Quaternary Science Reviews 28 (2009) 1831–1849

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Quaternary Science Reviews

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Modeling ice sheets from the bottom up

T. Hughes*

Department of Earth Sciences, Climate Change Institute, University of Maine, Bryand Global Sciences Center, Grove Street Extension, Orono, ME 04469-5790, USA article info abstract

Article history: Three facts should guide ice-sheet modeling. (1) Ice height above the bed is controlled by the strength of Received 6 November 2008 ice-bed coupling, reducing ice thickness by some 90 percent when coupling vanishes. (2) Ice-bed Received in revised form coupling vanishes along ice streams that end as floating ice shelves and drain up to 90 percent of an ice 25 May 2009 sheet. (3) Because of (1) and (2), ice sheets can rapidly collapse and disintegrate, thereby removing ice Accepted 6 June 2009 sheets from Earth’s climate system and forcing abrupt climate change. The first model of ice-sheet dynamics was developed in Australia and applied to the present Antarctic Ice Sheet in 1970. It treated slow sheet flow, which prevails over some 90 percent of the ice sheet, but is the least dynamic component. The model made top-down calculations of ice velocities and temperatures, based on known surface conditions and an assumed basal geothermal heat flux. In 1972, Joseph Fletcher proposed a six- step research strategy for studying dynamic systems. The first step was identifying the most dynamic components, which for Antarctica are fast ice streams that discharge up to 90 percent of the ice. Ice-sheet models developed at the University of Maine in the 1970s were based on the Fletcher strategy and focused on ice streams, including calving dynamics when ice streams end in water. These models calculated the elevation of ice sheets based in the strength of ice-bed coupling. This was a bottom-up approach that lowered ice elevations some 90 percent when ice-bed coupling vanished. Top-down modeling is able to simulate changes in the size and shape of ice sheets through a whole glaciation cycle, provided the mass balance is treated correctly. Bottom-up modeling is able to produce accurate changes in ice elevations based on changes in ice-bed coupling, provided the force balance is treated correctly. Truly holistic ice-sheet models should synthesize top-down and bottom-up approaches by combining the mass balance with the force balance in ways that merge abrupt changes in stream flow with slow changes in sheet flow. Then discharging 90 percent of the ice by ice streams mobilizes 90 percent of the area so ice sheets can self-destruct, and thereby terminate a glaciation cycle. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction produced elliptical ice-sheet profiles on a horizontal bed. Ablation rates were added and both accumulation and ablation rates were This review primarily traces the trajectory of my glaciological allowed to vary in later refinements (see Hughes, 1998, Figure 5.10). career, which began in 1968. Consequently, those who influenced In all these treatments, gravitational ice motion was resisted by my career the most are cited prominently. My apologies to other a basal shear stress proportional to the product of ice height above prominent glaciologists. A half-century ago, glaciology was being the bed and ice surface slope. The resulting ice surface was high and converted from a descriptive branch of geology to an analytical convex, even when moderate bed topography was included. Their branch of physics. Analytical reconstructions of ice sheets began dependence on the surface mass balance made them top-down with the parabolic profile of an ice sheet having a constant basal models that produced nearly steady-state ice sheets. The ice sheets shear stress on a horizontal bed (Nye, 1951). Next, the basal shear of Antarctica and Greenland today are nearly in steady state overall, stress was allowed to vary with ice velocity determined by within the accuracy of the surface mass balance. These ice sheets a constant surface accumulation rate and whether ice moved by have high convex surfaces where slow sheet flow prevails, as creep over a frozen bed (Haefeli, 1961) or by sliding over a thawed assumed in the analytical models. The Antarctic Ice Sheet often bed (Nye, 1959), using the newly published flow law (Glen, 1955) ends as floating ice shelves because ice accumulation over virtually and sliding law (Weertman, 1957a) of ice. These treatments its entire surface allows it to advance into the sea, where iceberg calving provides the primary ablation mechanism. Weertman * Tel.: þ1 207 581 2198; fax: þ1 207 581 1203. (1957b) provided the first analytical derivation of the low and E-mail address: [email protected] essentially flat surface of a floating ice shelf. In his derivation,

0277-3791/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2009.06.004 1832 T. Hughes / Quaternary Science Reviews 28 (2009) 1831–1849 gravitational ice motion is resisted by a longitudinal tensile stress The top-down models for sheet flow by Budd et al. (1971) and proportional to the height of ice floating above sea level. later top-down models were incompatible with important field data from Antarctica, much of it collected by tractor-train traverses 2. Modeling ice sheets from the top down during the International Geophysical Year (IGY) in 1958 and beyond. The traverses measured ice elevations, temperatures, and Numerical ice-sheet modeling was inaugurated by William accumulation rates at the surface and ice heights above the bed Budd, Richard Jenssen, and Uwe Radok in 1971. They developed along traverse routes. The data, traverse routes, and geographical a steady-state flowline model which they applied to the Antarctic features appeared on the 1970 map, Antarctica, published by the Ice Sheet in order to derive variations of temperature, stress, and American Geographical Society. Contoured bed topographic data velocity with depth, using measured ice heights above the bed, ice showed that most of the West Antarctic Ice Sheet was grounded elevations above sea level, ice surface accumulation rates, and ice below sea level on the Antarctic continental shelf (Bentley and surface temperatures (Budd et al., 1971). From these data, their Ostenso, 1961), leading Mercer (1970) to propose that it was an model plotted ice trajectories and timelines with depth along inherently unstable ‘‘marine’’ ice sheet. Contoured surface eleva- surface flowlines, and calculated either basal ice temperatures tion data showed the East Antarctic Ice Sheet had the convex below the melting point or basal ice melting rates at the melting surface produced by steady-state models of sheet flow, but the point for specified rates of the basal geothermal heat flux. Doubling West Antarctic Ice Sheet had a concave surface. Perhaps the West the geothermal heat flux converted a ubiquitously frozen bed into Antarctic Ice Sheet was far from steady state and in fact was in an a largely thawed bed. Widespread changes from a frozen to advanced stage of gravitational collapse that produced the low a thawed bed also resulted from moderate changes in conditions at floating ice shelves surrounding it. My response to this possibility the ice surface. An outer basal freezing zone was introduced beyond appeared in four monographs, in 1972, 1973, 1974, and 1975, under the inner basal melting zone in subsequent applications of the the acronym ISCAP (Ice Streamline Cooperative Antarctic Project), model to prevent widespread ice-bed decoupling as the basal water all of which posed the question, ‘‘Is the West Antarctic Ice Sheet layer thickened (Sugden, 1977). disintegrating?’’ Was the West Antarctic Ice Sheet not only Budd and Radok were meteorologists who saw interactions of collapsing into ice shelves, but would the ice shelves then disin- the ice surface with the atmosphere as the critical boundary tegrate into icebergs, thereby removing the West Antarctic Ice condition in modeling ice sheets. Budd et al. (1971) specified Sheet from the global climate system, and flooding the world ocean surface conditions in order to determine basal conditions. Theirs with icebergs that, in melting, would cool ocean surface water and was a top-down model in which the surface mass balance combines therefore reduce the ocean-to-atmosphere heat exchange that with the force balance to offset gravitational motion with basal drag drives atmospheric circulation? Could a new glaciation cycle then that resists motion. By that constraint, their model applied only to begin? slow sheet flow. This is known as the ‘‘shallow-ice’’ approximation (Hutter, 1983). Their pioneering work set the stage for developing 3. The Fletcher memorandum gridpoint ice-sheet models that were three dimensional and time dependent. Time-dependent modeling showed that present basal Incorporated in my ISCAP bulletins was a research strategy for thermal conditions are determined primarily by past surface studying dynamic systems proposed by Joseph Fletcher in an conditions, not present conditions, even if surface changes from internal memorandum when he headed the Office of Polar past to present are only moderate. These models also simulate only Programs at the National Science Foundation (Fletcher, 1972). slow sheet flow, which prevails over some 90 percent of ice sheets, Fletcher recommended research that answered six questions past and present. In sheet flow, gravitational flow is resisted directed at how any dynamic system operates. His six questions and primarily by basal drag, as quantified by the basal shear stress. my answers for the Antarctic Ice Sheet that also apply to all ice Since past surface conditions are poorly known for present ice sheets are: sheets, and unknown for former ice sheets, top-down models cannot deliver reliable basal conditions for 90 percent of the bed 1. What are its most dynamic parts? Answer: ice streams, beneath ice sheets. For example, when a state-of-the-art three including their calving fronts when ice streams end grounded dimensional time dependent top-down model was applied to the in water or as floating ice tongues often imbedded in ice Antarctic (Huybrechts, 1990, 1992) and Greenland (Huybrechts, shelves. 1994, 1996) ice sheets, it was unable to generate the amount and 2. What factors force motion in these parts? Answer: gravity distribution of basal water that has been mapped by radar sounding resisted by ice-bed coupling and atomic bonding in ice. in both Antarctica (Siegert et al., 1996) and Greenland (Oswald and 3. Which of these factors vary over time? Answer: ice-bed Gogineni, 2008), unless the model used a distribution of basal coupling and breaking atomic bonds during iceberg calving geothermal heat flux that forced a fit. Basal water controls ice-bed events. coupling, and therefore the height and stability of ice sheets. Basal 4. What physical processes cause the time variations? Answer: water cannot support a basal shear stress, so gravitational flow is processes that weaken or strengthen ice-bed coupling and that not resisted by basal drag when basal ice is no longer in contact lead to and cause calving of ice. with the bed. As a consequence, progressive reduction of ice-bed 5. Can these processes be quantified theoretically? Answer: yes, coupling by basal water converts the high convex surface of but the processes are poorly understood and the theories must a grounded ice sheet into the low and flat surface of a floating ice be holistic, including transitions from sheet flow to stream flow shelf. The ice sheet has destroyed itself when the ice shelf disin- to shelf flow and the dynamics of calving. tegrates into icebergs. Disintegration of an ice shelf is also 6. What experiments will test the theories? Answer: experiments a consequence of eliminating ice-bed coupling where the ice shelf designed to gather broad comprehensive data over the is grounded laterally in a confining embayment and where the ice Antarctic Ice Sheet, especially in West Antarctica and relying shelf is pinned locally to the sea floor, producing ice rumples or ice heavily on satellite technology, combined with field studies rises on the ice-shelf surface above each pinning point. Confined that include deep drilling and are concentrated on ice streams and pinned ice shelves are common where the Antarctic Ice Sheet and their calving fronts where rapid changes in behavior are advances into the sea and becomes afloat. observed, with all data being input to computer models T. Hughes / Quaternary Science Reviews 28 (2009) 1831–1849 1833

designed to replicate observed behavior and to project West Antarctica, based on mass-balance measurements available at behavior into the future when internal and external forcing the time (Bull, 1971). Perhaps a confined and pinned ice shelf can may change. buttress the ice streams supplying it. Then, if the ice shelf dis- integrated, perhaps ice streams would rapidly downdraw the The ISCAP bulletins applied my answers to Fletcher’s questions remaining ice sheet until it became afloat. If the bed under these ice in a research strategy for the Antarctic Ice Sheet as a dynamic streams sloped downward into the ice sheet, retreat of the ice-shelf system that was changing most noticeably in West Antarctica. My grounding line would force ice streams to retreat. Weertman (1974) answers were based on glaciological, geophysical, and geological published the essence of this retreat mechanism (Fig. 1) and field studies in Antarctica during and after the International Thomas (1977) quantified the processes driving retreat (Fig. 2). Geophysical Year in 1958. Much of the glaciological and geophysical Thomas and Bentley (1978) applied the mechanism to model research was directed by Charles R. Bentley and published in collapse on the former marine ice sheet in the Ross Sea Embayment Volume 16 of the Antarctic Research Series of the American when ice-bed uncoupling began 18,000 years ago with rising global Geophysical Union (Crary, 1971). Because ice streams discharge 90 sea level. They assumed the convex surface of sheet flow extended percent of Antarctic ice, instabilities in ice streams make the entire to the grounding line of Ross Ice Shelf, as it does for ice ridges Antarctic Ice Sheet inherently unstable and subject to rapid between ice streams. De Angleis and Skvarka (2003) documented collapse, collapse that was largely complete in its marine West retreat when part of Larsen Ice Shelf disintegrated. Antarctic sector that has a concave surface and is mostly grounded In the second mechanism for gravitational collapse, heads of below sea level. Field studies of glacial and marine geology in the concave ice streams retreat and drag the marine ice-shelf Dry Valleys of the Ross Sea embayment by Denton et al. (1968, grounding lines with them (Hughes, 1974, 1975). This seemed 1971), on East Antarctic outlet glaciers through the Transantarctic possible, based on the theory for cyclic surging mountain glaciers Mountains into the embayment by Mercer (1968, 1972), and in the developed by Robin and Weertman (1973), applied to account for Weddell Sea embayment by Anderson (1972) confirmed collapse of nearly flat sections, ‘‘pseudo ice shelves,’’ of West Antarctic ice former marine ice sheets in these parts of West Antarctica. Field streams reported by Robin et al. (1970b). Ice-bed uncoupling by studies of surface lowering along the Byrd Station Strain Network in deepening basal water under the flat sections would migrate the center of West Antarctica by Whillans (1972, 1973) provided upstream if the pressure gradient of basal water decreased upslope, direct evidence that collapse was still underway. causing the maxima in surface slope between each flat section to Newly developed airborne radar sounding technology was migrate upslope. This process may now be underway in West applied to the West Antarctic Ice Sheet, showing that the concave Antarctic ice streams (Bindschadler, 1997) and it causes the ice surface was a consequence of major ice streams (Robin et al., 1970a) streams to retreat. In both cases, collapse of the ice sheet is deter- and that basal water was abundant where these ice streams had mined by conditions at the bed, specifically ice-bed uncoupling. To a nearly flat surface, as though here the ice streams were afloat, capture these mechanisms, which depart radically from steady- making these portions ‘‘pseudo ice shelves’’ (Robin et al., 1970b). state conditions, models should be constructed from the bottom up. Concave ice streams draining the West Antarctic Ice Sheet supplied floating ice shelves, so ice streams seemed to be the vehicles by which gravitational collapse converted the high convex surface of 4. Reconstructing former ice sheets for CLIMAP nearly steady-state sheet flow into the low flat surface of shelf flow. The ISCAP bulletins were designed to test this hypothesis. Their The International Decade of Ocean Exploration (IDOE), 1970– contents were subsequently published in refereed journals 1980, was underway when my ISCAP bulletins were circulating and (Hughes, 1973, 1975, 1977) and as a book chapter (Hughes, 1998, Chapter 3). They presented a case for studying ongoing gravita- tional collapse of the West Antarctic Ice Sheet. Glaciologists began studying the ice streams mapped by Robin et al. (1970a). Early results were presented at a conference spon- sored by the American Association for the Advancement of Science (AAAS) at the University of Maine on 8–10 April 1980, as recorded by Horne (1980). These studies led to The West Antarctic Ice Sheet Initiative (WAIS) funded by the U.S. National Science Foundation (NSF) as a long-term investigation of this possibility (Bindschadler, 1991). Results over the next decade were presented at a Chapman Conference sponsored by the American Geophysical Union (AGU) and held at the University of Maine on 13–18 September 1998 (Bindschadler and Borns, 1998). More recent results are presented in Volume 77 of the AGU Antarctic Research Series (Alley and Bindschadler, 2001). WAIS workshops have been held annually since 1993. The ISCAP bulletins explored two mechanisms for gravitational collapse. The first mechanism was gravitational collapse progress- ing inward from marine margins of the West Antarctic Ice Sheet. Fig. 1. The marine ice instability at marine ungrounding lines of ice sheets (Weertman, Collapse was triggered by rising sea level, beginning 18,000 years 1974). Top left: the base of right-triangles are equal basal ice and water pressures, the ago, and accelerated by disintegration of floating ice shelves formed heights of these triangles are heights of ice and water above the base, the areas of these during collapse. Ross Ice Shelf occupies a confining embayment and triangles are opposing longitudinal gravitational driving forces in ice and water per is pinned to the sea floor at places identified by ice rumples and ice unit transverse width, the pulling force is the difference between these areas, shown as the dashed triangle. Top right: the longitudinal pulling force is the area of the dashed rises on the surface (Hughes, 1972, 1973). Surface velocities along triangles and is proportional to the square of ice thickness. Bottom: as the ungrounding the north–south leg of the Ross Ice Shelf Survey (Dorrer et al., 1969) line retreats, the pulling force increases on a downsloping bed and decreases on an were less than velocities for ice entering the Ross Ice Shelf from upsloping bed because the floating ice thickness increases and then decreases. 1834 T. Hughes / Quaternary Science Reviews 28 (2009) 1831–1849

sea ice) as albedo input, (2) the volume of ice sheets to obtain the reduced ocean surface area in the ocean-to-atmosphere heat exchange, and (3) the elevation of ice sheets to obtain the surface topography that may re-direct surface winds and the jet stream. As a glacial geologist, Denton could provide the areal extent of ice sheets, but he needed a glaciologist to determine their volume and elevation at the LGM, and a mechanism to collapse the West Antarctic Ice Sheet to obtain the Eemian sea level 6 m higher than at present at the LIM. The only numerical model for ice sheets available at that time was the flowline model Budd et al. (1971) had developed for the Antarctic Ice Sheet. My ISCAP bulletins high- lighted defects in that modeling approach, the primary defect being theirs was a top-down model of sheet flow in which the amount and distribution of basal water was very sensitive to the surface temperature and accumulation rates and the basal geothermal heat flux. None of this information was available for former ice sheets to be reconstructed for CLIMAP. However, these data are of minor importance in reconstructing former ice sheets because the primary result CLIMAP needed was ice elevations above the bed. That depends on the strength of ice-bed coupling, which could be determined from the glacial geology Denton was providing, a bottom-up approach. Bottom-up modeling requires distinguishing between ‘‘first- order’’ and ‘‘second-order’’ glacial geology. First-order glacial geology consists of a glacial imprint on the deglaciated landscape that becomes more pronounced with each cycle of Quaternary glaciation due to repeated processes of nearly steady-state glacial erosion and deposition. Second-order glacial geology is produced over time during the last glacial retreat. It overprints first-order glacial geology. Most glacial geologists at that time were mapping and dating second-order glacial geology. For this reason, our ice- sheet reconstructions based on first-order glacial geology were rejected when we presented them at the International Symposium on Dynamics of Large Ice Masses, sponsored by the International Glaciological Society and held in Ottawa, Canada, in 1978. We subsequently published our CLIMAP work in book form, The Last Great Ice Sheets (Denton and Hughes, 1981). It is necessary to Fig. 2. Processes triggering gravitational collapse of marine portions of an ice sheet present the rationale for our bottom-up approach based on first- (Thomas, 1977). The ungrounding line migrates across basal sills due to (a) ice thinning order glacial geology that determines ice-bed coupling and there- during an ice-stream surge, (b) lowering the sill due to glacial erosion or ongoing bed depression under the weight of ice, (c) rising sea level that lifts floating ice, (d) melting fore ice elevations above the bed. In the bottom-up approach, the the upper or lower surfaces of floating ice, and (e) accelerated calving causing a calving unknown surface conditions and basal geothermal heat flux at the bay to migrate up the ice stream. LGM and LIM, and the previous history of an ice sheet, are virtually irrelevant in determining the size and shape of former ice sheets. Ice-bed coupling weakens when a frozen bed becomes thawed an awareness was taking root that ice sheets may be the most beneath an ice sheet. For slow sheet flow, basal thawing lowers the vulnerable component of Earth’s climate system (Hollin, 1972). A ice surface by about 20 percent. Thawing begins in bed hollows, major part of IDOE was CLIMAP (Climate: Long-range Investigation, from which progressive thawing expands and eventually envelops Mapping, and Prediction). CLIMAP provided glaciologists with an bed hills as ice flows across a melting bed. When ice flows across opportunity to become part of the large scientific community a freezing bed, hilltops are frozen first and hollows last. Complete engaged in documenting and understanding global climate change. freezing raises the ice surface by about 25 percent and restores the George Denton at the University of Maine (UM) was charged surface above a frozen bed. Therefore, melting and freezing beds primarily with reconstructing ice sheets at the Last Glacial consist of a mosaic of frozen and thawed patches which are Maximum (LGM) for 18 ka BP, but also with disintegrating the determined primarily by bed topography. Ice surfaces above both marine West Antarctic Ice Sheet during the preceding Eemian frozen and thawed beds are high and convex. Melting and freezing Interglacial at the Last Interglacial Maximum (LIM) for 125 ka BP. zones along an ice-sheet flowline produce respectively more steep Denton recruited me for these tasks. The top-down model of ice and less steep ice surface slopes in the flow direction, but the sheets developed by Budd et al. (1971) was not suited to the CLI- overall ice surface remains convex (Fig. 3). MAP tasks, so I developed a bottom-up approach for reconstructing Once the bed is wholly thawed, continued basal melting will and disintegrating ice sheets. James Fastook and David Schilling submerge the wet bed, first in hollows but progressive submer- contributed mightily to this effort. We incorporated the marine ice gence eventually envelops hills as well. Since bed topography instability (Weertman, 1957a,b; Thomas, 1977) in conducting the includes linear channels formed by tectonic activity or by subaerial LIM experiment. and submarine erosion processes before the ice sheet existed, The primitive models of atmospheric circulation used by CLI- drowning of the bed will preferentially occur along channels. The MAP required three boundary conditions that only ice sheets could resulting ice-bed decoupling will allow overlying ice to move faster provide. These models needed (1) the areal extent of ice sheets (and along these channels because resistance to gravitational motion is T. Hughes / Quaternary Science Reviews 28 (2009) 1831–1849 1835

unclear whether stream flow begins when basal water submerges hills lower than a critical size, supersaturates till or sediments to a critical depth, or drowns bedrock bumps up to a ‘‘controlling obstacle size’’ in the Weertman (1957a) theory of basal sliding. Progressive reduction of ice-bed coupling along channels where basal water progressively drowns the topography of a hard bed, mobilizes the till or sediments of a soft bed, or both, will produce the lowering concave surface of ice streams that ends with the low flat surface of ice tongues in water or as the low convex surface of ice lobes on land. Basal water draining around the perimeter of an ice lobe allows partial ice-bed recoupling and gives the lobe its low convex surface. If ice tongues enter an embayment, they can merge to become a floating ice shelf that is grounded along the sides of the embayment or at basal pinning points within the embayment. In this way partial ice-bed coupling continues into the embayment, and allows the ice shelf to buttress the ice streams (Thomas, 1973a,b). Most West Antarctic ice streams enter the Ross Sea and Weddell Sea marine embayments, where their floating ice tongues merge to become the buttressing Ross and Ronne–Filchner ice shelves. The former LGM ice sheets of North America and Eurasia were centered on Hudson Bay in North America and over the Gulf of Bothnia, Barents Sea, and (in my opinion) Kara Sea in Eurasia, all isostatically depressed by the weight of overlying ice. Postglacial raised beaches and negative gravity anomalies are first-order features that locate these centers of ice spreading at the LGM when the ice load was greatest. They are first-order glacial features. Being marine water bodies originally, the ice domes that formed above them probably rested on a largely thawed bed in the deepest water. From these centers, ice moved across exposed Precambrian crys- talline shields from which the remaining overlying layer of sedi- mentary rock has been removed by Quaternary glacial erosion, after eons of subaerial erosion. Since these shields are spattered with lakes, which would have been thawed patches under the ice, the ice sheets moved across a freezing bed that was a mosaic of thawed and frozen patches. These eroded shields are first-order glacial features. Beyond the shields, rock and regolith eroded from the thawed patches are deposited over the largely un-eroded sedi- mentary rock cover, producing looping end moraines. This band of depositional moraines is a first-order glacial feature. The biggest looping moraines lie beyond troughs eroded in the sedimentary rock cover. Today, these troughs are often occupied by linear lakes on land (notably the Great Lakes in North America). Along marine ice margins, the troughs are linear straits and inter-island channels today. The troughs were occupied by ice streams at the LGM and are first-order glacial features. Fast stream flow produced a melting bed that cut across the freezing bed between ice streams. Each cycle of Quaternary glaciation reinforced the imprint of these first-order glacial features on the deglaciated landscape (Hughes, 1981a; Fig. 3. Response of ice elevation and ice trajectories to ice-bed coupling linked to basal Hughes et al., 1981). In North America, the Laurentide Ice Sheet thermal conditions (Denton and Hughes, 1981, Chapter 5). For sheet flow, the ice centered on Hudson Bay merged with a Cordilleran Ice Sheet which surface lowers 20 percent as a frozen bed thaws and rises 25 percent as a thawed bed had a frozen bed beneath ice divides along the crests of mountain freezes (broken surface lines). Frozen beds are white. Thawed beds are black. Basal ranges and a thawed bed in valleys on the flanks of these ranges, shear stresses sO are higher for creep (sO)C over a frozen bed and lower for sliding (sO)S over a thawed bed (broken tau lines). Top: ice elevations and trajectories as ice moves giving an overall melting bed from ice divides to ice margins as from a frozen bed on an upland plateau across melting and freezing beds to a frozen a first-order glacial feature. bed under a surface ablation zone on land. Bottom: ice elevations and trajectories as ice In the CLIMAP Eemian modeling experiment at the LIM, the moves from a thawed bed in a marine embayment across freezing and melting beds to a thawed bed under a marine ice stream that becomes afloat. Melting and freezing Thomas (1977) marine instability mechanism at ice-stream beds are shown as a mosaic of thawed (black) and frozen (white) patches. ungrounding lines was applied to the CLIMAP West Antarctic Ice Sheet at the LGM (Stuiver et al., 1981). Rising sea level since the LGM triggered the marine instability, causing ungrounding lines to weakened by the deepening basal water. Fast currents of ice in retreat up the concave CLIMAP ice streams (Fig. 4). The ice sheet these channels will become ice streams imbedded in the ice sheet, collapsed into ice shelves on its eastern and western flanks, especially toward ice margins where thinning sheet flow is forced producing the Weddell Sea embayment occupied by the Ronne– to increasingly conform with bed topography. Ultimately, up to 90 Filchner Ice Shelf and the Ross Sea embayment occupied by the percent of ice in an ice sheet is discharged by ice streams. It is Ross Ice Shelf. Subsequent collapse occurred on its northern flank, 1836 T. Hughes / Quaternary Science Reviews 28 (2009) 1831–1849

Fig. 4. : Gravitational collapse of the West Antarctic Ice Sheet during Terminations of Quaternary glaciation cycles modeled for CLIMAP at the LIM (Denton and Hughes, 1981, Chapter 10). Ice is downdrawn by progressive ice-bed uncoupling along ice streams occupying the shaded submarine troughs. Stages of collapse proceed from the glacial maximum (A), to progressive collapse in marine embayments of the Ross, Weddell, and Amundsen seas (B and C), to collapse of the central West Antarctic Ice Sheet (D), to disintegration of buttressing ice shelves produced during collapse (E). T. Hughes / Quaternary Science Reviews 28 (2009) 1831–1849 1837 producing an equally large embayment in the Amundsen Sea when a viscous response in the mantle and an elastic response in the ungrounding lines migrated up Thwaites and Pine Island glaciers lithosphere to changing surface loads. This allowed him to deter- into the heart of West Antarctica. Pine Island Bay is the beginning of mine mantle viscosities and lithosphere flexures which he then that embayment today. Final collapse occurred following disinte- used, in a circular way, to calculate the changing height (calculated gration of the confined and pinned ice shelves that formed as ice load) of ice at the end of these radii from the known changing ungrounding lines retreated. These ice shelves had buttressed the areal extent of ice sheets and changing sea level (measured water retreating ice streams. This LIM experiment could apply equally to load) during the last deglaciation. Glaciologists weren’t needed. He the present Holocene Interglacial, and projects ongoing collapse of published his ice elevations on the Internet at precisely the loca- the West Antarctic Ice Sheet into the future. Collapse is entirely due tions climate modelers preferred in their General Circulation to ice-bed uncoupling that progresses up lowering concave ice Models (GCMs) of Earth’s atmosphere. In my view, Peltier’s chal- streams and allows calving bays to discerp the downdrawn ice. lenge has been good for glaciology. Competition is always good, and Ice-bed uncoupling can also proceed from the interior of ice his approach compels glaciologists to examine more critically the sheets along ice streams to ice margins, as had been modeled for defects in their approach to ice-sheet modeling. the Laurentide Ice Sheet in Hudson Bay and its ice stream in Hudson The Achilles’ heel in Peltier’s approach was threefold: (1) his Strait (MacAyeal, 1993; Calov et al., 2002). When a frozen bed model depends on the rheology of Earth’s interior, a rheology thaws, ice-bed coupling is largely lost in these models, and the ice- which can never be known as accurately as the rheology of ice sheet collapses and spreads until surface lowering brings cold sheets, (2) only vertical motion in the mantle and lithosphere is interior ice into contact with the bed, causing the bed to freeze so treated, and (3) only a slow isostatic response is delivered for surface ice accumulation can restore the original ice elevation. changes in ice sheets when rapid climatic responses to rapid These ‘‘binge/purge’’ cycles can be tuned to mimic Heinrich (1988) changes in ice sheets are of most interest. In (1), Peltier treats ice events for quasi-periodic rapid discharges of icebergs from the sheets as a purely static load on Earth’s surface. As a consequence of Laurentide Ice Sheet. No changes in ice surface temperature and ignoring ice-sheet modeling, his ice sheets tend to be too thin to accumulation rates are required, not even changes linked to the account for the known lowered sea level at the LGM and, contrary lowering ice surface. Changing surface conditions take millennia to to the glacial geological record, his ice sheets are more like ice slabs affect the basal thermal regime (Whillans,1981), so surface changes of nearly constant thickness than even the idealized elliptical ice have little effect on these essentially bottom-up processes linked to sheets permit for constant accumulation rates. In (2), adding the abrupt ice-bed decoupling and recoupling. linear viscous radial extensions in Earth’s mantle to linear elastic The CLIMAP bottom-up approach to ice-sheet modeling focused flexures in Earth’s lithosphere allows Peltier to use Green functions attention on ice streams. Since ice streams discharge up to 90 to generate spherical harmonics on Earth’s surface that could be percent of ice from past and present ice sheets, modeling ice- linked to known changing sea levels worldwide as the loads of ice stream dynamics correctly can generate changes in the size and and water changed during the last deglaciation. A more robust shape of ice sheets that are big enough and fast enough to trigger model would allow elastic–viscoplastic lithosphere flexure, allow rapid changes in global climate and sea level. Changes of this kind lateral variations in nonlinear mantle viscosity as surface loads are well documented for former ice sheets (Denton and Hughes, changed, and allow nonlinear viscoplastic mantle creep to vary 1981; Mayewski et al., 1997), and even now are becoming manifest laterally and interact with moving lithosphere plates and mantle in present ice streams in Antarctica (Thomas et al., 2004) and convection currents (Koons and Kirby, 2007). In (3), Peltier Greenland (Thomas, 2004). Top-down models controlled by the provided no mechanisms for controlling ice elevations by speci- surface mass balance are often unable to advance the Laurentide Ice fying the strength of ice-bed coupling, coupling that weakens Sheet below the Great Lakes at the Last Glacial Maximum (LGM), drastically as basal ice melts and ultimately allows ice sheets to which did happen, without also advancing the Cordilleran Ice Sheet destroy themselves. Rapid changes in the size and shape of ice almost to Mexico, which didn’t happen. This is because ice eleva- sheets in response to basal decoupling are not possible in Peltier’s tions at the center of the Laurentide Ice Sheet over Hudson Bay model, so it cannot be used in studying causes of abrupt climate must be high enough, from a positive mass balance, to force sheet change. Peltier has modified his old approach to include mecha- flow across the Great Lakes. With ice streams occupying the deep nisms for ice-bed decoupling, based on glacial geology (Peltier, troughs of the Great Lakes, the southern Laurentide margin is 2004; Tarasov and Peltier, 2004). rapidly advanced by greatly reducing ice-bed coupling when The CLIMAP ice sheets also had an Achilles’ heel. The gravita- summer meltwater in the surface ablation zone reaches the bed, tional driving stress for sheet flow in ice sheets is proportional to probably through crevasses (Zwally et al., 2002). This process the product of ice height above the bed and ice surface slope. This would be greatly accelerated by heavy summer rainfall over the driving stress was equated with the basal shear stress resisting southern Laurentide ablation zone (Bromwich et al., 2004). gravitational motion in the CLIMAP ice sheets. Generating the The ablation zone, not the accumulation zone, controls advance of concave CLIMAP ice streams by letting the basal shear stress the ice margin by controlling ice-bed decoupling. In the ablation, decrease along ice streams also made the gravitational driving zone, therefore, surface conditions can directly influence basal stress decrease, because both ice height above the bed and ice conditions, with no time lag. Such is not the case in the accumulation surface slope decrease along a concave ice stream. That was unre- zone. alistic because marine ice streams typically end as floating ice tongues which may or may not be imbedded in confined and pin- 5. The Peltier challenge ned ice shelves. For floating ice, the gravitational driving stress is proportional to ice height above water and it is equated with the The CLIMAP reconstructions of ice sheets at the LGM were used longitudinal tensile stress in an ice tongue (Weertman, 1957b). This by climate modelers throughout the decade of the 1980s until 1994, tensile stress is reduced when the ice tongue is imbedded in an ice when W. Richard Peltier presented another approach to recon- shelf confined laterally and pinned locally to the bed (Thomas, structing LGM ice sheets in his paper, ‘‘Ice Age Paleotopography’’ 1973a,b). Therefore, resistance to gravitational motion in an ice (Peltier, 1994). He had developed models of global isostasy in which stream should allow a downstream transition from basal shear to the lengths of Earth radii changed as the load of ice and water over longitudinal tension, with the tensile stress actually reaching up ice Earth’s surface changed during glaciation cycles. His radii had streams and pulling ice out of the ice sheet. Including this stress in 1838 T. Hughes / Quaternary Science Reviews 28 (2009) 1831–1849 the longitudinal force balance introduced the ‘‘pulling power’’ of ice and side shear and longitudinal tension and compression. Basal streams. shear dominates in slow sheet flow, longitudinal tension dominates in unconfined shelf flow, but all stresses appear in stream flow and 6. The ‘‘pulling power’’ of ice streams confined shelf flow. The floating fraction of ice is the part of the ice overburden that is supported by basal water and for which longi- The bed was not only fully thawed under the CLIMAP ice tudinal gravitational motion is resisted by longitudinal tension in streams, basal water progressively drowned bed topography the ice. The remaining part is supported by the bed, so that longi- downstream. Progressive drowning would produce a ‘‘floating tudinal gravitational motion is resisted by basal and side shear and fraction’’ of ice that increased downstream. In hollows it might longitudinal compression, all of which occur along ice streams and even produce the ‘‘pseudo ice shelves’’ along West Antarctic ice in confined or pinned ice shelves. streams reported by Robin et al. (1970b). When ice was fully afloat, Physically, the floating fraction of ice can be an ambiguous the ice stream would become an ice shelf. Therefore, the concave quantity at the bed. Is it the submerged fraction of rolling bed surface profiles of ice streams were bottom-up reflections of major topography, supersaturated regions of basal sediments or till, the ice-bed uncoupling as the floating fraction increased from essen- drowned fraction of bedrock bumps that are smaller than the tially zero under the high convex surface of sheet flow to essentially controlling obstacle size in theories of basal sliding, or all of these? one under the low flat surface of shelf flow. The Achilles’ heel in the The important point is that the floating fraction is linked to parts of CLIMAP ice streams could be ‘‘healed’’ by making the floating the bed that are unable to resist longitudinal gravitational motion fraction of ice the primary variable in modeling stream flow. Sheet by generating basal or side shear or longitudinal compression. flow alone cannot collapse an ice sheet and terminate a glaciation Perhaps the best way to illustrate the floating fraction of ice is cycle. Stream flow can because most of the ice, up to 90 percent, is with a cartoon that shows areas of wet thawed or drowned beds in pulled out by ice streams. In his treatment of ice shelves, Weertman black and areas of dry frozen beds in white, with flow radiating (1957b) showed that the tensile gravitational ‘‘pulling’’ stress in from both marine and terrestrial ice domes, including vertical a flat ice shelf is determined by the height of ice floating above longitudinal cross-sections along selected flowline transects that water, not by the product of ice thickness and ice surface slope, show ice trajectories (Fig. 5). In slow sheet flow, the bed is generally which is zero for a flat ice shelf. So as the floating fraction of ice wet under a marine ice dome over an embayment and dry under increases along an ice stream, the gravitational driving stress a terrestrial ice dome over a plateau. Ice streams begin near the changes from being determined by the product of ice thickness and marine dome but begin far from the terrestrial dome. Ice streams surface slope (which applies to sheet flow) to being determined by moving landward end as ice lobes and ice streams moving seaward the height of ice floating above water (which applies to shelf flow). end as ice shelves. Between ice streams, a freezing zone surrounds Gravitational thinning in Weertman’s ice shelf is resisted by the marine dome and a melting zone surrounds the terrestrial a longitudinal tensile stress that is a ‘‘pulling stress’’ on the ice dome, with these zones consisting of a mosaic of wet and dry streams that supply an ice shelf. If stream flow is transitional patches. Wet patches become elongated lakes or marine channels between sheet flow and shelf flow, this pulling stress should extend in flow directions as ice velocity increases. Dry patches that thaw up ice streams and ‘‘pull’’ ice out of ice sheets. Linking the floating become elongated drumlins or roches moutonees as velocity fraction of ice along an ice stream to the decrease in basal shear increases. Eskers form in the braided drainage systems near ice stress that resists sheet flow and the increase in longitudinal margins and under ice lobes. pulling stress that resists shelf flow quantifies how ice streams pull ice out of ice sheets. I called this ‘‘the pulling power of ice streams,’’ 7. Modeling a glaciation cycle from the bottom up pulling power being the product of the longitudinal gravitational pulling force and the longitudinal ice velocity (Hughes, 1992). Bottom-up ice-sheet modeling allows reconstructing a full Bottom-up ice-sheet modeling linked to the pulling power of ice glaciation cycle based on first-order glacial geology (Hughes, 1996). streams has been developed further since then (Hughes, 1998, This was presented at a symposium honoring Johannes Weertman 2003, 2009a,b). It remains a work in progress. The pulling power when he turned seventy. Since the symposium volume did not concept is now being applied to ice streams in Greenland and reach many glaciologists, the work was presented again in Ice Antarctica that have suddenly increased their ice discharge in Sheets (Hughes, 1998), which described the glacial geology on pages recent years, behavior for which conventional glaciology theory 239–250 in Chapter 9 and produced the reconstructed ice sheets on had no explanation solely in terms of sheet flow resisted by basal pages 307–317 in Chapter 10. In these reconstructions, the basal drag. Those working on this problem all take the pulling power of shear stress balanced gravitational forcing in sheet flow, but in shelf-like flow into account, including the acceleration of marine stream flow the basal shear stress was gradually replaced by the ice streams when a buttressing ice shelf disintegrates (e.g., Van der longitudinal tensile stress along ice streams as providing primary Veen, 1985, 1987; MacAyeal, 1989; Hulbe and MacAyeal, 1999; resistance to gravitational forcing. Northern Hemisphere ice sheets Hindmarsh and LeMeur, 2001; Marshall et al., 2002; MacAyeal were reconstructed for six stages in a generalized Quaternary et al., 2003; Hulbe et al., 2004; Thomas, 2004; Thomas et al., 2004; glaciation cycle based on the last cycle: (1) the initial advance of ice Dupont and Alley, 2005a,b; Marshall, 2005; Schoof, 2007; Hofstede, sheets, (2) their extent during interstadials, (3) their advance from 2008). interstadial to stadial positions, (4) their extent during stadials, (5) In my approach to pulling power, as it exists now, ice-bed their retreat from stadial to interstadial positions, and (6) their uncoupling produces a floating fraction of ice under ice streams collapse during Termination of the glaciation cycle. Ice elevations that increases from being essentially zero for a frozen bed or small above the bed depend mainly on ice-bed coupling assigned to each for a thawed bed where sheet flow predominates, to unity or close stage. to unity as stream flow develops, and remains at or close to unity Pleistocene ice sheets were nucleated at high northern latitudes when ice streams merge with floating ice tongues or confined and in both terrestrial and marine environments where permafrost is pinned ice shelves, but decreases toward zero when ice streams continuous, even on broad Arctic continental shelves (Hughes, end as ice lobes grounded on land and beneath which basal water 1986a). Therefore these ice sheets had a high height-to-area ratio can escape (Hughes, 2009a,b). All stresses in the direction of ice during their initial advance because a frozen bed maximized ice- flow depend on the floating fraction of ice. These stresses are basal bed coupling. T. Hughes / Quaternary Science Reviews 28 (2009) 1831–1849 1839

Fig. 5. : The flow regime linked to ice-bed coupling for an idealized ice sheet (Hughes, 1998, Chapter 9). Left: surface ice flowlines (solid lines) and the surface equilibrium line (dashed line) linked to wet bed conditions where ice-bed coupling is weakened in sheet flow (mosaics of black patches) and largely lost in stream flow (solid black areas). Right: flowline transects along the main ice divide (AA), from the main ice saddle (BB), from an ice dome above a marine embayment (CC), and from an ice dome above a highland plateau (DD), showing ice trajectories (solid lines) for beds that are wet (W), dry (D), freezing (F), and melting (M), and regions of debris-charged refrozen basal ice (shaded areas).

Initial advance was primarily over Precambrian shields studded deposited as ice-rafted sediments after icebergs calved from marine with lakes. These lakes would have been thawed patches under the ice streams, and were deposited as looped moraines at the lobate spreading ice sheet. The largest lakes arc around the edges of the termini of terrestrial ice streams. Successive recessional moraines shields and are furthest from centers of ice spreading, so they would form after terrestrial ice streams shut down and their lobes would be interconnected pro-glacial lakes during interstadials retreated during transitions from stadials to interstadials. when the advancing ice sheets halted long enough to produce an When ice lowered sufficiently, calving bays would migrate up isostatically depressed trough along their landward margins at the stagnating marine ice streams and eventually carve out marine edge of the shields. embayments that were formerly centers of ice spreading. Calving Beyond the shields and surrounding the centers of ice spreading bays carve out the heart of the accumulation zone of an ice sheet, are straits and inter-island channels extending seaward and the arc and forcedforcedtermination of the glaciations cycle. Calving also of large linear lakes extending landward. These linear troughs discerps terrestrial ice margins ending in proglacial lakes. would have been occupied by ice streams that advanced the ice- Following deglaciation, sites of greatest isostatic rebound and of sheet margins during transitions from interstadials to stadials. strongly negative gravity anomalies would identify sites of major These transitions are triggered when complete thawing of the bed domes at the glacial maximum, as distinct from the last remaining takes place under the centers of spreading on the shields. Thawing ice domes on land after final gravitational collapse of ice saddles in occurs because the increasing ice overburden finally crushes marine embayments. basal ice into water, causing reduced ice-bed coupling (crushing is These then constitute the first-order features useful in modeling physically observed as a reduction in ice volume and melting a glaciation cycle: (1) the present distribution of permafrost, (2) the temperature). The resulting partial gravitational collapse shot ice present distribution of lakes on Precambrian shields, (3) troughs streams into the surrounding troughs during stadials. The exposed radiating seaward and landward from these shields, (4) the areal shields have ubiquitous erosion features produced by ice sliding over extent of ubiquitous glacial scouring on shields that over time a wholly thawed bed. This proves that thawed patches originally exposes the shields, (5) lobate recessional moraines at the ends of confined to the smattering of lakes on the shields expanded over landward troughs beyond the shields, and (6) centers of greatest the entire shield at glacial stadials, causing general ice-sheet postglacial isostatic rebound. Each of these six features is associated lowering and spreading, without a great change in ice mass. with a particular stage of a glaciation cycle, and they should As ice over the shields lowered, colder ice would move toward therefore guide ice-sheet modeling of the cycle from the bottom up. the bed and restore the previous pattern of thawed patches in In this modeling activity, ice-bed coupling deduced from these a frozen matrix. That would shut down the ice streams and allow features is the primary control on ice elevations above the bed. interior ice to thicken. Rock and rubble eroded from shields and Surface temperatures and accumulation rates, which are largely along the linear troughs when these areas were wholly thawed were unknown, are very much secondary controls on ice elevations. If 1840 T. Hughes / Quaternary Science Reviews 28 (2009) 1831–1849 the first-order glacial geology and geomorphology that controls 1987, 1992, 2009) is used by Canadian and Scandinavian glacial each stage of the glaciation cycle are converted into correct varia- geologists to provide a glaciological explanation for glacial geology tions in ice-bed coupling, the resulting elevations of the ice sheets they map that was produced by the Laurentide and Scandinavian above the bed will be correct, regardless of conditions at the ice ice sheets during the last glaciation cycle. Since I am familiar with surface, prior changes in the size and shape of the ice sheets, or the UMISM, I will use it as an example of the advantages of combining pattern of the geothermal heat flux over the glaciated landscape top-down and bottom-up modeling approaches. UMISM generates under the ice sheets. ice sheets in the map plane, using specified surface temperatures The key to bottom-up modeling of ice sheets is linking glacial and accumulation or ablation rates as model input, obtained by geology and geomorphology to ice-bed coupling that dominates direct measurement for present ice sheets and as output from each stage of the glaciation cycle. In addition to identifying which models of atmospheric circulation or by other means for past ice first-order features of the glaciated landscape control a given stage, sheets. In this sense it is a standard top-down model. Temperatures second-order glacial features can be ‘‘peeled off’’ the first-order at depth are calculated vertically from specified surface tempera- landscape in layers that ‘‘stack’’ these features in a time trans- tures and accumulation or ablation rates, and are linked to verti- gressive manner from the youngest to the oldest (Boulton et al., cally averaged horizontal ice velocities by way of the flow law of ice, 1985; Boulton and Clark, 1990; Hughes, 1998, Figures 9.24, 9.25, such that mass and energy are conserved. The temperature field is and 9.26). Joan Kleman of Stockholm University has taken this used to determine if the bed is frozen or thawed and, if thawed, to approach to a new level by developing a time–space topology that calculate basal melting and freezing rates. For a thawed bed, sliding peers into the past along all flowline transects of former ice sheets velocities can be calculated from a variety of sliding laws modified until that ‘‘look’’ is blocked by more recent events of glacial erosion to convert the calculated amount of basal water into a water depth or deposition that erase the older features (Fig. 6). These second- that progressively drowns bedrock bumps or supersaturates order features are often glacial lineations of various kinds that align deformable till and sediments. Both processes decouple ice from in directions of ice flow at the time the lineations were imprinted the bed. The combined velocities of basal sliding, a mobilized on the landscape. They show whether the bed was frozen, thawed, deformable bed, and vertically averaged creep in ice become the or a mosaic of frozen and thawed patches that correlate with basal horizontal ice velocity in the mass-balance equation. A positive or freezing and melting zones. By applying time–space topology to negative mass balance at model gridpoints requires ice to thicken various parts of the former Scandinavian and Laurentide ice sheets, or thin at these sites to sustain mass continuity. Ice thickening or Kleman has taken the concept of ice-bed coupling under former ice thinning rates over time are obtained from the difference between sheets at various times in their history to a new level (Kleman, the surface accumulation or ablation rates of ice (and basal freezing 1990, 1992, 1994a,b, 2008; Kleman et al., 1992, 1994, 1997, 2008; or melting rates) and thickening or thinning rates of ice due to Kleman and Borgstrom, 1996; Kleman and Stroeven, 1997; Kleman convergence or divergence of ice flow caused by gravitational and Hattestrand, 1999; Kleman and Glasser, 2007; De Angelis and motion. The rheology of a soft deforming bed is not included in Kleman, 2008; Clark and Stokes, 2001; Stokes et al., 2007; Stokes UMISM, but it is generally included in other top-down models et al., 2009). (Marshall, 2005). Treatments of deforming beds under West With this bottom-up approach, there is no need to know the Antarctic ice streams appear in Volume 77 of the AGU Antarctic past climate history that determines the surface temperatures and Research Series (Alley and Bindschadler, 2001), which includes accumulation rates, and partly determines the basal geothermal a comprehensive study of wet deforming till by Kamb (2001). Also heat flux, at a given stage in the glaciation cycle. Top-down models see discussion by Hooke (2005, Chapters 7 and 8). that trace their pedigrees to Budd et al. (1971) depend critically on UMISM and other top-down models based on the shallow ice all these conditions. Yet it remains true that the changing size and approximation progressively add velocities obtained from flow and shape of ice sheets during a full glaciation cycle cannot be modeled sliding laws to get the cumulative mass-balance velocity and the ice without using the top-down approach. thickness profile used in the mass balance for transporting ice in the direction of the downsloping ice surface, for prescribed rates of 8. Combining top-down and bottom-up modeling strategies surface ice accumulation and ablation entered as model input. As noted by Van der Veen (1987) and Hofstede (2008), however, the Modeling ice sheets needs the top-down approach because that concave profile of ice streams introduces reduced velocities in the approach employs the mass-balance equation that controls how ice flow and sliding laws which depend on a gravitational driving sheets advance and retreat over time. When snow precipitation stress proportional to the product of ice height above the bed and accumulates year by year over highland plateaus or on sea ice, so ice surface slope in the shallow ice approximation. For a positive surface snow is compressed into ice at depth, the ice eventually surface accumulation rate, the mass-balance ice velocity increases becomes thick enough to thin and spread under its own weight. as the driving stress decreases, because both ice thickness and slope Merger of these ice spreading centers can produce an ice sheet with decrease along an ice stream as the surface goes from convex for terrestrial and marine portions (Hughes, 1986a, 1998, Figure 1.9). sheet flow to concave for stream flow. This introduces negative Marine portions formed initially from sea ice are necessary for longitudinal strain rates that reduce ice velocities based on the grounded ice to occupy marine embayments so terrestrial portions driving stress, whereas ice velocities based on the mass balance formed initially in highlands will not end by calving along marine increase continuously for the calculated downslope ice thickness shorelines. Gravitational thinning of ice is more than compensated profile. Poorly known quantities in the flow and sliding laws, by continued surface accumulation of snow, so spreading ice especially in sliding laws, can be ‘‘tuned’’ to force the two velocities advances until landward ice margins melt and marine or lacustrine to match. This problem points to a breakdown in the shallow ice ice margins calve as fast or faster than ice moves forward. This is the approximation that may be corrected by gradually replacing first-order effect of the mass balance. A bottom-up modeling resistance by basal drag with resistance by longitudinal tension, approach is not useful until an ice sheet is already in place, so ice- both linked to the flotation height of ice through the longitudinal bed coupling can be treated as a first-order effect in controlling ice gravitational force (Hughes, 2009a,b). The amount of basal water elevations above the bed. calculated in UMISM and a version of the Johnson (2002) model of A bottom-up application of the top-down University of Maine subglacial hydrology are used to move basal water from sources to Ice Sheet Model (UMISM) developed by James Fastook (Fastook, sinks. The gravitational driving stress, proportional to the product Fig. 6. Geomorphic systems associated with an ice sheet (Kleman, 1994a,b, Figure 7, reproduced with permission). Top: an ice sheet is sectioned to show velocity profiles for creep over a frozen (dry) bed and sliding over a thawed (wet) bed, flowline trajectories, the surface mass-balance regime, and the basal thermal regime. Dry bed, wet bed, and marginal meltwater geomorphic systems are shown for one-dimensional symmetry. Bottom: a demonstration of how time–space histories can be deduced by observing geomorphic landforms along transects such as I–II. The actual history is shown in (a), the line-of-sight along a given transect is shown in (b), what can be seen along all possible lines-of-sight is shown in (c), and what can be seen in sectors A, B, C, and D is shown in (d). 1842 T. Hughes / Quaternary Science Reviews 28 (2009) 1831–1849 of ice thickness and surface slope, is balanced only by the local basal reduction of ice thickness for grounded ice results in a rise in global shear stress in the shallow ice approximation. Therefore, UMISM is sea level. To accomplish this surface lowering by way of ice streams, a sheet-flow model even thought the mass balance and subglacial resistance to gravitational spreading has to change from being hydrology combine to concentrate ice motion along linear basal dominated by the basal shear stress for sheet flow at the head of ice depressions that generate stream flow. To introduce longitudinal streams to being dominated by the longitudinal ‘‘pulling’’ stress tension that allows ice streams to pull ice out of ice sheets, UMISM when ice streams become afloat as shelf flow. has a parameter called a ‘‘Weertman’’ that represents the pulling power of marine ice streams at their (un)grounding lines, with 9. Modeling basal thermal conditions under values increasing from zero to unity down an ice stream or as present ice sheets a buttressing ice shelf disintegrates, with unity providing the pulling force for an unconfined ice shelf, following Weertman Nearly all studies of ice streams have been in West Antarctica, (1957b). The ‘‘Weertman’’ generates concave ice-stream profiles in where the bed is largely below sea level (Alley and Bindschadler, UMISM, and thereby aleviates the problem of negative longitudinal 2001). These ice streams lie on soft deforming marine sediments strain rates noted by Van der Veen (1987) and Hofstede (2008).A mobilized into till. Maintaining ice streams of this kind requires more complete treatment of this problem requires solution of the a continual supply of soft sediments if the ice streams remain full momentum/equilibrium equation, as Sargent (2009) has done. largely in place. West Antarctic ice streams are believed to be Her solution can be imbedded in UMISM for stream flow and retreating along submarine troughs from positions near the confined or pinned shelf flow. continental shelf edge at the LGM (Fig. 3), with soft marine sedi- Glacial geology, often undated, reveals changing directions of ice ments occupying the troughs (Anderson, 1999). This may have flow and changing basal thermal conditions before and after the strongly biased our understanding of ice-stream dynamics. Many if LGM. These data are then used as ‘‘targets’’ that UMISM attempts to not most Antarctic and Greenland ice streams today pass through ‘‘hit’’ by adjusting variables in the model within acceptable limits. fjords as outlet glaciers, and may remain in place in their fjords These variables are parameters in the flow and sliding laws of ice, even as ice margins advance and retreat. This was also the case for conditions at the ice surface (temperatures and rates of ice accu- many ice streams draining former ice sheets. For such ice streams, mulation or ablation), and conditions at the bed (temperatures, soft marine sediments should have been removed by glacial water volume and distribution, rates of basal freezing or melting, erosion. Without mechanisms for resupplying sediments, these and the geothermal heat flux). For a Scandinavian application, see would be rock-floored ice streams. Very little fieldwork has focused Na¨slund et al. (2003a,b). For Laurentide applications, see Clarhall on these ice streams. The study of a former rock-floored stream by (2002) and De Angelis (2007). In this way, the concept of first-order Stokes and Clark (2003) on the Dubawnt swarm should inform and second-order glacial geology developed for CLIMAP is used by fieldwork; also see De Angelis (2007). Our concepts of ice-stream UMISM to target basal processes and understand the glacial history dynamics may need major revisions when these studies are done. of former ice sheets. Essential data for studying ice streams in their full complexity MacAyeal (1992) developed the first model for turning West include gridded seismic sounding of the kind being done for Rut- Antarctic ice streams on and off in an irregular way, as basal till ford Ice Stream in West Antarctica (Smith and Murray, 2009), and thaws and loses its cohesion under thickening ice and refreezes to applications of seismic streamer technology to ice streams. Equally regain cohesion under thinning ice. In top-down models, changes important is deep radar sounding capable of mapping the amount in ice-bed coupling are determined primarily by changing the and extent of basal water, and therefore of ice-bed coupling and amount and distribution of basal water that is produced by changes perhaps even the geothermal heat flux, as has been done along in the surface temperature and mass balance over time, for a given radar flightlines crisscrossing the northern Greenland Ice Sheet distribution of the basal geothermal heat flux. Modeling changes in (Oswald and Gogineni, 2008). Jakobshavn Isbrae, Kangerdlugssuaq the size and shape of ice sheets over time must employ a top-down Glacier, and Helheim Glacier are probable rock-floored ice streams approach, because bottom-up modeling calculates ice elevations that occupy Greenland fjords and have doubled their velocities in sustained by ice-bed coupling anchored to basal thermal condi- recent years (Stearns and Hamilton, 2007; Joughin et al., 2008). tions. These conditions ultimately depend on surface conditions for These should be mapped by seismic and radar sounding, and a specified pattern of the basal geothermal heat flux. Surface monitored continually by satellite sensors. Deep drilling to the bed, conditions change during a glaciation cycle, so the basal thermal as done for West Antarctic ice streams, is possible (Engelhardt and conditions will also change, including the geothermal heat flux, Kamb, 1997, 1998). which changes as the insulating ice thickness changes over time. Basal thermal regimes can be mapped under present ice sheets For rapid surface lowering of the kind modeled for Termination of by linking ice elevations to ice-bed coupling. Using this linkage, a glaciation cycle, these long-term effects are less important than basal thermal zones (frozen, freezing, melting, and thawed condi- repeating episodes of basal freeze–thaw processes of the MacAyeal tions at the bed) were mapped under the Antarctic Ice Sheet where (1992) kind or episodes of repeating discharge of impounded basal surface and bed topography were known with sufficient accuracy water, as proposed by Erlingsson (1994, 2006, 2008) for rapid along flowbands in sheet flow, taking variable flowband widths and drainage of subglacial lakes. Sudden drainage of this kind has been surface accumulation rates into account (Wilch and Hughes, 2000). reported by Stearns et al. (2008) at the head of Byrd Glacier in East Martin Siegert compared places where the bed was determined to Antarctica, with a rapid increase in ice velocity, probably as dis- be wholly thawed with the known locations of subglacial lakes charged water causes additional ice-bed uncoupling to propagate detected by radar, and found a strong match (Siegert, 2001). The down the ice stream. geothermal heat flux was not needed in this approach. Gravity easily thins thick ice after it loses contact with the bed, Johnson (2002) developed a finite-element map-plane model of so a grounded ice sheet 3 km thick under its interior ice domes is subglacial hydrology, based on a solution of the Manning equation, only 300 m thick after it becomes afloat, for the same surface mass that he coupled to UMISM to map flow of basal water from sources balance. This is because, for the same ice thickness, the longitudinal to sinks beneath the Antarctic Ice Sheet for various rates of the tensile stress for floating ice is ten times larger than the basal shear geothermal heat flux. His model generated subglacial lakes that stress for grounded ice, so reducing ice thickness by a factor of 10 matched those detected using radar (Siegert, 2001), and located equates the two stresses that resist gravitational spreading. Any where other subglacial lakes would be, with some subsequently T. Hughes / Quaternary Science Reviews 28 (2009) 1831–1849 1843 found. His model also drove subglacial water toward bed channels, the advective ice flow, but it could be measured in principle. If it is resulting in enhanced ice-bed decoupling so stream flow developed found to occur and to be significant, then thermal convection in ice above the channels. Pulses in discharge of impounded should be included in ice-sheet models. subglacial water generated pulses in stream flow. Linking ice elevations to ice-bed coupling for stream flow was 10. Modeling West Antarctic collapse from the bottom up used to map the floating fraction of ice beneath Byrd Glacier, one of the fastest East Antarctic ice streams with the biggest ice drainage The six kinds of first-order features useful in modeling ice sheets basin (Reusch and Hughes, 2003). Surface wave-like undulations from the bottom up to capture stages in past glaciation cycles are having the appearance of stacked terraces correlated with the also useful in capturing basal conditions for the Greenland and floating fraction of ice along the bed, after correcting for variations Antarctic ice sheets at present. The present size of these ice sheets of bed topography. As with sheet flow, surface elevations in stream should be close to stage 5, when the ice sheets are in recession from flow are primarily determined by the degree of ice-bed coupling. forward positions during the LGM. Whether these ice sheets The relatively level portions of the wave-like surface of Byrd Glacier continue recession to stage 6 and a full Termination remains an were matched with the greatest floating fraction of ice, thereby open question. The West Antarctic Ice Sheet is the leading candi- quantifying the concept of ‘‘pseudo ice shelves’’ for the level date for this event. portions of West Antarctic ice streams proposed by Robin et al. In stage 5, interior ice has lowered, bringing cold upper ice into (1970b) and based on radar sounding. This correlation remained contact with the bed and partly freezing a ubiquitously thawed bed after a ‘‘bookkeeping’’ error was corrected (Hughes, 2009b). at state 4 so that isolated thawed patches remain, most promi- Hofstede (2008) applied the ice-bed coupling concept to model nently as subglacial lakes in bedrock hollows. That this condition recent lowering of Jakobshavn Isbrae in Greenland, following now exists beneath the Greenland Ice Sheet has been demonstrated disintegration of a buttressing ice shelf in Jakobshavn Isfjord in from basal radar data (Oswald and Gogineni, 2008). Subglacial 2002. He used a flowline version of the flowband model that lakes, also located by radar sounding, are widespread beneath the partitions a geometrical representation of the longitudinal gravi- Antarctic Ice Sheet (Siegert et al., 1996). Johnson (2002) developed tational driving force among local basal and side shear stresses, a model of subglacial hydrology coupled to UMISM that generated longitudinal tensile stresses upstream, and longitudinal compres- numerous known subglacial Antarctic lakes. Robin Bell, in a lecture sive stresses downstream, all resisting gravitational flow and all at the Center for Remote Sensing of Ice Sheets (CReSIS) at the linked to the floating fraction of ice (Hughes, 2009a,b). The flowline University of Kansas on 1 November 2006 titled, ‘‘East Antarctica: was along a flightline down the center of Jakobshavn Isbrae that An Ice Sheet Controlled By Lakes and Mountains,’’ explored the delivered surface and bed radar reflections. Side shear stresses implications of these bed conditions for ice-sheet stability if vanish along the center of ice streams, so the effects of side and subglacial lakes can link up and generate ice streams. Stearns et al. basal drag were assigned to the basal shear stress. Hofstede’s (2008) presented evidence from precision ice elevation changes model was able to reproduce the known thinning of Jakobshavn mapped by Earth-orbiting satellites showing that subglacial lakes Isbrae after its ice shelf disintegrated. An increase in the floating linking up under Byrd Glacier right now are causing a substantial fraction of ice accompanied surface lowering, giving a direct link increase in ice velocity. between a reduction of ice thickness and a reduction of ice-bed With these new methods for mapping the amount and extent coupling over time. of basal water, including linked subglacial lakes (‘‘pseudo ice Aitbala Sargent, a doctoral student advised by James Fastook, shelves’’), it now becomes possible for top-down models to use has modified the MacAyeal/Morland equations for ice shelves to basal water as bottom-up input and calculate the geothermal heat include a basal shear stress to get equations that also apply to ice flux as output, as Koons and Kirby (2007) have done for tectonic streams. This amounts to a solution of the full equilibrium/ systems. Then sites of high geothermal heating can be used as momentum equations that modelers have long sought, since the sources for basal water in bottom-up modeling of subglacial ice-shelf application already includes stresses in the map plane for instabilities which, coupled to models of subglacial hydrology, side shear and for longitudinal and transverse tension and allow basal water to ‘‘carry’’ the overlying ice at accelerating rates. compression (Sargent, 2009; Sargent and Fastook, 2008). The Sar- Transport of basal water from geothermal sources to sinks at ice- gent equations apply to ice streams in a standard top-down ice- shelf grounding lines should generate stream flow in overlying sheet model. Resistance to gravitational stream flow is reduced in ice. Ice streams, therefore, become the primary vehicles for a bottom-up application that increases the ‘‘Weertman’’ floating transporting subglacial water from sources to sinks, sinks being fraction under ice streams and eliminates basal pinning points both marine ice tongues, possibly imbedded in ice shelves, and under ice shelves. This solution is being imbedded in UMISM. terrestrial ice lobes from which basal water drains away. This Among innovative ways of thinking, may I include thermal demonstrates that ice streams must be modeled correctly. convection below the density inversion of the Antarctic and The best natural laboratory for modeling the interaction Greenland ice sheets as the origin of ice streams (Hughes, 2009c)? between top-down and bottom-up process is the West Antarctic Ice A gravitational buoyancy force below the density inversion gener- Sheet, which is in an advanced stage of gravitational collapse, and is ates a buoyancy stress that is about one-third the gravitational underlain by a Cenozoic volcanic province in its central subglacial driving stress for advective sheet flow. The buoyancy stress is highlands. A probable high geothermal heat flux in the highlands therefore theoretically large enough to cause thermal convective would provide the source of subglacial water that enters ice flow in ice below the density inversion. When superimposed on streams moving toward sinks for that water (Blankenship et al., advective flow, the pattern of convective flow would be aligned in 2001). Long fast concave ice streams drain West Antarctic ice on the direction of advective flow. The most efficient pattern would be three sides, east, west, and north (Fig. 7). Again, modeling ice for warm basal ice to rise in narrow curtains that would be the streams and subglacial hydrology correctly becomes paramount. lateral shear zones of ice streams, with ice sinking slowly in the ice Both require modeling basal processes correctly. Data input from stream between shear zones. That lowers the surface of ice streams the bed is as important as data input from the surface. and allows basal water to flow toward ice streams, thereby further The West Antarctic Ice Sheet was one-third of the Antarctic Ice decoupling ice from the bed and increasing stream flow. The cycling Sheet at the LGM (Fig. 1), but after Holocene gravitational collapse convective flow would spiral downstream at a rate controlled by of its Ross Sea and Weddell Sea sectors, it is only one-tenth of the 1844 T. Hughes / Quaternary Science Reviews 28 (2009) 1831–1849

Fig. 7. : The Antarctic Ice Sheet today. Broken lines show ice drainage divides for Pine Island Glacier (P) and Thwaites Glacier (T) draining West Antarctic ice into Pine Island Bay, Foundation Ice Stream (F) and Mercer Ice Stream (M) draining East Antarctic ice through the Bottleneck into Ronne–Filchner and Ross ice shelves, Byrd Glacier (B) draining East Antarctic ice into Ross Ice Shelf, and Lambert Glacier (L) draining East Antarctic ice into Amery Ice Shelf. Heavy dashed lines show projected retreat routes of Pine Island and Thwaites glaciers through the Bottleneck into East Antarctica as they downdraw and collapse the West Antarctic Ice Sheet.

Antarctic Ice Sheet today (Fig. 7). During collapse, ice poured into Glacier, will retreat and may eventually reach the two entrances to both the Ross Sea and Weddell Sea sectors from three sides, south, the Bottleneck on the east and west sides of Thiel Mountains east, and west, and left from the remaining side, north. Today, East (Fig. 7). Then they may merge with Foundation and Mercer ice Antarctic ice enters only through the Bottleneck and ice leaves West streams and continue to migrate into East Antarctica and discharge Antarctica on its north, east, and west flanks. It has gone from an unknown amount of East Antarctic ice (Fig. 8). In the worst-case gaining ice on three sides and losing ice on one side to gaining ice scenario, they could throw the entire East Antarctic Ice Sheet into on one side and losing ice on three sides. This has to be an unstable a negative mass balance that may put it on the road to total grav- situation, and today the West Antarctic Ice Sheet is in an advanced itational collapse and a 65 m rise in sea level. That answers the stage of gravitational collapse. Ice leaving on the east and west sides ‘‘how high’’ question. is buttressed by the respective Ronne–Filchner and Ross ice shelves, An answer to the ‘‘how fast’’ question depends on what happens but ice leaving on the north side enters the ice-free Pine Island Bay to the ice shelves that buttress the West Antarctic Ice Sheet as it polynya in the Amundsen Sea, which provides no buttressing. For continues to collapse. Holocene collapse began about 7000 BP this reason Pine Island Bay may be ‘‘the weak underbelly of the (Anderson and Shipp, 2001), apparently continues today, and West Antarctic Ice Sheet’’ (Hughes, 1981b). produced the huge Ross and Ronne–Filchner ice shelves that How much of the remaining West Antarctic Ice Sheet will buttress most of the remaining grounded one-third of the ice sheet. collapse and how soon? How high and how fast will global sea If Pine Island Bay remains an ice-free polynya as it opens to the level rise? How much East Antarctic ice will pour through the south, following downdrawn retreat of Pine Island and Thwaites Bottleneck as West Antarctic ice lowers? The Bottleneck (60 W– glaciers, these ice streams should continue to move at their present 135 W at 84 S) is the junction connecting the grounded east and velocities of several kilometers per year, and may even speed up as west components of the Antarctic Ice Sheet (Fig. 7). Thiel Moun- they merge with Foundation and Mercer ice streams, pass through tains in the middle of the Bottleneck diverts most East Antarctic ice the Bottleneck, and tap into much higher East Antarctic ice. As they into Ronne–Filchner Ice Shelf by way of Foundation Ice Stream on pull out East Antarctic ice, discharge of East Antarctic ice by other the east and into Ross Ice Shelf by way of Mercer Ice Stream on the ice streams supplying Ronne–Filchner and Ross ice shelves should west. Little East Antarctic ice now enters West Antarctica, so the diminish and may even stop, if these ice streams through the West Antarctic Ice Sheet can be considered as losing ice on three Transantarctic Mountains have high fjord-like headwalls, as does sides and gaining ice only from the top. The rise in sea level will be Byrd Glacier, the largest ice stream (Hughes, 1998, Figure 3.20). In 3–5 m if the ice sheet collapses to sea level, depending on whether that case, the Ronne–Filchner and Ross ice shelves will be deprived buttressing ice shelves disintegrate (Fig. 1). This is not the worst- of ice input from these ice streams and the resulting negative mass case scenario. balance may lead to catastrophic ice-shelf disintegration. The ice If the West Antarctic Ice Sheet collapses into Pine Island Bay, as streams that remain active will then be unbuttressed and can pull anticipated, two giant ice streams, Pine Island Glacier and Thwaites out even more East Antarctic ice. T. Hughes / Quaternary Science Reviews 28 (2009) 1831–1849 1845

Fig. 8. : Possible partial gravitational collapse of the East Antarctic Ice Sheet. Present ice elevations are contoured every 0.5 km. Broken contour lines show possible ice elevations before partial gravitational collapse of East Antarctic ice into Amery Ice Shelf, and after partial gravitational collapse of East Antarctic ice through the Bottleneck into an ice-free West Antarctica. Heavy black lines enclose possible collapsed portions of the East Antarctic Ice Sheet. Collapse obtained using Equation 11.34 in Hughes (2009b).

Far fetched? Look at Byrd Glacier, the largest of these ice streams calves when they meet (Kenneally and Hughes, 2006). This is the through the Transantarctic Mountains (Fig. 7). It has a huge case in an arid polar environment and it is an essentially bottom-up drainage basin, as large as the West Antarctic Ice Sheet, drawing ice process because bottom crevasses fracture up to 90 percent of the from the three highest interior ice domes of East Antarctica. Glacier ice thickness. If surface melting is extensive, water-filled top erratics and scoured bedrock that can be dated using cosmogenic crevasses can propagate through the whole ice thickness. Johannes nuclides are found all along its fjord through the mountains at Weertman was first to treat both cases (Weertman, 1973, 1980). elevations 1000 m above Byrd Glacier. So Byrd Glacier has thinned These and other processes are active in calving bays (Thomas, 1977; and lowered by at least 1000 m, and that is why it has acquired such Hughes, 2002; MacAyeal et al., 2003). a vast ice drainage basin. Yet it is still buttressed by Ross Ice Shelf. Jakobshavn Isbrae drains about seven percent of the Greenland The ‘‘how fast’’ question is answered by removing buttressing ice Ice Sheet, has long been the fastest known ice stream, and becomes shelves. Calving bays accomplish this task most rapidly (Thomas, afloat in Jakobshavn Isfjord. A calving bay has migrated some 30 km 1977; Hughes, 2002; MacAyeal et al., 2003; Hulbe et al., 2004). up the fjord since 1850 AD, the end of the Little Ice Age (Weidick and Bennike, 2007). Several seasons of observations suggested 11. Calving bays a series of positive feedback mechanisms called the Jakobshavn Effect (Hughes, 1986b). The ‘‘pulling power’’ of an ice stream in Lambert Glacier lies across the East Antarctic ice divide opposite extending flow produces ubiquitous surface crevasses which the Bottleneck (Fig. 7). Lambert Glacier discharges into Amery Ice absorb much more solar energy than does smooth surface ice, Shelf, which has a drainage basin even larger and much lower than thereby enhancing surface melting so surface meltwater can reach the drainage basin of Byrd Glacier. Ice may have lowered 3000 m the bed by way of the ubiquitous crevasses, accelerate basal sliding where Lambert Glacier enters Amery Ice Shelf (Fig. 8). If we treat and, once ice becomes afloat, accelerate the calving rate. These Amery Ice Shelf as a giant ice stream that has lost contact with its processes have accelerated in recent years. The calving bay has bed, retaining partial basal contact only along Lambert Glacier and carved away the ice shelf and the unbuttressed ice stream is other tributary ice streams, then it enters an ice-free polynya in the moving nearly twice as fast (Thomas, 2004; Joughin et al., 2008). Indian Ocean. Like Pine Island and Thwaites glaciers, and unlike Jay Zwally and five colleagues first observed the connection Byrd Glacier, it is then unbuttressed and can pull out much more ice between increased midday summer melting and increased ice as a result. That would explain why it has such a broad and low ice velocity on the smooth ice surface just north of Jakobshavn Isbrae drainage basin, and that is what could happen across the East (Zwally et al., 2002). This is now called the Zwally Effect and it Antarctic ice divide if Pine Island and Thwaites glaciers passed, should accelerate most Greenland ice streams. It may have caused unbuttressed, through the Bottleneck and into East Antarctica. rapid advance of the southern margin of the Laurentide Ice Sheet at When ice downdrawn by ice streams become afloat, water-filled the LGM, where a climate simulation generated heavy summer bottom crevasses can extend upward close to sea level and air-filled rainfall (Bromwich et al., 2004). The Zwally Effect is unlikely in the top crevasses can extend downward close to sea level. Floating ice cold Antarctic environment, but other positive feedbacks in the 1846 T. Hughes / Quaternary Science Reviews 28 (2009) 1831–1849

Jakobshavn Effect should be active if a calving bay follows Pine discerped by large oceans swells that are known to pass under Island and Thwaites glaciers into the Bottleneck. As East Antarctica Antarctic ice shelves (MacAyeal et al., 2006). These mechanisms ice pouring through the Bottleneck lowers, the calving bay may allow ice sheets to trigger rapid changes in the ocean and atmo- migrate up Pine Island and Thwaites glaciers and begin to carve out sphere. The West Antarctic Ice Sheet is where all these interacting the heart of the East Antarctic Ice Sheet. The heart of the Laurentide top-down and bottom-up processes can be studied and then Ice Sheet was carved out 8000 years ago when a calving bay modeled to simulate its ongoing gravitational collapse. migrated up a giant ice stream in Hudson Strait and entered Hud- son Bay (Hughes et al., 1977; Denton and Hughes, 1981, Figure 8.8). 12. Discussion With most of its accumulation zone gone, the Laurentide Ice Sheet collapsed in 200 years, leaving three isolated ice caps on land above This review is based primarily on my own experiences in sea level. glaciology in a career now spanning four decades. It began at The All aspects of the above scenario take place with no change in Ohio State University in 1968. I had no knowledge of glaciology. surface temperatures and accumulation or ablation rates except Paterson (1969), in his first edition of The Physics of Glaciers, those associated with the lowering ice surface. Therefore, the provided my initial education. In 1970 the American Geographical scenario is controlled by conditions at the base of ice and bottom- Society published its map, Antarctica, which changed my perspec- up modeling is required to capture the ice-sheet response to tive forever. The first-order concave profile of the West Antarctic Ice changing these conditions. For the floating ice shelves, changing Sheet, in sharp contrast to the overall convex profiles of the East basal conditions by enhanced basal melting frees basal ice from Antarctic Ice Sheet and the Greenland Ice Sheet, led me to suspect pinning points that enable the ice shelves to buttress ice streams that it was in an advanced stage of gravitational collapse. If so, was supplying them with ice. Enhanced basal melting is now wide- collapse ongoing? The Fletcher (1972) memorandum provided the spread under Antarctic ice shelves (Jacobs et al., 1996; Rignot and anchor for answering that question. I composed and circulated four Jacobs, 2002). ISCAP bulletins designed to address the question, using a quantum Calving bays can remove the pinning points by removing the ice jump in data from Antarctic meteorology, geophysics, glaciology, shelves, which is what triggered the doubled discharge from glacial geology, and marine geology, dating from the International Jakobshavn Isbrae in Greenland after its ice shelf disintegrated in Geophysical Year in 1958. 2002. This was first demonstrated by NASA glaciologist, Robert My bulletins led to my move to the University of Maine in 1974, Thomas (Thomas, 2004). Hofstede (2008) used a bottom-up where I was given the task of reconstructing global ice sheets at the approach to model the same observations. LGM and to model total gravitational collapse of the West Antarctic As basal melting allows ice streams to pull more Antarctic ice Ice Sheet at the LIM as part of CLIMAP during the 1970–1980 into the sea, the resulting rise in sea level can also float Antarctic ice International Decade of Ocean Exploration (Denton and Hughes, shelves free from their basal pinning points. Basal conditions that 1981). To accomplish that task, I had to develop a way to recon- would allow ice streams to discharge more ice center around struct ice sheets from the bottom up, based on ice-bed coupling expanding the supply of water under the ice streams. This would deduced from glacial geology at the LGM, rather than employ a top- allow the ice streams to both widen and lengthen. Even without down model that depended on unknown surface conditions. expanding the water supply, glacial erosion of a till or sediment Simulating total collapse of the West Antarctic Ice Sheet at the LIM blanket, and then of bedrock pinning points that project up into brought James Fastook into glaciology, and began a collaboration basal ice, will free overlying ice to slide more swiftly over whatever that continues to this day. Fastook developed his University of basal water is available. Maine Ice Sheet Model (UMISM), which combines the essentials of If an ice stream joins an ice shelf at a bedrock sill, retreat of the top-down and bottom-up modeling. ice-shelf grounding line over the sill increases the ice thickness, and Peltier (1994) inspired me to more forcefully re-direct my therefore the height of ice floating above sea level. The ‘‘pulling’’ thinking from the CLIMAP view that ice sheets at the LGM and the force of the ice shelf on the ice stream increases as the square of this LIM defined boundary conditions bracketing maximum perturba- height, so more ice is pulled out and the grounding line may retreat tions of a fundamentally stable global climate. After CLIMAP, I even faster if the bed continues to deepen (Fig. 1). This retreat over began to view ice sheets as fundamentally unstable and able to sills can be caused by a surging ice stream that thins ice upstream, control climate change, especially rapid climate change, through by lowering of the sill due to delayed isostatic sinking and glacial the instabilities inherent in ice sheets. I found these instabilities as erosion, by rising sea level, and by melting basal ice, all with no residing primarily in the periphery of ice sheets, allowing the ice- change in surface conditions unrelated to surface lowering (Fig. 2). sheet interiors to remain relatively stable until the instabilities These are the processes that give ice sheets a measure of inde- were able to penetrate the core regions and Terminate a glaciation pendence from other components of Earth’s climate system, yet cycle, thereby warming global climate. This led me to assign allow ice sheets to control the system by discharging enormous particular features of first-order glacial geology and related amounts of ice into the ocean in a short time. It takes eighty degrees geomorphic features on a deglaciated landscape to specific stages of sensible heat in ocean surface water to supply latent heat needed of a glaciation cycle, not just to the LGM (Hughes, 1996, 1998). Then to melt one gram of an iceberg, and these icebergs can be tens to ice-bed decoupling under the central core lowered the interior ice thousands of cubic kilometers in volume, weighing billions of surface and shot out ice streams at the LGM without a large change metric tons. If they are discharged by the hundreds to thousands in in ice volume, whereas in the CLIMAP ice sheets, an increase in ice a few centuries, they may trigger sustained global cooling over that volume caused the ice sheets to advance to the LGM ice margins. time (Hughes, 2004). The heat needed to bring these icebergs up to I now believe the CLIMAP simulation of gravitational collapse of the melting point, and then melt them, is supplied primarily by the West Antarctic Ice Sheet, in order to account for sea level 6 m ocean surface water to the depth of the draft of the icebergs. This higher at the LIM, should have been allowed to propagate through heat is then unavailable to sustain the ocean-to-atmosphere heat the Bottleneck into the East Antarctic Ice Sheet. This propagation exchange that drives global climate. Rapid disintegration of ice can also take place during the present interglacial, with an shelves (MacAyeal et al., 2003; Hulbe, et al., 2004) and calving of unknown increase in sea level. Rapid collapse of sectors of the giant icebergs (Kenneally and Hughes, 2006) should become Greenland and Antarctic ice sheets is the focus of my contribution to a major focus of glaciological research. Ice shelves may also be CReSIS. This is a new problem to solve that will draw on insights T. Hughes / Quaternary Science Reviews 28 (2009) 1831–1849 1847 gained from both top-down and bottom-up modeling strategies. Clarhall, A., 2002. Glacial Erosion Zonation – Perspectives on Topography, Land- Future models should include the dynamics of calving bays to forms, Processes and Time. Doctoral dissertation, Stockholm University. Clark, C.D., Stokes, C.R., 2001. Extent and basal characteristics of the M’Clintock complete the destruction of ice sheets, as was the case for the Channel Ice Stream. Quaternary International 86, 81–101. Laurentide Ice Sheet and may become possible for the East Antarctic De Angelis, H., 2007. Paleo-Ice Streams in the North-Eastern Laurentide Ice Sheet. Ice Sheet with prolonged climate warming. A central part of this Doctoral dissertation, Stockholm University. De Angelis, H., Kleman, J., 2008. Paleo-ice-stream onsets: examples from the north- strategy should be coupling the isostatic response to rapid lowering eastern Laurentide Ice Sheet. Earth Surface Processes and Landforms 33, 560–572. of ice sheets to a holistic model that treats the lithosphere and De Angelis, H., Skvarka, P., 2003. 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