracy of ICESat measurements is the entire ice plain is likely to result in 4. E. Rignot, R. Thomas, Science 297, 1502 (2002). determined mainly by errors in laser pointing and by further glacier acceleration and additional 5. E. Rignot, Science 281, 549 (1998). forward scattering in thin clouds, with the latter 6. E. Rignot, J. Glaciol. 47, 213 (2001). edited from the data on the basis of distortion of increases in total ice discharge. The bed of 7. A. Shepherd, D. Wingham, J. Mansley, Geophys. Res. laser return waveforms. We estimated pointing the ice plain deepens from 600 m (below Lett. 29 (no. 10), 1364 (2002). errors for the September–November 2003 and the sea level at the grounding line) to 1300 m 8. E. Rignot, D.Vaughan,M.Schmeltz, T. Dupont, 2004 measurements to be G5 arc sec by comparing (25 km further inland) (Fig. 4A). This large D. MacAyeal, Ann. Glaciol. 34, 189 (2002). ICESat data with aircraft surveys over undulating 9. T. Kellogg, D. Kellogg, J. Geophys. Res. 92, 8859 terrain in Antarctica, Greenland, and arid parts of thickness slope, together with rapid glacier (1987) . the western United States. Most of our Amundsen motion, favors advection thickening by 10. T. Hughes, ISCAP Bull. (Ohio State University), no. glacier surveys were over slopes less than 2-, several tens of meters per year as thicker 1 (1972). resulting in slope-induced errors less than T0.5 m. 11. J. Mercer, Nature 27, 321 (1978). Consequently, estimated (largely random) errors in ice moves seaward. Even so, glacier thin- 12. R. Thomas, Geogr. Phys. Quat. 31, 347 (1977). elevation changes are less than T0.6 m. ICESat ning dominates, indicating that thinning by 13. P. Huybrechts, Ann. Glaciol. 14, 115 (1990). pointing errors for February–March 2003 data were longitudinal stretching exceeds advection 14. R. Hindmarsh, Ann. Glaciol. 23, 105 (1993). larger (up to 40 arc sec) and are not included in this 15. H. DeAngelis,P.Skvarca, Science 299, 1560 analysis. thickening. Inland of the ice plain, the bed (2003). 24. A. Shepherd, D. Wingham, J. Mansley, H. Corr, is almost horizontal for 250 km, and if 16. R. Thomas et al., J. Glaciol. 49, 231 (2003). Science 291, 862 (2001). the grounding line retreats to this deeper 17. R. Thomas, J. Glaciol., in press. 25. H. Corr, C. Doake, A. Jenkins, D. Vaughan, J. Glaciol. 18. W. Krabill et al., J. Geodyn. 34, 357 (2002). 47, 51 (2001). region, advection of thicker ice will decrease 19. S. Gogineni et al., J. Geophys. Res. 106, 33761 26. R. Thomas, E. Rignot, P. Kanagaratnam, W. Krabill, by an order of magnitude, allowing more rapid (2001). G. Casassa, Ann. Glaciol., in press. thinning. All surveyed glaciers have similar 27. I. Joughin, E. Rignot, C. Rosanova, B. Lucchitta, J. 20. M. Lythe, D. Vaughan, J. Geophys. Res. 106, 11335 (2001). Bohlander, Geophys. Res. Lett. 30 (no. 13), 1706 (2003). deep beds with ice plains close to flota- 21. E. Rignot et al., Ann. Glaciol., in press. 28. H. Liu, K. Jezek, B. Li, J. Geophys. Res. 104, 23199 tion in regions more than 10 km upstream 22. J. Zwally et al., J. Geodyn. 34, 405 (2002). (1999). from their 1996 grounding-line positions 23. NASA’s ICESat (Ice, Cloud, and Land Elevation 29. We dedicate this work to the memory of Niels Satellite) was launched into near-polar orbit in Gundestrup, who did much to make the project (Fig. 2), making all of them vulnerable to February 2003 carrying a laser altimeter (22)that possible despite serious illness. We thank pilots, a rapid and widespread response to thick- operated during February–March and September– crew, technicians, and staff from the Armada de ness change. November 2003 and during February–March 2004. Chile, University of Kansas, NASA/WFF, and CECS The catchment regions of Amundsen Sea Some of our aircraft surveys were along ICESat who helped make the surveys over Antarctica; the orbits, and all crossed orbit tracks at many locations. British Antarctic Survey, NSF, U.S. Geological glaciers contain enough ice to raise sea level Overlapping (by 50%) planar surfaces, or ‘‘platelets,’’ Survey, NOAA, Direcci´ on Meteorol´ ogica de Chile, by 1.3 m (6). Our measurements show them were fit to the È1200 aircraft measurements on and the field team at Base Carvajal for providing collectively to be 60% out of balance, suf- each side of the aircraft within a 70-m along-track weather reports/forecasts and GPS data that made distance, generally with root-mean-square fit of 10 possible the flights and subsequent accurate trajec- ficient to raise sea level by 0.24 mm/year. cm or less. These were compared with ICESat tory calculations; A. Shepherd for estimates of Although these glaciers are the fastest in footprint elevations by extrapolating elevations from elevation change rates (7); NASA’s ICESat Project Antarctica, they are likely to flow consider- any platelet within 200 m distance (using the for satellite laser altimeter elevation measurements; platelet slope), yielding almost 7000 comparisons. and T. Hughes and an anonymous reviewer for ably faster once the ice shelves are removed The major source of aircraft survey errors was laser suggesting improvements to the paper. Supported and glacier retreat proceeds into the deeper pointing, with errors magnified by the 15- off-nadir by CECS, through Fundaci´ on Andes and the Millen- part of glacier basins. scan angle. For the È1000-m altitude flown and a nium Science Initiative, and by NASA’s Cryospheric typical maximum roll error of 0.05- (effects of pitch Processes Program. E.R. performed his work at JPL errors are averaged out in the platelet calculation), under a contract with this program. References and Notes the resulting elevation error is G0.3 m, with opposite 1. Intergovernmental Panel on Climate Change, IPCC signs on each side of the surveyed swath. Because 27 April 2004; accepted 9 September 2004 Third Assessment Report, Climate Change 2001: The we averaged about four adjacent ICESat/aircraft Published online 23 September 2004; Scientific Basis (Cambridge Univ. Press, Cambridge, comparisons that included data from each side of 10.1126/science.1099650 2001). the aircraft, effects of these roll errors should be Include this information when citing this paper. 258 8 OCTOBER 2004 VOL 306 SCIENCE www.sciencemag.org Accelerated Sea-Level Rise from West Antarctica R. Thomas et al. Science 306, 255 (2004); DOI: 10.1126/science.1099650 This copy is for your personal, non-commercial use only. 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Accelerated Sea-Level Rise from West Antarctica

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R EPORTS 11. A. T. Bell, Science 299, 1688 (2003). comparable reaction conditions for high-surface-area Explanations for the exceptional activity of 12. N. Lopez et al., J. Catal. 223, 232 (2004). supported Au (35). Au nanostructures often include a direct role 13. M.-S. Chen, A. K. Santra, D. W. Goodman, Phys. Rev. 28. G. C. Bond, D. T. Thompson, Catal. Rev. Sci. Eng. 41, played by the support, such as at the perimeter B 69, 155404 (2004). 319 (1999). 14. Materials and methods are available as supporting 29. F. Cosandey, T. E. Madey, Surf. Rev. Lett. 8,73 of the Au-support interface (7, 9, 28, 29). Our material on Science Online. (2001). data show that the TiO support is crucial as a 15. M.-S. Chen, D. W. Goodman, in preparation. 30. J. A. Rodriguez et al., J. Am. Chem. Soc. 124, 5242 dispersant and a promoter of the Au overlayer, 16. Annealing multilayer Ti O or TiO on a Mo(112)- (2002). 2 3 2 but that TiO itself cannot be directly c(22)-[SiO ] surface induces an interface restructur- 31. A. R. Lupini et al., in preparation. ing of the [SiO ] surface followed by decomposition or 32. Z.-P. Liu, X.-Q. Gong, J. Kohanoff, C. Sanchez, P. Hu, involved in the bonding of O or CO, because desorption at 1200 K. Phys. Rev. Lett. 91, 266102 (2003). in the Au-(11) and -(13) structures, the Au 17. Q. Guo, W. S. Oh, D. W. Goodman, Surf. Sci. 437,49 33. L. M. Molina, M. D. Rasmussen, B. Hammer, J. Chem. overlayer precludes access to the Ti cation (1999). Phys. 120, 7673 (2004). 18. The Mo-O phonon feature is at 78 meV, as deduced 34. M.-S. Chen, D. W. Goodman, unpublished data. sites by the reactants. That an optimum from the Mo(112) surface (Fig. 3A, curve a); the 35. T. V. Choudhary et al., J. Catal. 207, 247 (2002). reactivity is seen for the (13) Au structure comparable feature for Ti O is at 88 meV (17). A 36. We thank Y. Kuroda (Okayama University) for his 2 3 2þ strongly suggests that a combination of the single phonon at 70 meV, corresponding to Ti -O- valuable assistance as a visiting scientist and W. T. &þ Pd , was observed on a Pd-TiO complex (34) (fig. Wallace, B. K. Min, K. Gath, E. Ozensoy, C.-W. Yi, first- and second-layer Au sites is necessary to 3þ S7). This feature for a Ti -O-Mo species should Z. Yan, and K. Luo for fruitful discussions, com- promote reaction between CO and O .The appear between 78 and 88 meV. ments, and the supporting data displayed online. 3þ interaction of the first-layer Au with Ti of 19. C. Ruggiero, P. Hollins, Surf. Sci. 377–379, 583 Supported by the U.S. Department of Energy, &– (1997). Office of Basic Energy Sciences, Division of the support, yielding Au , likely is crucial for 20. F. Boccuzzi et al., J. Catal. 202, 256 (2001). Chemical Sciences; the Robert A. Welch Founda- activation of O (3, 4, 21–23). CO, however, 21. E. Wahlstro ¨m et al., Phys. Rev. Lett. 90, 026101 tion; and the Texas Advanced Technology Program has been shown to adsorb strongly on the Au (2003). under grant no. 010366-0022-2001. 22. A. Vijay, G. Mills, H. Metiu, J. Chem. Phys. 118, 6536 bilayer structure (5, 6). For the more general (2003). Supporting Online Material case of Au/TiO , mono- and bilayer Au 23. N. Lopez et al., J. Catal. 223, 232 (2004). www.sciencemag.org/cgi/content/full/1102420/DC1 islands in the G4-nm diameter range form as 24. P. D. Holmes, B. E. Koel, J. Vac. Sci. Technol. A 8, Materials and Methods a result of Au atoms nucleating initially at 2585 (1990). Figs. S1 to S7 3þ 25. D. A. Outka, R. J. Madix, Surf. Sci. 179, 351 (1987). Ti defect sites (2), then with the addition of 26. M. Haruta et al., J. Catal. 144, 175 (1993). 7 July 2004; accepted 16 August 2004 more Au, evolve into Au bilayer islands 27. The factor of 45 is based on a reaction rate for the Published online 26 August 2004; j5 -1 -2 stabilized by the bonding between the inter- Au-(13) structure of 1.82  10 molIs Im 10.1126/science.1102420 j7 -1 -2 3þ compared to a rate of 4.0  10 molIs Im at Include this information when citing this paper. facial Au atoms and Ti defects that accumulate at the Au-TiO interface (21–23, 30). The mono- and bilayer Au islands seen Accelerated Sea-Level Rise from for Au on TiO (110) (2) have also been observed for Au supported on high-surface- area TiO by Lupini and co-workers (31). 2 West Antarctica These mono- and bilayer Au structures appear 1,2 2,3 2 4 R. Thomas, E. Rignot, G. Casassa, P. Kanagaratnam, to be truncated analogs of the extended Au- 2 4 5 1 4 (11) and Au-(13) structures described C. Acun ˜ a, T. Akins, H. Brecher, E. Frederick, P. Gogineni, 6 1 4 2,7 here. In the arrangement of the (13) surface, W. Krabill, S. Manizade, H. Ramamoorthy, A. Rivera, 1 1 1 1 6 all the first-layer atoms of the bilayer Au R. Russell, J. Sonntag, R. Swift, J. Yungel, J. Zwally structure are accessible to the reactants, a morphology that may very well contribute to Recent aircraft and satellite laser altimeter surveys of the Amundsen Sea its exceptional catalytic activity. sector of West Antarctica show that local glaciers are discharging about 250 Recent density functional theoretical cal- cubic kilometers of ice per year to the ocean, almost 60% more than is culations have shown that O adsorbs pref- accumulated within their catchment basins. This discharge is sufficient to erentially and readily dissociates at the raise sea level by more than 0.2 millimeters per year. Glacier thinning rates Au-TiO interface (32). However, as we noted near the coast during 2002–2003 are much larger than those observed during earlier, for the Au-(13) structure, access by the 1990s. Most of these glaciers flow into floating ice shelves over bedrock O to the Au and Ti interface is precluded. In up to hundreds of meters deeper than previous estimates, providing exit any case, if O activation is promoted by the routes for ice from further inland if ice-sheet collapse is under way. Au-(11) sites and CO adsorbs on the Au- (13) sites, theory predicts a relatively small Perhaps half the present increase in global 1990s, nonpolar glaciers accounted for an barrier for the CO-O reaction (33). sea level of È1.8 mm/year is caused by estimated 0.4 mm/year (2)and Greenland melting of terrestrial ice (1). During the for È0.15 mm/year (3). Although data from References and Notes Antarctica are still sparse, they suggest a 1. M. Haruta, N. Yamada, T. Kobayashi, S. Ijima, J. 1 net loss from West Antarctica equivalent to EG&G Inc., NASA Goddard Space Flight Center Catal. 115, 301 (1989). È0.2 mm/year and approximate balance in (GSFC)/Wallops Flight Facility (WFF), Building N- 2. M. Valden, X. Lai, D. W. Goodman, Science 281, 1647 159, Wallops Island, VA 23337, USA. Centro de East Antarctica, where uncertainty remains (1998). Estudios Cientıficos (CECS), Avenida Arturo Prat 514, 3. D. W. Goodman, J. Catal. 216, 213 (2003). large (4). Substantial grounding line retreat Casilla 1469, Valdivia, Chile. Jet Propulsion Labora- 4. C. Chusuei, X. Lai, K. Luo, Q. Guo, D. W. Goodman, (5, 6), thinning (7), and acceleration (8) Top. Catal. 14, 71 (2001). tory (JPL), 4800 Oak Grove Drive, Pasadena, CA have been observed on glaciers flowing 5. D. C. Meier, D. W. Goodman, J. Am. Chem. Soc. 126, 91109, USA. Radar Systems and Remote Sensing 1892 (2004). Laboratory, University of Kansas, Lawrence, KS into the Amundsen Sea, with small ice 6. V. A. Bondzie, S. C. Parker, C. T. Campbell, Catal. Lett. 66045, USA. Byrd Polar Research Center, Ohio State shelves now but larger ones in the past 63, 143 (1999). University, Columbus, OH 43210, USA. Code 972, (9). These glaciers flow into ice shelves 7. M. Haruta, CATTECH 6, 102 (2002). NASA-GSFC, Greenbelt, MD 20771, USA. Departa- 8. J. J. Pietron, R. M. Stroud, D. R. Rolison, Nano Lett. 2, over beds well below sea level, and mento de Geografı ´a, Universidad de Chile, Casilla 545 (2002). 3387, Santiago, Chile. sustained thinning would allow them to 9. J. Guzman, B. C. Gates, J. Am. Chem. Soc. 126, 2672 float free from bedrock, potentially easing *To whom correspondence should be addressed. (2004). 10. M. M. Schubert et al., J. Catal. 197, 113 (2001). E-mail: [email protected] resistive forces acting on upstream ice www.sciencemag.org SCIENCE VOL 306 8 OCTOBER 2004 255 Downloaded from Downloaded from Downloaded from Downloaded from www.sciencemag.org www.sciencemag.org www.sciencemag.org www.sciencemag.org on May 4, 2015 on May 4, 2015 on May 4, 2015 on May 4, 2015 R EP ORTS and thereby leading to further glacier sheets are susceptible to near-coastal pertur- 16 km of ice over a catchment area of acceleration. bations are correct (10–12). Using our ice- 393,000 km (21). The extent to which ice shelves affect the thickness measurements, together with ve- Satellite radar altimetry data show all dynamics of tributary glaciers remains an locity estimates for 1996 (SMI, HAY, KOH, surveyed glaciers to have thinned rapidly unresolved controversy within glaciology. POP) and 2000 (PIG, THW) derived from during the 1990s (5, 22), with thinning rates Early suggestions that ice-shelf weakening interferometric synthetic aperture radar decreasing from È2, 3, and 5 m/year near would result in increased discharge from (InSAR), we find that the entire ice-sheet PIG, THW, and SMI grounding lines, the ice sheet (10–12) require ice-shelf Bback sector Eearlier estim ates ( 4– 6) only respectively, to È0.1 m/year hundreds of forces[ to affect glacier dynamics over long addressed individual glaciers^ bounded by kilometers inland. Comparison of our mea- distances. If correct, this implies that and including PIG and KOH discharged surements with surface elevations derived Bmarine ice sheets[ with beds deep below 253 T 5km /year of ice at the time of the from satellite laser altimeter data acquired sea level may be vulnerable to rapid velocity measurements, compared to a total by NASA_s ICESat (22, 23) in late 2003 and collapse if their deep beds extend to the annual snow accumulation equivalent of 160 T early 2004 shows thinning for each of the coast and if buttressing ice shelves are removed. But if glacier behavior is deter- mined mainly by local conditions, it is almost immune to distant perturbations (13, 14). The behavior of the Amundsen Sea glaciers, recent acceleration of tributary glaciers soon after ice-shelf breakup along the east side of the Antarctic Peninsula (15), and rapid acceleration of Greenland_s fastest glacier, Jakobshavn Isbrae, after thinning and breakup of its floating tongue (16, 17) may help to resolve this issue, which is particularly important because it could imply far more rapid ice discharge than currently predicted (1) from Antarctica in a warmer climate. Understanding these observations and predicting future glacier behavior requires detailed measurements of surface elevation and ice thickness, but the remoteness of the Amundsen Sea glaciers limited comprehen- sive measurements until late 2002, when surveys were made from Punta Arenas by CECS aboard a Chilean Navy P-3 aircraft equipped with NASA sensors, including a conically scanning laser altimeter (18) and ice-sounding radar (19). Four flights yielded measurements of surface elevations (to T0.4 m) at a dense array of 2-m laser footprints within a swath È500 m wide, and ice thickness to T20 m, along a total flight track of 3500 km. Surveys included Pine Island (PIG), Thwaites (THW), Haynes (HAY), Pope (POP), Smith (SMI), and Kohler (KOH) glaciers (Fig. 1), where our measurements show much deeper bedrock near the coast than had been estimated earlier (20). For the flight running mainly close to the coast between PIG and KOH, the bedrock was on average 400 m deeper than previous estimates (Fig. 2), reaching more than 1 km deeper for the SMI and KOH glacier troughs where no data had previously been obtained, with beds up to 2 km below sea level that may connect to the Byrd Subglacial Basin (BSB). Flights over Fig. 1. Part of West Antarctica, showing ice velocities (on a logarithmic scale) derived from 1996 PIG show its northern side to be shallower European Remote Sensing Satellites 1 and 2 (ERS-1/2) interferometric radar data (21), overlaid on than earlier estimates, but its main trunk and a radar image from Radarsat (28). The four CECS/NASA flight lines over Pine Island (PIG), Thwaites particularly its southern tributaries are (THW), Haynes (HAY), Pope (POP), Smith (SMI), and Kohler (KOH) glaciers, and Crosson (CRO) deeper further inland, suggesting another and Dotson (DOT) ice shelves, are shown in white, brown, yellow, and green lines. Boundaries of link to the BSB and the potential for ice- catchment basins are marked as thin broken black lines. Grounding line positions in 1996 inferred sheet collapse if suggestions that marine ice from ERS-1/2 are shown as thin continuous black lines. Inset shows location in Antarctica. 256 8 OCTOBER 2004 VOL 306 SCIENCE www.sciencemag.org R EPORTS four flight lines (Fig. 3), with average values effect of large radar footprints (several our conclusion that these measurements ranging from 0.4 m/year (for the flight kilometers, versus laser footprints of È2m show a real increase in thinning rates. primarily over PIG tributaries) to 1.8 m/ for aircraft and 60 m for ICESat), which Earlier observations (25) showed the year (for the flight crossing the seaward results in underestimation of high thinning seaward 25 km of PIG to be a grounded ends of THW, HAY, POP, SMI, and KOH), rates along narrow channels occupied by Bice plain[ with surface elevation less than and a mean value of 1.0 m/year for all some outlet glaciers, and partly because of a 30 m above flotation elevation. Our 2002 flights. Although the short time interval real increase in thinning. Our results from measurements include resurvey of a 1998 implies that elevation changes less than a 2002–2003 and 2004 show thinning along airborne survey (25), with estimated surface few tens of centimeters per year may simply the entire main trunk of PIG (Fig. 4A), elevation errors of T0.7 m, and show surface reflect natural fluctuations in snow accumu- averaging È1.2 m/year between 100 and lowering by 20 to 30 m across the ice plain lation rates, our results show many areas 300 km inland from the grounding line, or during the 5-year interim. Similar high with changes greater than 1 m/year and a double the value from satellite radar altim- thinning rates are also shown by comparison pattern of rapid thinning over fast-moving etry (24) for the period 1992–1999 in an of our measurements with those from ICE- parts of surveyed glaciers. If the 1 m/year area of smoother near-horizontal ice where Sat along an orbit track that followed our average thinning typifies conditions within the radar measurements should give reliable flight line across the ice plain (Fig. 4C). If the È60,000 km encompassed by our estimates. Further west, flights over THW, this continues, most of the ice plain should survey, this alone represents a volume loss SMI, and KOH show rapid thinning within float free from its bed within the next 5 of È60 km /year from only 15% of the total 50 km of the coast (Fig. 4B), decreasing years. catchment area. Although this estimate is with increasing distance from the sea. This Flotation of parts of the ice plain may approximate, it is consistent with losses follows a similar pattern to that inferred have been responsible for a 9% PIG velocity inferred from mass-budget calculations if from satellite radar altimetry (7), but with increase (8) between 1996 and 2001 (26), average thinning over the rest of the catch- higher thinning rates within 50 km of representing almost half the 22% increase ment area is 0.1 m/year, as inferred from grounding lines. Further inland, within the between 1974 and 2000 (27). Here, we com- satellite radar altimetry data (22). THW catchment basin, our results agree pared 24-day repeat data collected by Thinning rates near the grounding lines with those from radar altimetry data (7) RADARSAT-1 in January–February 2003 of all surveyed glaciers reach local maxima indicating little change in thinning rates. and in April 2001, using a speckle-tracking exceeding 5 m/year, but with high spatial This region has surface characteristics very technique, to show that PIG velocity in- variability resulting from rapid forward similar to those of inland regions of PIG, creased another 3.5 T 0.5% during that time motion of the undulating surface. These where our results show thinning rates period. This new observation of continued values are much higher than the earlier approximately double those from earlier velocity increase is consistent with the estimates, partly because of the smoothing radar altimetry data, lending confidence to increase in thinning rates inferred from our Fig. 2 (left). Bed topography, H (20), of Amundsen Sea glaciers, omitting the location of the Byrd Subglacial Basin. Fig. 3 (right). Thinning rates, elevations above sea level (color scale at lower left), and differences, dH/dt, interpolated from ERS-1/2 radar altimetry comparisons (7)over dH, between this and our measurements along flight tracks (color scale Amundsen Sea glaciers, with overlaid values obtained by comparing at lower right). Sections of floating ice along flight tracks are shown in CECS/NASA airborne and ICESat satellite laser altimetry data. Thin black red. Grounding line positions in 1996 are shown in black. BSB marks lines mark locations of profiles shown in Fig. 4. www.sciencemag.org SCIENCE VOL 306 8 OCTOBER 2004 257 R EP ORTS A C PIG THW HAY POP SMI KOH PIG -2000 -4000 -6000 0 100 200 300 400 0 50 100 150 200 250 300 020 40 60 Distance (km) Fig. 4. Surface and bed elevations, shown on the y axis in meters (black starting from the north. Thinning is largest along the channels of ice lines), and rates of elevation change in millimeters per year [red for discharge occupied by glaciers, as indicated by the ice velocity shown in earlier estimates from ERS-1/2 radar altimetry data (7); blue for our yellow in meters per year on the y axis. High spatial variability in our results], plotted against distance along sections of flight tracks shown in estimated thinning rates is apparent near the PIG grounding line [(A) and Fig. 3 for measurements (A) along PIG from the ice shelf to the interior; (C)], where down-glacier motion of high-amplitude surface undulations (B) along the coast crossing THW, HAY, POP, SMI, and KOH from east to causes excessive local thickening at some locations and thinning at west; and (C) across the main trunk of PIG near the grounding line others nearby. 2. M. Dyurgerov, Polar Geogr. 25, 241 (2001). largely canceled. Other error sources contribute elevation measurements on PIG. Flotation of 3. W. Krabill et al., Science 289, 428 (2000). about T0.2 m. Accuracy of ICESat measurements is the entire ice plain is likely to result in 4. E. Rignot, R. Thomas, Science 297, 1502 (2002). determined mainly by errors in laser pointing and by further glacier acceleration and additional 5. E. Rignot, Science 281, 549 (1998). forward scattering in thin clouds, with the latter 6. E. Rignot, J. Glaciol. 47, 213 (2001). edited from the data on the basis of distortion of increases in total ice discharge. The bed of 7. A. Shepherd, D. Wingham, J. Mansley, Geophys. Res. laser return waveforms. We estimated pointing the ice plain deepens from 600 m (below Lett. 29 (no. 10), 1364 (2002). errors for the September–November 2003 and the sea level at the grounding line) to 1300 m 8. E. Rignot, D.Vaughan,M.Schmeltz, T. Dupont, 2004 measurements to be G5 arc sec by comparing (25 km further inland) (Fig. 4A). This large D. MacAyeal, Ann. Glaciol. 34, 189 (2002). ICESat data with aircraft surveys over undulating 9. T. Kellogg, D. Kellogg, J. Geophys. Res. 92, 8859 terrain in Antarctica, Greenland, and arid parts of thickness slope, together with rapid glacier (1987) . the western United States. Most of our Amundsen motion, favors advection thickening by 10. T. Hughes, ISCAP Bull. (Ohio State University), no. glacier surveys were over slopes less than 2-, several tens of meters per year as thicker 1 (1972). resulting in slope-induced errors less than T0.5 m. 11. J. Mercer, Nature 27, 321 (1978). Consequently, estimated (largely random) errors in ice moves seaward. Even so, glacier thin- 12. R. Thomas, Geogr. Phys. Quat. 31, 347 (1977). elevation changes are less than T0.6 m. ICESat ning dominates, indicating that thinning by 13. P. Huybrechts, Ann. Glaciol. 14, 115 (1990). pointing errors for February–March 2003 data were longitudinal stretching exceeds advection 14. R. Hindmarsh, Ann. Glaciol. 23, 105 (1993). larger (up to 40 arc sec) and are not included in this 15. H. DeAngelis,P.Skvarca, Science 299, 1560 analysis. thickening. Inland of the ice plain, the bed (2003). 24. A. Shepherd, D. Wingham, J. Mansley, H. Corr, is almost horizontal for 250 km, and if 16. R. Thomas et al., J. Glaciol. 49, 231 (2003). Science 291, 862 (2001). the grounding line retreats to this deeper 17. R. Thomas, J. Glaciol., in press. 25. H. Corr, C. Doake, A. Jenkins, D. Vaughan, J. Glaciol. 18. W. Krabill et al., J. Geodyn. 34, 357 (2002). 47, 51 (2001). region, advection of thicker ice will decrease 19. S. Gogineni et al., J. Geophys. Res. 106, 33761 26. R. Thomas, E. Rignot, P. Kanagaratnam, W. Krabill, by an order of magnitude, allowing more rapid (2001). G. Casassa, Ann. Glaciol., in press. thinning. All surveyed glaciers have similar 27. I. Joughin, E. Rignot, C. Rosanova, B. Lucchitta, J. 20. M. Lythe, D. Vaughan, J. Geophys. Res. 106, 11335 (2001). Bohlander, Geophys. Res. Lett. 30 (no. 13), 1706 (2003). deep beds with ice plains close to flota- 21. E. Rignot et al., Ann. Glaciol., in press. 28. H. Liu, K. Jezek, B. Li, J. Geophys. Res. 104, 23199 tion in regions more than 10 km upstream 22. J. Zwally et al., J. Geodyn. 34, 405 (2002). (1999). from their 1996 grounding-line positions 23. NASA’s ICESat (Ice, Cloud, and Land Elevation 29. We dedicate this work to the memory of Niels Satellite) was launched into near-polar orbit in Gundestrup, who did much to make the project (Fig. 2), making all of them vulnerable to February 2003 carrying a laser altimeter (22)that possible despite serious illness. We thank pilots, a rapid and widespread response to thick- operated during February–March and September– crew, technicians, and staff from the Armada de ness change. November 2003 and during February–March 2004. Chile, University of Kansas, NASA/WFF, and CECS The catchment regions of Amundsen Sea Some of our aircraft surveys were along ICESat who helped make the surveys over Antarctica; the orbits, and all crossed orbit tracks at many locations. British Antarctic Survey, NSF, U.S. Geological glaciers contain enough ice to raise sea level Overlapping (by 50%) planar surfaces, or ‘‘platelets,’’ Survey, NOAA, Direcci´ on Meteorol´ ogica de Chile, by 1.3 m (6). Our measurements show them were fit to the È1200 aircraft measurements on and the field team at Base Carvajal for providing collectively to be 60% out of balance, suf- each side of the aircraft within a 70-m along-track weather reports/forecasts and GPS data that made distance, generally with root-mean-square fit of 10 possible the flights and subsequent accurate trajec- ficient to raise sea level by 0.24 mm/year. cm or less. These were compared with ICESat tory calculations; A. Shepherd for estimates of Although these glaciers are the fastest in footprint elevations by extrapolating elevations from elevation change rates (7); NASA’s ICESat Project Antarctica, they are likely to flow consider- any platelet within 200 m distance (using the for satellite laser altimeter elevation measurements; platelet slope), yielding almost 7000 comparisons. and T. Hughes and an anonymous reviewer for ably faster once the ice shelves are removed The major source of aircraft survey errors was laser suggesting improvements to the paper. Supported and glacier retreat proceeds into the deeper pointing, with errors magnified by the 15- off-nadir by CECS, through Fundaci´ on Andes and the Millen- part of glacier basins. scan angle. For the È1000-m altitude flown and a nium Science Initiative, and by NASA’s Cryospheric typical maximum roll error of 0.05- (effects of pitch Processes Program. E.R. performed his work at JPL errors are averaged out in the platelet calculation), under a contract with this program. References and Notes the resulting elevation error is G0.3 m, with opposite 1. Intergovernmental Panel on Climate Change, IPCC signs on each side of the surveyed swath. Because 27 April 2004; accepted 9 September 2004 Third Assessment Report, Climate Change 2001: The we averaged about four adjacent ICESat/aircraft Published online 23 September 2004; Scientific Basis (Cambridge Univ. Press, Cambridge, comparisons that included data from each side of 10.1126/science.1099650 2001). the aircraft, effects of these roll errors should be Include this information when citing this paper. 258 8 OCTOBER 2004 VOL 306 SCIENCE www.sciencemag.org Accelerated Sea-Level Rise from West Antarctica R. Thomas et al. Science 306, 255 (2004); DOI: 10.1126/science.1099650 This copy is for your personal, non-commercial use only. 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