J ournal rifGlaciolog)" Vo t. 44, No . 146, 1998

Rapid sea-level rise fronl a West Antarctic ice-sheet collapse: a short-term. perspective

CHARLES R. BEl TLEY Geophysical and Polar Research Cenle ]; University qfWisconsin- N/adison, Madison, Wisconsin 53706, US.A.

ABSTRACT. Will wo rld wide sea level soon ri se rapidly because of a shrinkage of the Wes t Antarctic ice sheet (WAIS)? H ere I give a personal perspective of that probability. The crucial question is not whether large changes in ice mass can occur, but how likely it is that a large, rapid change, say a several-fold increase in the 20th-century rate of abo ut 2 mm a-I, will occur in the next century or two from a West Antarctic cause. Twenty years ago Weertma n proposed that a m a rine ice sheet is inherentl y unsta bl e. But Weertman's a nalysis was based on a simple m odel of a marine ice sheet that did not include fas t-flowing, wet-based ice streams, which a re now known to domina te the grounded ice sheet. Modern a nalyses do not definitively determine just how ice stream s affect the stability of the \ VAIS, but it can at least be a id that there is no compclling the­ oreti cal reason to expect a rapid rise in sea level from the WAIS triggered by ice-shelf thinning. Of the three m ain ice-drainage sys tems in the "VAIS, the one that fl ows into Pine Is­ land Bay might be a particul a rl y likely site for accelerated fl ow since there is no ice shelf to res train the inflowing ice streams, ye t measurements show that this system is not sig nifi­ cantly out of m ass balance. If the "Ross Embayment" system, which has undergone several sudden glacia l reorgani zations in the las t thousand years, were unsta ble one might expect a history of la rge changes in the total outflow of ice into the Ross Ice Shelf, ye t the tota l outflow in the "Ross Embay ment" has remain ed relati vely uncha nged despite the la rge internal perturbati ons, a fact that points to a tabl e, not an unstable, system. Study o f the third maj or dra inage from the "VAIS, into the Ronne Ice Sh elf, also suggests that there is no gross discorda nce between the present velocit y vectors and fl ow tracers in the ice shelf, although the evidence is limited. In the light of the evidence for recent stabili t y, it is di flI cult to see how climate wa rm i ng (whether anthropogeni c or natural) could trigger a coll apse of the WAIS in the next cen­ tury or two. Thus, I beli eve that a rapid ri se in sea level in the next century or two from a Wes t Antarctic cause could only occur if a natu ra l (not induced ) collapse of the WAIS were imminent. Based on a concept of pse udo-ra ndom co ll apse once per major glacia l cycl e, I es timate the chances of that ta be on the o rder of onc in a thousand.

1. INTRODUCTION "rapid" ri se in sea level to be a rate of a t least I m per century (10 mm a \ fi ve times the 20th-ce ntury rate. (At the end of Will world wide sea level soon rise rapidly because of a this paper I co nsider briefl y a rise of tw ice the present rate.) shrinkage of the Antarctic ice sheet? That is a question of A rate of 10 mm a 1 corres pond s to a n additional input to 3 wides pread interes t and great societal import; it is also a the ocean of 3000 km of water per year, about one a nd a matter about which there is great uncertainty. A reader of ha lf times the total present-day outflow from the Antarctic reports in the popular press a nd of some documents promot­ ice sheet. ing pa rticul a r research agendas might conclude that such Glaciologists genera ll y agree that a marine ice sheet, a n event is likely and consequently that the coasta l areas of one that res ts on a bed well below sea level, is much m ore the world are in imminent da nger of inundati on. In the In­ likely to undergo a rapid change tha n o ne lying on a higher tergovernmental Panel on Climate Change Second Assess­ bed. Although there a re sec ti ons of the East Anta rctic ice ment Report, Warriek and others, (1996), after a brief but sheet that arc marine, I will focus on the "Vest Antarctic ice up-ta-date review, stated that "estimating the likelihood of sheet (\rVATS ) because it appears to be the most vulnerable, a coll apse [of the West Antarctic ice sheet] during the next being open to the ocean on three sides, a nd because it com­ century is not yet possible". \Vhile I agree with that conclu­ prises the bulk of the marine ice. If the entire WAIS were sion in a forma l se nse, I nevertheless beli eve that the rele­ discha rged into the ocean, sea level wo uld ri se by 5 or 6 m. vant informati on at hand po ints strongly towa rds the In fact, the \VAIS surely has not been co nstant in size unlikelihood of a collapse event in the nex t century or two. throughout the Pl eistocene ice age . M a rine seismic studies H ere I give a perso nal perspecti ve of present knowledge, show that it expanded across the conti nental shelf m any arriving at a n order-of-magnitude quantitati ve estimate of times (Al onso, and others, 1992), whereas the subglacia l oc­ what I mean by "unlikelihood". currence of algal remains suggests that at other times the For the sake of quantitative disc uss ion, I will defin e a m a in WAIS inl and ice was gone (Scherer, 1991). The crucia l

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ques tion in the present context, however, is not whether shelves rapidly, the \VAIS would collapse in as little as a cen­ large changes in ice mass have occurred, but how rapidly tury by a catas trophic grounding-line retreat, causing sea they can occur and how likely is it that a large, rapid change I evel to rise at a mean rate of some 50 mm a- I. will occur soon. The idea of marine-ice-sheet instability, however, was Like other large ice sheets, the WAIS is not a single dy­ based on a simplistic model ofa m a rine ice sheet in which a namic entity; it comprises three maj or and several minor discontinuity in physics at the grounding line, together with drainage systems that have separate regimens and are un­ a required co ntinuity in the ice thickness, led to the modeled likel y all to acceleratc at once. But to consider the worst- case instability. The reality of the WAIS is that its activity is scena ri o, ass ume that the whole \VAIS behaves as a unit. The dominated by fast-flowing, wet-based ice streams whose 3 WAIS produces about 320 km a 1 (water equivalent) of iee cha racteristics blend graduall y into those of the ice shelves outflow (Bentley and Giovinetlo, 1991); thus, to achieve a sea­ a nd whose response times to changes at the grounding line level-rise rate of 10 mm a \ a tenfold increase in the contri­ appear to be very rapid (Alley and VVhillans, 1991). bution of the \VAIS wo uld be needed. Approximately half of Just what the dynamic consequences of the ice streams the WAIS is below sea level a nd conseq uently already dis­ are for the stability of the \ VAIS is much in dispute. On the pl aces ocean water. Consequently, there are two conceptual one ha nd, the inherent ability of ice streams to transport ice cnd-member models of how the ice would be removed. In rapidly from the interior to the ocean indicates, in the view onc, the fl ow is such as to remove a ll the ice above buoyancy of some glaciologists, a n enhanced capability for a dras­ and none below, res ulting in the end in an ice shelf that is just ti call y accelerated output flu x. A contrary view is that the barely aflo at everywhere. In that ex treme, all the ice re­ rapid response time of ice streams removes the flux imba l­ moved wo uld contribute to sea-l evel rise. In the other ex­ a nce at the grounding line th at was the basis of the instabil­ treme, a ll the ice is removed; only half then contributes to ity model and that the purported g rounding-line instability sea level. In any real case, the situati on wou ld be intermedi­ very well may not exist. ate: some of the ice below sea level wo uld be removed along Recent work is equivoca l. The best model study to date is with that above. Thus the increase in outflow that would be that by TvlacAyea l (1992) of the possible behavior of the required for a "rapid" rise in sea level (as defined above) \ VAIS over the last million years, because it incorporates becomes a factor of 10- 20. ice streams and their slippery beds into the numerical model This would be a huge ch a nge: glaciers and ice streams (a lthough not in an entirely reali stic way). That study leads that now typically flow at sp eed s of 0.5- 2 km a- I would have to the conelusion that the WAIS might indeed have disap­ to accelerate to an average of 10 or 20 km a \ substantially p eared completely at times in the past; the maximum rate faster than any present-day g lacier anywhere in the wo rld . o f collapse in his model yielded a contribution to sea-level An increase of this magnitude, whether internally gener­ rise about equal to the present-day rate. A particul a rly ated or in response to a elimaticall y produced alteration in important aspect of M acAyeal's model resu lts is that the col­ the boundary conditions, could come about only from a lapses occur at times irreg ul a rl y spaced relative to the ice­ massive instability. What theoreti cal or observational ev i­ age cyele, not preferentially during interglacial periods, dence is there to sugges t the existence of such a n instability? because of the long time constant of the subglacial system. His modeling yielded three coll apses in 10 6 years; if each lasted a few thousand years (e.g. a 6 m rise in sea level at 2. THEORETICAL STUDIES 2.5 mm a- I) a coll apse would have been in progress on the order of I % of the time. However, when a collapse is not Twenty years ago the concept was developed that a marine a l ready in progress, as at present, the chance that one will ice sheet is inherently unstable, because of a mism atch in be initiated within the next century, say, is about 3 x 100 a/ ice-mass flu x at the junction, known as the grounding lin e, 106 a, i.e. only about 0.03% . between the grounded ice (ice resti ng on a solid bed ) and MacAyeal's (1992) model also produced iceberg pulses, 3 the adj acent floating ice shelves (Weertman, 1974). The mis­ with flu xes of 1000- 3000 kl11 a 1 (3- 10111111 a 1 sea-level match would result in a change in the thickness of the ice at rise) las ting a few decades, about ten times during the mil­ the grounding line that would cause the grounding line li on-year model run. These too occurred pse udo-randomly, either to advance or to retreat and, on a fl at bed or one that so the chance of being within a century of such a pulse beeomes deep er toward the center of th e ice sheet (the case would be about 0.1 % . with the \tVAIS ), to continue adva ncing or retreating either These quantitative res ults are, of course, not precise, and to the edge of the continental shelf (unstabl e advance) or should not be taken literall y, as som e approximations in into the central interior until a ll the ice was afloat (unstable M acAyeal's model remain crude, particularl y those relating retreat). Whether this junction, the "grounding line", ad­ to the bed and to horizontal advection, which is not consid­ vances or retreats depends in the theo ry upon the initial ered, thus precluding thermodyna mic feedback with the depth of water at the grounding li ne: when the height of sea fl ow from upstream (MacAyeal, 1992). I cite them only to level is less than a critical value, a in glacial times, unstable show that even in a m odel that produces collapses, collapses advance resul ts, but a high sea level, as at present, leads to are inherently infrequent, are not synchronized with the unstable retreat. The \VAIS owes its ex istence, in an exten­ climate cycle owing to the long response time of the ice sion of that concept, to frictional drag from a combination of sheet, and do not necessarily produce a drastic change in grounded "pinning points" in the ice shelves and shear res is­ the rate of sea-l evel rise. But it is likely that these general tance a long the sides of their enelosing embaym ents. That cha racteri stics of collapses will carry over to other, future drag creates a "back press ure" from the ice shelves, which m odels, because or the importance of basal processes, a nd restrains the outflow of the g ro unded inland ice (Thomas hence long and probably non-linear response times, to ice­ and Bentley, 1978; Thomas a nd others, 1979). If oceanic sheet dynamics. warmlllg a nd circul ation cha nges were to thin the ice Recent theoretical treatments tend to support the idea of

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stability rather than instability of a marine ice sheet. In system that Oows into Pine Island Bay on the Amundsen these analyses the ice shelves turn out to be unimportant to Sea. This might be a particularly likely site for accelerated the maintenance of the inland ice. One o[ the reasons [or fl ow since there are indications that an ice shelf filling the thi is the removal from the models o[ an unnecessary con­ bay may have disappeared in recent geologic time (Kellogg straint that the ice thickness must be continuous across the and Kellogg, 1986), thus reducing the restraint on the i nflow­ grounding line (Hindmarsh, 1993), with the consequence ing ice streams (Pine Island and Thwaites Glaciers). But that even total removal of the ice shelves might not res ult measuremel1lS indicate a mass balance o[ this system that is in a large increase in the outflow (personal communication not significantly different from zero (Bentley and Giovinet­ from R. C. A. Hindmarsh, 1997). Furthermore, several to, 1991; Lucchitta and others, 1995). Measurements of flow studies indicate that the transition zone between the rates on the Ooating parts of these two ice streams, as meas­ grounded and floating ice does not play an important role; ured by satellite-i mage techniques, do indicate increases in particular, that it does not act as a source of instability over the last decade or two (Ferrigno and others, 1993; Luc­ (Van der Veen, 1985; Herterich, 1987; Huybrechts, 1990; chitta and others, 1995), but the reality of these increases is Barcillon and MacAyeal, 1993; Hindmarsh, 1993, 1996; uncertain (personal communication from B. K. Lucchitta, Lestringant, 1994a, b; personal communication from P. 1997). Furthermore, velocity increases on the Thwaites Huybrechts, 1997). One study indicated that tripling the Glacier tongue, at least, are most likely due to the dramatic present-day melt rate beneath the West Antarctic ice shelves shortening of the tongue over recent decades (Ferrigno and would lead to a rise in sea level of only 0.3 m in 500 years others, 1993). So far only one set of velocity measurements (Huybrechts and Oerlemans, 1990). has been made on the grounded ice of these glaciers. A recent development that does suggest a dramatic in­ Additional evidence on the insensitivity of glacier input stability is the evidence for sudden, massive outpourings of flux to ice-shelf back-pressure comes from outside the vVAIS. icebergs from the Laurentide (Canadian) ice sheet during The small Wordie Ice Shelfin the Antarctic Peninsula (Fig. I) the last ice age ("Heinrich events"; Broecker, 1994). has virtually disappeared over the last decade (Doake and MacAyeal (1993) modeled these events by a thermally acti­ Vaughan, 1991), yet recent measurements reveal no speed-up vated growth- collapse cycle of ice in the Hudson Bay drain­ in the flow rates of the glaciers fl owing into it (Vaughan, age system; in his recent work he finds a maximum rate of 1993). Also, a small field experiment on the Ekstrbm Ice 3 mass Ou x during a collapse on the order of 3000 km a I Shelf (Fig. I) conducted in 1993- 94 showed that the trans­ (personal communication from D. R. MacAyeal, 1997). ition zone between grounded and Ooating ice is very narrow That, coincidentall y, is just the amount needed for a "rapid" (its width is on ly a few times the ice thickness), which sug­ rise in sea level (by my definition). Furthermore, the Hud­ gests that the Oow regi mes in the two region are essentiall y son Bay drainage system is comparable in area to the drain­ independent of each other (r-.layer and Huybrechts, 1994; age system of the "Ross Embayment" (the portion of the personal communication from P. Huybrechts, 1997). WAIS that discharges into the Ross Ice Shelf); if the Ross In the "Ross Embayment", field evidence (distribution Ice Shelf did not exist (MacAyeal still believes that ice and depths of buried crevasses) reveals that sudden reorga­ shelves are importal1l and essential), the WAIS ice stream nizations of th e ice streams have occurred in the last 1000 would be in exactly the configuration used for the Hudson years. Ice Stream C abruptly stagnated (Retzlaff and Bent­ Strait modeling (personal communication from D. R. ley, 1993), one of the boundaries of neighboring Ice Stream B MacAyeal, 1997). But MacAyeal's theory depends upon underwent a sudden lateral jump of 10 km or so (Bentley transformation of the bed of a potential ice stream from and others, in press), and part of Ice Stream A became frozen to melting. This process is not applicable to the West quiescent (personal communication from S. Shabtaie, Antarctic ice streams since they are already melting at their 1997). In addition, parts of the system are grossly out of beds and consequently should be in the "purge" state, yet balance today: Ice Stream B is hyperactive (strongly nega­ nine out of ten exhibit a positive or near-zero mass balance tive mass balance), Ice Stream C is stagnant (strongly posi­ (flux in, by snow accumulation, minus flux out, by ice flow ) tive mass balance) and Crary Ice Rise (an island of (Bentley and Giovinetto, 1991; Lucchitta and others, 1995). It grounded ice in the Ross Ice Shelf near the mouth of Ice is questionable whether a large increase in the speed of ice­ Stream B) is growing and changing the regional velocity stream movement could be induced (Radok and others, fi eld (MacAyeal and others, 1987; Bindschadler, 1993). 1987). Of course, MacAyeal's (1993) model for the Heinrich If the "Ross Embayment" systen1 were unstable, one events may not be conceptuall y correct (for example, Hulbe might expect these major dislocations to have caused some has developed a model of ice shelves, in which debris-rich large changes in the outflow of ice into the Ross Ice Shelf. basal ice is protected by the accretion of saline marine ice, Observations of the Ross Ice Shelf itself can be used to test that could yield Heinrich layers without any glacial surges this possibility for the last 1500 years, because tracers of past (Hulbe, 1997); if a different mechanism obtained, perhaps it Oow preserved in the shelf can be compared with the pres­ would also apply to the WAIS. Nevertheless, no model of the ent-day flow of the ice (Bentley and others, 1979; Neal, 1979; Heinrich events that would also imply WAIS instability has Bentley, 1981; Jezek, 1984). Large surges of the inland ice been put forward to date. The theoretical basis for claims of would be recorded as distortions of the flow tracers. Indeed, instability remains weak. one strikillg deformation of the Oow tracers has been found (Fig. 2); analysis indicates that perhaps 800 years ago there was a large pulse of ice from the vicinity of lee Streams A 3. FIELD OBSERVATIONS and B, perhaps due to differential variations in ice-stream discharge or perhaps due to the incorporation of a large ice Thus the modeling results yield no compelling evidence for "raft" (Casassa and others, 1991 ), but it also shows that the "rapid" sea-level rise from the "VAIS. Is that also the case for Oow after that pulse reverted to what it was beforehand. the glaciological field evidence? Consider first the drainage There is no sign of a major change in outflow at the time of Downloaded from https://www.cambridge.org/core. 24 Sep 2021 at 10:09:09, subject to the Cambridge Core terms of use. 159 Journal qfGlaciology

65°5 Ekstrom Ice Shelf East Antarctica

West Antarctica

Fig.i. Map cif the inland ( grounded) ice sheet qfAntarctica , showing su'ljace elevations ( black contour lines; heights in km ), mountainous regions (dark gray) and sections cif the ice sheet where the bed is above (medium gray ) and below ( light gray) sea level. The heavy black lines divide the principal drainage systems. The grounding lines qf the Ross and Filchner- Ronne Ice Shelves are designated by the black dotted lines.

the reorganizations ofIce Streams Band C, about a century perhaps partly in response to climatic warming about and a half ago. 10 000 years ago at the end of the last ice age (Whillans, Taken as a whole, the WAIS "Ross Embayment" system is 1983; Hamilton and Whillans, 1997). White and Steig (1997) nearly in mass balance (Bentley and Giovinetto, 1991). interprel diverging trends in oxygen isotope ratios in ice­ Furthermore, the proportion of ice flowing into the Ross core records from Byrd Station and Taylor Dome (in the Ice Shelf from East and West Antarctica, respectively, has Transantarctic Mou ntains near McMurdo Sound) as indi­ remained approximately constant over the full 1500 years. cating a drop in surface elevation at Byrd Station of about This is shown by the concordance between flowlines and 400 m over the last 6000 years (';::;j7 cm a- I). This concept is flow tracers all the way across the grid eastern part of the in accord with glacial geologic studies by G. H. Denton and ice shelf, where the flow is from East Antarctica (Fig. 3). his group, mostly in the region around McMurdo Sound, This concordance implies that the total outflow in the "Ross which indicate that the grounding line in the "Ross Embay­ Embayment" has remained relatively unchanged despite ment" retreated slowly to its present position over the last the large internal perturbations, since a large change in that 7000 years or so (Hall and Denton, 1996). outflow at some time within the last 1500 years would have Interestingly, the first results from radar altimetry using caused a corresponding distortion in the flowlines from East the European remote-sensing satellites ERS-l and ERS-2 Antarctica, the total outflow from which presumably has suggest that the ice surface in central West Antarctica has been fairly steady over time. Nearly constant total flow from been slowly rising (at a few cm a- I) over the mid-1990s the WAIS at times oflarge internal disturbances points to a (Wingham and others, 1997). Elsewhere in West Antarctica stable, not a n unstable, system. north of 81.5 ° S (the limit of coverage of the ERS satellites) Glaciological studies of several types have all indicated the surface height has not changed observably,' i.e. by more that there has been no drastic change over the last than a few cm a- I. There is no suggestion in the radar alti­ 30000 years in the height or flow of the ice sheet at Byrd Sta- metry of a large imbalance of either sign anywhere in this ~ tion, which lies in the West Antarctic interior within the region, which includes the drainage basins of the Ronne "Ross Embayment" (Whillans, 1976, 1983; Raynaud and Ice Shelf and Thwaites and Pine Island Glaciers. Whillans, 1982). Field measurements on the surface indicate Study of the third major drainage from the WAIS, into that the ice there and elsewhere in the interior "Ross Embay­ the Ronne Ice Shelf, also suggests that there is no gross dis­ ment" is now thinning slowly (by a few to about 10 cm a \ cordance between the present velocity vectors and flow tra- Downloaded160 from https://www.cambridge.org/core. 24 Sep 2021 at 10:09:09, subject to the Cambridge Core terms of use. Bentley: Rapid sea -level riseJrom a I/Vest Antarctic ice-sheet collapse

(1 80°) variability. Based on the studies by M acAyeal cited above, o a nd on the more genera l idea of an unsynchroni zed coll apse occurring about once every glacial cycle, r es timate the cha nce of that to be on the order of one in a thousand. Ob­ vio usly, this is not a number to be taken literall y; neverthe­ less, I beli e\'e it puts the threat of rapid sea-level rise from a WAIS coll apse in a realisti c perspective. Bindschadl er (1997) argues that neither climate cha nge nor the res ponse of a n ice sheet thereto is truly random; the problem is that models of ice-sheet dynamics arc still woe­ fully inadequate. That is true, but it does not provide a ny help in es timating the likelihood of collapse. My point (Bentley, 1997) is that a major catas trophe would have to be­ fa ll the WArS to cause a "rapid" rise in sea level and that there is no theoretical or observationa l reason to susp ect tha t such a catastrophe is imminent, if, indeed, one can occur at all. I use a pse ud o-random m odel based on the wo rk of MacAyeal (1992) to obtain a rough es timate of the probabi lit y of a collapse event, because there is nothing bet­ ter to use; improve meI1ls in the phys ics of the ice-dynamic \ m od els are, a priori , just as likely to yield a lesser likelihood ~-----+------,~7Y------'1-~~~9 ° of coll apse as a greatc-r one. Bindsc hadler (1997) further po ints to major changes in the WArS that have occurred in the past, including the last 11 000 years, as part of the world­ wide retreat fr om the L ate Glacial M aximum. But the evi­ Fig. 2. Sketch map offeatures identified on advanced very dence he himself cites indicates that a 10- 20 m contribution high-resolution radiometer image1)! in the central Ross Ice to sea level was spread out over mos t of that time, so that the Shelf ( the grid coordinates are the sa me as in Figure 3). The corresponding mean rate of sea-I e\'el rise was onl y 2 m m a - \ hatc/zed area bounded by curvilinearflow stripes is inte1jJreted even in th at maj or evenl. That ha rdly points to a much to be caused either by an ice raft or by a la1ge differen tial var­ la rger rate of co ll apse in the near future. iation in the ou!Jlow of Ice Streams A and B some 800years Fina ll y, it is perhap, worth noting that (other things ago. From Casassa alld oth ers (199I). bei ng equal) the more rapid and a brupt, and rh erefore cers in the ice shelf, although the evidence is limited (per­ sho rter-l as ting, a hypothetica l rise in sea level (supposed to sonal communicati on from C. S. M. Doa ke, 1997). occur at some time in a glacial cycle) is ta ken to be, the less likely it is to be initiated within a pa rtic ula r short period of time, such as the next century. 4. THE FUTURE

In the li ght of the e\·idence for recent stability, it is difficult 5. CONCLUSIONS to see how climate warming (wh ether anthropogenic or nat­ I n the la t thousand years, the total o utflow in the "Ross Em­ ural) could trigger a coll apse of the WArS in the next cen­ bayment" has remained relativel y uncha nged des pite la rge tury or two. Ice sheets take thousands of years to respond to inLerna l perturbati ons. That is the m a rk of a stable, not a n changes in surface temperature, because it takes that long unstabl e, system. for the temperature changes to penetrate cl ose to the bed, There is no evidence to suggest, or theoreti cal reason to a nd onl y there could increasing temperatures affect the fl ow suppose, that other drainage sys tems within th e WArS be­ rates (Whilla ns, 1977). Oceanic wa rming co uld cause thin­ have in a fund amentall y different way, ning of the ice sheh-es, but recent g loba l- circul ati on-model Consequentl y, the li kelihood of a ",-a pid" ri se in sea level studies have suggested th at oceanic warming in the far in the nex t two centuries as a res ult o f a nthropogeni call y Southern O cean from, say, an enha nced greenho use effect caused climate ehange acting on the "Ve t Antarcti c ice sheet wo uld be delayed by centuries compared to the res t of the is va nishingly small. The likeli hood of a naturall y caused world, because of the large-scale sinking of surface waters "rapid" rise is on the order of one in a thousand, around Anta rctica (l\lanabe a nd others, 199 1). Furthermore, These conclusions a re weakened o nly slightl y if "rapid" for reasons a lready cited, it is d o ubtful th at ice-shelf thin­ is redefin ed as twice rather than fi ve times the present rate, ning wo uld have a ny drasti c effect on the inland ice. because even to produce that le se r flu x the WArS wo uld The onl y cl imati c change tha t is likel y to have an im­ have to accelerate its outllow by a faetor of four or fi ve, mediate effect on the Antarcti c ice sheet is the increase in which would still require strongly unstabl e behav ior, prec ipitati on that probabl y will accompany wa rmer air temperatures. Increased precipitation will increase the mas input onto the ice sheet witho ut causing any balancing ACKNOWLEDGEMENTS increase in outflow for centuries. In the short term, the ice sheet probabl y will grow slowly (relative to its current state The a uthor is grateful to G S, M. Doa ke, R. G A. Hind­ of mass balance) (FOl· tuin and O erIemans, 1992). m a rsh, P. Huybrechts, C. H. Hulbe, B. K . Lucchitta, D. R . There rema ins the poss ibility that the WAIS will shrink M acAyeal and S. Shabtaie for thei I' p erso nal communica­ rapidly in the next century or two, owing to its own natural tions, several of them in each ca e. M y thanks also to T. M.

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J4 '" :ib· J7\ (g) I 1 K7 "KO "K9 go 8° K.t ... .. K''---.. \ ---- &oaCg) / I \ ', S, I L9 \ LI\ \ I ...... M3 , - ~ ! '\ - ~.' !. '! \ MI3 I N4 "- NB \ 10° \ 1- 10°

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! I 12°S 12°S V~LO~I~~ SCA~E --.-LI """00'METERS PER YEAR 1 -'~_--L. J _---'- _ _.. __ '- J , - ---1...... _ 6°W 4° 2° 0 2° 4°E

Fig. 3. Veloeify vectors on the R oss Ice Shelf, determinedfrom Doppler satellite tracking data (arrows;Jrom Thomas, and others, 1984), andflowlines based on variations in radar-echo strength (dashed lines). The heavy line in the central part rif the map represents the boundary between East and West Antarctic ice. The grid coordinate system has its origin at the South Pole, with north toward Greenwich. From Bentley andJ ezek (1981).

Whillans for hi s critical comments on th e manuscript. This ciologica l investigati ons of the Ross Ice Shelfin the U.S. programme. J work was pa rtially supported by U.S. National Science R. Soc. N.Z, 11 (4), 355- 372. Bemley, C. R., J. W. Clough, K. C. Jezek and S. Shabtaie. 1979. lee-thickness Foundation grant No. OPP92-20678. This is contribution patterns and the dynamics of the Ross Ice Shelf, Antarctica . .7 GlacioL., No. 577 of the Geophys ical a nd Polar Research C enler, Un i­ 24(90),287- 294. versity on t\T isconsin- Madi son. Bentley, C. R., P D. Burkholder, T. S. Clarke, C. Liu and N. Lord. In press. Geophys ical experiments on ridge Bl/B2, Si plc Coast. An/m·et. ] Us. Bindschadlel; R. 1993. Siple Coast Project research of Crary Ice Rise and the mouths of Ice Streams Band C, \ Vest Antarctica: review and new REFERENCES perspecti,·es. J GlacioL. , 39(133), 538-.~52. Bindschadler, R. 1997. L etters. West Antarctic ice shee t coll apse? Science. Alley, R. B. and I. M. Whillans. 1991. Changes in the West Antarctic ice 276 (5313),662-663. shee l. Science, 254(503+),959- 963. Broecker, IN. S. 1994. M assive iceberg di scha rges as trigge rs for global cli­ Alonso, B. ,]. B. Anderson,j.l. Diaz and L. R. Banek. 1992. Pliocenc Pl eis­ mate change. Nature, 372(6505), 421- 424. tocene seismic stratigraphy of the Ross Sea: evidence for multiple ice Casassa, G., K. C. J ezek, J. Turner and I. M. Whillans. 1991. Relict now sheet grounding episodes. In Elliot, D. H., ed. Contributions 10 Antarctic re ­ stripes on the Ross lee Shelf. Ann. Glaciol., 15, 132 138. search Ill. \ Vashinglon, DC, American Geophysical Union, 93-103. (Ant­ D oake, C. S. M. and D. G. Vaughan. 1991. R apid disilllegration of the Wor­ arctic Research Series 57.) die Ice Shelf in response to atmospheric warming. Nature , 350(6316), Barci lon, V and D. R. MacAyea l. 1993. Steady now of a viscous ice stream 328 330. across a no-slip/free-slip transition at the bed. J Glaciol. , 39(131), 167- 185. Ferri gno, J. G., B. K. Lucchitta, K. F. Mullins, A. L. Alli son, R.]. Ali en and Bentley, C. R. 1981. Variations in vall ey glacier activit y in the Tj'ansantarctic W. C. Gould. 1993. Velocit y mcasuremems and changes in position of Mountains as indicated by associated now bands in the Ross lee Shelf. Thwaites Glacier/iceberg tongue from aerial photography, Landsat Intmwliollal Association ()[ Hydrological Sciences Publication 131 (Symposium images and NOAA AVHRR data. Ann. Glaciol. , 17, 239- 2+4. at Canberra 1979 - Sea Level, lee alld Climalic Change ), 247 251. Fm'tuin, J. P. F andJ. Oerlemans. 1992. An atmospheric model for simulat­ Bentl ey, C. R. 1997. Letters. West Amarctic ice sheet collapse? Science, ing the mass ba lance and temperature of the Antarctic ic e sheel. Z 276 (5313), 663- 664. Gletscherkd. GlazialgeoL., 26 (1 ), 1990, 31- 56. Bentley, C. R. a nd M. B. Giovinetlo. 1991. l\lass ba lance of Antarctica and H a ll , B. L. and G. H. D enton. 1996. Deglacia l chronology (or the western sca \c"el change. In\l'e ll cr, G., C. L. VVilson ancl B. A. B. Severin, eds.lnler­ Ross Sea: implicati ons for the behavior of the \Vest Antarctic ice sheet. national Conference on Ihe Role ()[Ihe Polar Regions in Global Change: proceedings [Abstracl.) GeoL. Soc. Am. Abslr. Programs, 29(7), 57. ()[a conference held]une 1/- 15, 1990 al the University ()[ Alaska Fairballks. J01. 11. Hamilton, G. S. and 1. M. Whillans. 1997. G l'S glaciology in Antarctica. Fairbanks, AK, Uni versity of Alaska. Geophys ical [nstilUte/Center for [Abstract] EOS, 78(17), Spring Meeting Supplement, SIOO. Global Change and Arcti c System R esearch, 481- 488. H erterich, K. 1987. On the now within the transition zonc between ice sheet Bentley, C. R . and K. C. ,lezek. 198 1. RISS, RISP and RIGGS: post-IGY gla- a nd iee she lf. In Van derVeen, C.]. a ndJ. Oerlemans, eds. Dynamics ()[the

Downloaded16 from2 https://www.cambridge.org/core. 24 Sep 2021 at 10:09:09, subject to the Cambridge Core terms of use. BentLey: Rapid sea -LeveLris eJrom a Hlest Antarctic ice-sheet coLLapse

lI esl Antarclir ice sheel. Dordrecht , etc., Kluwer Academic Publ ishers. Radok, U. , D..Jenssen and B. M c I nnes. 1987. Olllhe surgillg polelllial if/J ola.r ice 185-202. slream s. Alltarclic surges - a dear and presenl danger? Boulder, CO, Univer­ H indmarsh, R. C. A. 1993. Qualitative dynamics of marine ice sheets. III sity of Colorado. Cooperative InstilU te for Resea rch in Em'ironmental Peltier. \V R. , ed. Ice ill th.e climale syslem. Berlin, NC., Springer-Verlag, Sciences; Melbourne, Uni\'crsil Y of ;-'lclbou rne. ~ I cteo r o l og)' Depa rt­ 67- 99. (:\,ATO ASI Series I: Global Environmental Cha nge 12.) ment. ( US Department of Energy Report DOE/ ER/60197-HI.) H indma rsh, R. C. A. 1996. SlOchasti c perturbation of di\'ide position. Ann. Raynaud , D. a nd I. ~r. \\·hilla ns. 1982. Air content of the Byrd core a nd past Glariol., 23, 94- 104. changes in the Wes t Antarctic ice shee t. Arm. Glaciol., 3,269- 273. Hulbe, C. L. 1997. An ice shelf mec hanism fo r H einrich layer production. RetzlaIT, R . a nd C. R. Bentley. 1993. Timing of stagnati on of Ice Stream C, Pa leoreanography, 12. 7ll 717. "Ves t Anta rctica, from short-pulse radar studies of buried surface cre­ Huybrechts, 1'. 1990. The Anta rctic ice sheet during the las t glacial- intergla­ vasses. ]' Glaciol. , 39(133 ), 553- 561. cial cycle: a three-dimensional experiment. AIlII. Glariol., 14, 11 5-119. Scherer, R. P. 1991. Quaternary a nd Tertiary microfoss ils from beneath Ice H uybrechts, P. a ndj. Ocrlem a ns. 1990. Response of the Antarctic ice sheet Stream 13: ev idence for a d ynamic West Antarctic ice sheet history. to future greenhouse warming. Climale Djll., 5 (2),93- 102. Palaeogeogr., Palaeorlimalol., Pa laeoeeol., 90(4), 395412. J ezek, K. C. 1984. Recent cha nges in the dynamic condition of the Ross Ice Thomas, R . H. a nd C. R. Bentley. 1978. A model for H o locenc retreat of the Shelf, Anta rcti ca.] Geophys. RfS., 89(131 ), 409- 4 16. West Antarctic ice sheet. Qjlal. Res., 10 (2), 150- 170. Kell ogg, D. E. a nd T. B. K ell ogg. 1986. Bi otic provinces in modern Amund­ Thomas, R. T1. , T. j. 0. Sanderson a nd K. E. Rose. 1979. EffecI of cl imatic sen Sea sediments: implications for glacia l history. All ta rcl. ] Us. , 21 (5), warming on the West Anta rcti c ice shee t. . \'ature, 277 (5695),355 358. 154--156. Thomas, R . H. , D. R. ~ l ac Ayca l , D. H. Eilers and D. R. Caylord. 1984. Cla­ Lestringa nt, R. 199+a. Etude numcrique et mathematique des tra nsitio ns ciological studies on the Ross Ice Shelf, Anta rcti ca, 1973 1978.111 H ayes, a bruptes "fort froltement - faib le frottement": appl ication a ux tra nsi­ D. a nd C. R. Bentley, eds. The Ross lee Shelf: glaciology and geo/Jhysics. ti ons "calotte pola ire ice shelf': (These de doctoral, Un ive rsit eJoseph Washing ton, DC, Ame ri can Geophysical Union, 21- 53. (Antarcti c Fourier- Grenobl e I.) Research Series +2.) Lestringa nt, R. 1994b. A two-dimensional finite-element study of now in Van der Veen, c. j. 1985. Response of a marine ice sheet to changes at the the transiti on zone between an ice sheet a nd a n ice shelf AIlI!. Glaciol. , g rounding line. QJlal. Res., 24(3), 257- 267. 20,67- 72. Vaughan, D. C. 1993. Implicati o ns of the brea k-up of\\'ordie lee Shdf, Ant­ Lucchitta, B. K. , C. E. Rosanm'a and K. E Mullins. 1995. Velocities of Pine a rctica for sea "'·el. Anlarcl. Sri., 5 (+),403- 408. Island Gl acier, Wes t Antarctica. from ERS-I SAR im ages. Anll. Glaciol., Wa rrick, R . A. , C. le Pro\'ost, ~l. E ~ l c i er, J. O erle ma ns a nd P. L. Wood­ 21,277- 283. worth. 1996. Changes in sca leve l. In Houghton, .J. T., L. G. M. Fi lho, M acAyeal, D. R. 1992. Irregular osc ill ati ons of the West Antarctic ice sheet. B. A. Call ander, N. H a rris, A. Kaltenberg a nd K . Maskell , eds. Climale :Valllre, 359(6390),29- 32. change 1995: Ihe sciellce if dill/all' change. Cambridge, elC.. Ca mbridge ~l acAyea l , D. R. 1993. Binge/purge oscill ations of the Laurentide ice sheet Uni\'ersity Press, 359-+05. as a cause of the ;>.iorth Atl ami c's Hcinrich e\·ents. Paleoreanograp/i)I, 8 (6), \\'eenma n, j. 197+. Stability of th" juncti on of an ice shect a nd an ice shelf. ] 775- 78+. Glaciol., 13 (67),3 11. i\l acAyeal, D. R. , R. A. Bindschadler, S. Shabtaie, S. Stephenso n and C. R. Whilla ns, 1. }' l. 1976. Radio-ccho layers and the recent stability of the \lVes t Bentl ey. 1987. Force, mass, and energy budgets of the Crary Ice Rise Antarctic ice shee t. . \'cllllre, 264(.1582), 152-155. complex. Amarelica. ] Glaciol. , 33(11 4), 218- 230. [Correction in] Gla­ \\,hi ll a ns, I. ~1. 1977. The equation of continuity a nd its applicati on to the ciol .. 35(11 9), 151-152] ice sheet near .. Byrd" Stat io n, t\ I1l ilrctica. ] Glaciol. , 18 (80),359 37 1. ~ I anabe. S., R.J. StouOe r, ~r.j. Spclman a nd K. Br),a n. 199 1.1i-a nsient re­ \\'hi ll a ns. 1. ~l. 1983. Glaciological parameters. their measurement a nd sponse of a coupled ocean- atmosphere modclto gradual changes of at­ significance. Ice mo\'cme nt. III Robin, G. de Q, I'd. The climalic record in mospheri c CO". Pa rt 1: Annual mean response. ] Clilllale, 4 (8), 785 818. /J O"" ice sheels. Cambridge. I.' tc., Cambridge Univcrsity Press,70 77. ~ J ayer, C. and P. Huybrechts. 1994. The Ek stro m grounding line ex peri­ Whitc,.J.\V C. and E.]. Steig. 1997. Climate change in the Southern l-lemi­ ment - first results. III Oener, H., ed. Filrhn er ROllne Ite Shelf Programme. sphere Holocene as \'iew"d from high latitude isotopic records. Re/JorIB. Bremerll

MS received 30 J une 1997 and accepted in revisedJonn 17 November 1997

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