《生物技术与人类》课程教学资源（经典阅读）Twenty-first-century warming of a large Antarctic ice-shelf cavity by a redirected coastal current

LETTER doi:10.1038/nature.11064 Twenty-first-century warming of a large Antarctic ice-shelf cavity by a redirected coastal current Hartmut H.Hellmer,Frank Kauker2,Ralph Timmermann',Juirgen Determann'&Jamie Rae3 The Antarctic ice sheet loses mass at its fringes bordering the nearly undiluted from the continental shelfbreak into~1,000-m-deep Southern Ocean.At this boundary,warm circumpolar water can trenches underlying the floating extensions of ice streams that drain override the continental slope front,reaching the grounding line the West Antarctic Ice Sheet'4.Some ice streams from this ice sheet through submarine glacial troughs and causing high rates of also feed the 449,000-km2 Filchner-Ronne Ice Shelf (Fig.1),forming melting at the deep ice-shelf bases.The interplay between ocean the southern coast of the Weddell Sea.These ice streams pass over currents and continental bathymetry is therefore likely to influence mountain ranges and thus would not face an increase in basal melting future rates of ice-mass loss.Here we show that a redirection of the as the grounding line retreats.However,major ice streams entering the coastal current into the Filchner Trough and underneath the Filchner-Ronne Ice Shelf discharge large catchment basins of the East Filchner-Ronne Ice Shelf during the second half of the twenty-first Antarctic Ice Sheet.Once afloat,this ice interacts with the waters of century would lead to increased movement of warm waters into the the Weddell Sea. deep southern ice-shelf cavity.Water temperatures in the cavity We forced the Bremerhaven Regional Ice-Ocean Simulations would increase by more than 2 degrees Celsius and boost average (BRIOS)model'6 with the atmospheric output of two versions of the basal melting from 0.2 metres,or 82 billion tonnes,per year to HadCM3 climate model (Table 1).Whereas HadCM3-A is the base- almost 4 metres,or 1,600 billion tonnes,per year.Our results, line simulation used in perturbed physics ensembles,HadCM3-B is a which are based on the output of a coupled ice-ocean model forced model configuration with an interactive carbon cycle and vegetation, by a range of atmospheric outputs from the HadCM35 climate and is used in the ENSEMBLES project's.We used the output of two model,suggest that the changes would be caused primarily by an simulations of the twentieth century (HadCM3-A (1900-1999)and increase in ocean surface stress in the southeastern Weddell Sea HadCM3-B (1860-1999))and the Intergovernmental Panel on due to thinning of the formerly consolidated sea-ice cover.The projected ice loss at the base of the Filchner-Ronne Ice Shelf repre Climate Change scenarios E1 (2000-2199)and A1B(2000-2099/ 2199)(Table 1).These scenarios are characterized by different sents 80 per cent of the present Antarctic surface mass balance" carbon dioxide emissions,with atmospheric concentrations reaching Thus,the quantification of basal mass loss under changing climate conditions is important for projections regarding the dynamics of 09 Antarctic ice streams and ice shelves,and global sea level rise. The Weddell Sea(Fig.1)is dominated by a cyclonic gyre circulation that allows Circumpolar Deep Water to enter only from the east' Within the southern branch of the gyre,the water mass can be iden- tified as the Weddell Sea's temperature maximum at a depth of ~300m.The temperature decreases from 0.9C at the Greenwich 45°W meridian?to 0.6C off the tip of the Antarctic Peninsula.Only traces of the relatively warm water penetrate the broad southern continental Weddell Sea shelf,reaching the Filchner-Ronne Ice Shelf front with a temperature of-1.5C(ref.10).However,no indications exist that this water mass advances far into the ice-shelf cavity".Instead,locally formed high- salinity shelf water at the surface freezing temperature (about -1.89C)fuels a sub-ice-shelf circulation that brings the heat to the Longitude deep southern grounding line,where the base of the ice shelf touches Luitpold the ground.High-salinity shelf water is the densest water mass in the Weddell Sea,and is formed by brine rejection during sea-ice formation Filchner on a southward-sloping continental shelf.The need for a dense water ce Shelf mass to transport heat to the grounding line was used as an argument for the Filchner-Ronne Ice Shelfto be protected in a warmer climate2 This hypothesis assumes that rising atmospheric temperatures reduce 90°W sea-ice formation and,thus,the densification of the shelf water masses. This view considers solely the formation of dense continental shelf Figure 1 Map of Weddell Sea bathymetry south of 60S.Bathymetry is water masses in a warmer climate,but less-consolidated sea-ice cover based on RTopo-1(ref.29)with a colour contour interval of 500m.Inset might also influence the Weddell Sea circulation,including the course represents the model domain,with the red dashed line showing the map location within the circumpolar Southern Ocean.The solid yellow arrow marks of the coastal current. the present course of the coastal current in the Weddell Sea.The possibility of The marine-based West Antarctic Ice Sheet has the potential to pulsing into the Filchner Trough(FT)is marked by the dashed yellow arrow. contribute 3.3m to the global eustatic sea-level rise'3.Its ice shelves The region bounded by the dashed red line provided the integrated and mean fringing the Amundsen Sea are exposed today to Circumpolar Deep values in Fig.3.The solid grey line off the coastline indicates the ice-shelf front. Water with temperatures of more than 1C.This water mass cascades AP,Antarctic Peninsula;BI,Berkner Island. Alfred Wegener Institute for Polar and Marine Research,D-27570Bremerhaven Germany.0ASys,Lerchenstrasse 222767 Hamburg Germany.Met Office Hadley Centre,Exeter EX1 3PB,UK 10 MAY 2012 VOL 485 NATURE 225 2012 Macmillan Publishers Limited.All rights reserved

LETTER doi:10.1038/nature11064 Twenty-first-century warming of a large Antarctic ice-shelf cavity by a redirected coastal current Hartmut H. Hellmer1 , Frank Kauker1,2, Ralph Timmermann1 , Ju¨rgen Determann1 & Jamie Rae3 The Antarctic ice sheet loses mass at its fringes bordering the Southern Ocean. At this boundary, warm circumpolar water can override the continental slope front, reaching the grounding line1,2 through submarine glacial troughs and causing high rates of melting at the deep ice-shelf bases3,4. The interplay between ocean currents and continental bathymetry is therefore likely to influence future rates of ice-mass loss. Here we show that a redirection of the coastal current into the Filchner Trough and underneath the Filchner–Ronne Ice Shelf during the second half of the twenty-first century would lead to increased movement of warm waters into the deep southern ice-shelf cavity. Water temperatures in the cavity would increase by more than 2 degrees Celsius and boost average basal melting from 0.2 metres, or 82 billion tonnes, per year to almost 4 metres, or 1,600 billion tonnes, per year. Our results, which are based on the output of a coupled ice–ocean model forced by a range of atmospheric outputs from the HadCM35 climate model, suggest that the changes would be caused primarily by an increase in ocean surface stress in the southeastern Weddell Sea due to thinning of the formerly consolidated sea-ice cover. The projected ice loss at the base of the Filchner–Ronne Ice Shelf repre￾sents 80 per cent of the present Antarctic surface mass balance6 . Thus, the quantification of basal mass loss under changing climate conditions is important for projections regarding the dynamics of Antarctic ice streams and ice shelves, and global sea level rise. The Weddell Sea (Fig. 1) is dominated by a cyclonic gyre circulation that allows Circumpolar Deep Water to enter only from the east7 . Within the southern branch of the gyre, the water mass can be iden￾tified as the Weddell Sea’s temperature maximum at a depth of ,300 m. The temperature decreases from 0.9 uC at the Greenwich meridian7 to 0.6 uC off the tip of the Antarctic Peninsula8 . Only traces of the relatively warm water penetrate the broad southern continental shelf9 , reaching the Filchner–Ronne Ice Shelf front with a temperature of 21.5 uC (ref. 10). However, no indications exist that this water mass advances far into the ice-shelf cavity11. Instead, locally formed high￾salinity shelf water at the surface freezing temperature (about 21.89 uC) fuels a sub-ice-shelf circulation that brings the heat to the deep southern grounding line, where the base of the ice shelf touches the ground. High-salinity shelf water is the densest water mass in the Weddell Sea, and is formed by brine rejection during sea-ice formation on a southward-sloping continental shelf. The need for a dense water mass to transport heat to the grounding line was used as an argument for the Filchner–Ronne Ice Shelf to be protected in a warmer climate12. This hypothesis assumes that rising atmospheric temperatures reduce sea-ice formation and, thus, the densification of the shelf water masses. This view considers solely the formation of dense continental shelf water masses in a warmer climate, but less-consolidated sea-ice cover might also influence the Weddell Sea circulation, including the course of the coastal current. The marine-based West Antarctic Ice Sheet has the potential to contribute 3.3 m to the global eustatic sea-level rise13. Its ice shelves fringing the Amundsen Sea are exposed today to Circumpolar Deep Water with temperatures of more than 1 uC. This water mass cascades nearly undiluted from the continental shelf break into ,1,000-m-deep trenches underlying the floating extensions of ice streams that drain the West Antarctic Ice Sheet14. Some ice streams from this ice sheet also feed the 449,000-km2 Filchner–Ronne Ice Shelf (Fig. 1), forming the southern coast of the Weddell Sea. These ice streams pass over mountain ranges and thus would not face an increase in basal melting as the grounding line retreats. However, major ice streams entering the Filchner–Ronne Ice Shelf discharge large catchment basins of the East Antarctic Ice Sheet15. Once afloat, this ice interacts with the waters of the Weddell Sea. We forced the Bremerhaven Regional Ice–Ocean Simulations (BRIOS) model16 with the atmospheric output of two versions of the HadCM3 climate model (Table 1). Whereas HadCM3-A is the base￾line simulation used in perturbed physics ensembles17, HadCM3-B is a model configuration with an interactive carbon cycle and vegetation, and is used in the ENSEMBLES project18. We used the output of two simulations of the twentieth century (HadCM3-A (1900–1999) and HadCM3-B (1860–1999)) and the Intergovernmental Panel on Climate Change scenarios E1 (2000–2199)19 and A1B (2000–2099/ 2199)20 (Table 1). These scenarios are characterized by different carbon dioxide emissions, with atmospheric concentrations reaching 1 Alfred Wegener Institute for Polar and Marine Research, D-27570 Bremerhaven, Germany. 2 OASys, Lerchenstrasse 28a, 22767 Hamburg, Germany. 3 Met Office Hadley Centre, Exeter EX1 3PB, UK. 90° W 45° W 0° 70° S 80° S Weddell Sea AP Ronne Ice Shelf BI FT Filchner Ice Shelf Luitpold Coast Berkner Bank Longitude Latitude Figure 1 | Map of Weddell Sea bathymetry south of 606 S. Bathymetry is based on RTopo-1 (ref. 29) with a colour contour interval of 500 m. Inset represents the model domain, with the red dashed line showing the map location within the circumpolar Southern Ocean. The solid yellow arrow marks the present course of the coastal current in the Weddell Sea. The possibility of pulsing into the Filchner Trough (FT) is marked by the dashed yellow arrow. The region bounded by the dashed red line provided the integrated and mean values in Fig. 3. The solid grey line off the coastline indicates the ice-shelf front. AP, Antarctic Peninsula; BI, Berkner Island. 10 MAY 2012 | VOL 485 | NATURE | 225 ©2012 Macmillan Publishers Limited. All rights reserved

RESEARCH LETTER Table 1 List of BRIOS model experiments for example,Weddell gyre transport',sea-ice thickness distribution Model Simulation Period and drift in the Weddell and Amundsen seas?425,and sea-ice concen- HadCM3-A 20th century 1900-1999 tration related to iceberg drift?. HadCM3-A A1B 2000-2099 Ocean characteristics of the simulations forced with the output of HadCM3-B 20th century 1860-1999 both HadCM3-A and HadCM3-B for the twentieth century agree well HadCM3-B A1B 2000-2199 HadCM3-B E1 2000-2199 with those from hindcasts using the NCEP-reanalysis27.In the follow- ing,we focus on the results of the runs forced with the output from Atmospheric forcing was extracted from the results of the climate models HadCM3-A and HadCM3-B. HadCM3-A forcing extends only until 2099 and is not available for the scenario E1.Scenarios E1 and HadCM3-B for the AlB scenario,because this scenario provides stronger A1B are characterized by different carbon dioxide emissions,with atmospheric concentrations signals and only HadCM3-B extends to the end of the twenty-second reaching 450 parts per million by volume (p.p.mv.)and 700p.p.m.v.by the year 2100.respectively. century,covering a period of 200 years.For the simulated present-day period,a slope front separates shelf water at the surface freezing point 450 p.p.m.v.and 700 p.p.m.v.by the year 2100,respectively.BRIOS is a from relatively warm water that is advected to the southern Weddell coupled ice-ocean model that resolves the Southern Ocean at latitudes Sea by the coastal current.However,starting in around 2036,pulses of south of =50s zonally with a resolution of 1.5 and warm water sporadically cross the 700-m-deep sill of the Filchner meridionally with a resolution of 1.5X cos().The water column is Trough at its eastern flank(Fig.1)but do not reach the southern ice- variably divided into 24 terrain-following layers.The sea-ice com- shelf front(Fig.2a).As early as 2070,water warmer than 0C begins to ponent is a dynamic-thermodynamic snow-ice model with heat enter the Filchner Trough continuously(Fig.2b),reaching the ground- budgets for the upper and lower surface layers2 and a viscous-plastic ing lines of the southern tributaries 6 years later (Fig.2c).After a rheology2.BRIOS considers the ocean-ice-shelf interaction under- further 14 years,the whole trough plus the southern half of the neath ten Antarctic ice shelves'42>with time-invariant thicknesses, Ronne Ice Shelf cavity are filled with water of open-ocean origin assuming the flux divergence to be in equilibrium with both the surface (Fig.2d).This corresponds to a warming of the deep southern cavity and the basal mass balance.The model has been successfully validated by more than 2 C.The sporadic flow of warm water into the Filchner by the comparison with mooring and buoy observations regarding, Trough during the twenty-first century,as well as its southward a2037 b2075 15 5°w 30°VW 30W de ongi 45°W 45°W 82°S 90°S 82°S 90°S c2081 d2095 15°W 15°W 30°W 30°W 45°W 82S 90S 90:W 82°S 90°s Latitude -2.0-1.8-1.6-1.4-1.2 -1.0-0.80.60.4-0.2-0.0) Figure 2Simulated evolution of near-bottom temperatures in the Weddell Ice Shelf front.It fills the deeper part ofthe Filchner Ice Shelfcavity and enters the Sea.a-d,Values are from 60 m above bottom for the period 2030-2099 of the Ronne Ice Shelf cavity near the grounding line south of Berkner Island in 2081 HadCM3-B/A1B scenario.Warm pulses into the Filchner Trough(2037;a)are (c).By 2095(d),warm water fills most ofthe bottomlayer ofthe Filchner-Ronne followed by a return of the shelf water masses to the cold state typical for present Ice Shelf cavity,reaching a quasi-steady state.We note that a trend in the water conditions.The final (unrevoked)destruction ofthe slope front starts in 2066;by mass properties of the interior Weddell Sea is not associated with any of these 2075(b),the tongue of slightly modified warm deep water reaches the Filchner processes.The solid grey line off the coastline indicates the ice-shelf front. VOL 485 10 MAY 2012 2012 Macmillan Publishers Limited.All rights reserved

450 p.p.m.v. and 700 p.p.m.v. by the year 2100, respectively. BRIOS is a coupled ice–ocean model that resolves the Southern Ocean at latitudes south of w 5 50u S zonally with a resolution of 1.5u and meridionally with a resolution of 1.5u 3 cos(w). The water column is variably divided into 24 terrain-following layers. The sea-ice com￾ponent is a dynamic–thermodynamic snow–ice model with heat budgets for the upper and lower surface layers21 and a viscous-plastic rheology22. BRIOS considers the ocean–ice-shelf interaction under￾neath ten Antarctic ice shelves16,23 with time-invariant thicknesses, assuming the flux divergence to be in equilibrium with both the surface and the basal mass balance. The model has been successfully validated by the comparison with mooring and buoy observations regarding, for example, Weddell gyre transport16, sea-ice thickness distribution and drift in the Weddell and Amundsen seas24,25, and sea-ice concen￾tration related to iceberg drift26. Ocean characteristics of the simulations forced with the output of both HadCM3-A and HadCM3-B for the twentieth century agree well with those from hindcasts using the NCEP-reanalysis27. In the follow￾ing, we focus on the results of the runs forced with the output from HadCM3-Bfor theA1B scenario, because this scenario provides stronger signals and only HadCM3-B extends to the end of the twenty-second century, covering a period of 200 years. For the simulated present-day period, a slope front separates shelf water at the surface freezing point from relatively warm water that is advected to the southern Weddell Sea by the coastal current. However, starting in around 2036, pulses of warm water sporadically cross the 700-m-deep sill of the Filchner Trough at its eastern flank (Fig. 1) but do not reach the southern ice￾shelf front (Fig. 2a). As early as 2070, water warmer than 0 uC begins to enter the Filchner Trough continuously (Fig. 2b), reaching the ground￾ing lines of the southern tributaries 6 years later (Fig. 2c). After a further 14 years, the whole trough plus the southern half of the Ronne Ice Shelf cavity are filled with water of open-ocean origin (Fig. 2d). This corresponds to a warming of the deep southern cavity by more than 2 uC. The sporadic flow of warm water into the Filchner Trough during the twenty-first century, as well as its southward a 2037 b 2075 c 2081 d 2095 Longitude 90° W 74° S 82° S 90° S 90° W 74° S 82° S 90° S 90° W 74° S 82° S 90° S 90° W 74° S 82° S 90° S Latitude –2.0 –1.8 –1.6 –1.4 –1.2 –1.0 –0.8 –0.6 –0.4 –0.2 –0.0 (°C) 15° W 0° 30° W 45° W 60° W 75° W 15° W 0° 30° W 45° W 60° W 75° W 15° W 0° 30° W 45° W 60° W 75° W 15° W 0° 30° W 45° W 60° W 75° W Figure 2 | Simulated evolution of near-bottom temperatures in the Weddell Sea. a–d, Values are from 60 m above bottom for the period 2030–2099 of the HadCM3-B/A1B scenario. Warm pulses into the Filchner Trough (2037; a) are followed by a return of the shelf water masses to the cold state typical for present conditions. The final (unrevoked) destruction of the slope front starts in 2066; by 2075 (b), the tongue of slightly modified warm deep water reaches the Filchner Ice Shelffront. Itfills the deeper part of the Filchner Ice Shelf cavity and enters the Ronne Ice Shelf cavity near the grounding line south of Berkner Island in 2081 (c). By 2095 (d), warm water fills most of the bottom layer of the Filchner–Ronne Ice Shelf cavity, reaching a quasi-steady state. We note that a trend in the water mass properties of the interior Weddell Sea is not associated with any of these processes. The solid grey line off the coastline indicates the ice-shelf front. Table 1 | List of BRIOS model experiments Model Simulation Period HadCM3-A 20th century 1900–1999 HadCM3-A A1B 2000–2099 HadCM3-B 20th century 1860–1999 HadCM3-B A1B 2000–2199 HadCM3-B E1 2000–2199 Atmospheric forcing was extracted from the results of the climate models HadCM3-A and HadCM3-B. HadCM3-A forcing extends only until 2099 and is not available for the scenario E1. Scenarios E1 and A1B are characterized by different carbon dioxide emissions, with atmospheric concentrations reaching 450 parts per million by volume (p.p.m.v.) and 700 p.p.m.v. by the year 2100, respectively. RESEARCH LETTER 226 | NATURE | VOL 485 | 10 MAY 2012 ©2012 Macmillan Publishers Limited. All rights reserved

ETTER RESEARCH propagation,is also suggested by results of the finite-element model The warming of the whole Filchner-Ronne Ice Shelf cavity by more FESOM2 when forced with the HadCM3-B/A1B output(Supplemen- than 2Cboosts average basal melting from0.2 m yr to 4m yrat the tary Information).FESOM is a coupled ice-ocean model that also takes end of the twenty-first century,with the maximum exceeding 50myr ice shelves into account,but it has a different architecture and a reso near the deep southern grounding line.The values correspond to a lution that allows the simulation of eddies.Therefore,the model is jump in basal mass loss from 82 Gt yr to ~1,600 Gt yr(Fig.3c), expected to react more intensely to moderate perturbations in atmo- which represents 64%of the simulated circumpolar total.This total sphere and sea ice.Owing to the higher resolution of the marginal seas increases within two decades from~1,000 Gtyr to ~2,500 Gt yr (~10km)in FESOM,the warm water pulses reach the interior of the In contrast,basal mass loss beneath the Ross Ice Shelfremains constant Filchner-Ronne Ice Shelf cavity less diluted(Supplementary Fig.4) at~80 Gt yr.A similar drastic change in Filchner-Ronne Ice Shelf and thus cause earlier significant increases in basal mass loss basal mass loss and circumpolar ice-shelf basal mass loss also happens (Supplementary Fig.5). in the simulations(Table 1)forced with the AlB output of HadCM3-A, The analysis of the forcing fields and the BRIOS output reveals that but with a delay of 10 years,and the El output of HadCM3-B,but the redirection of the coastal current in the southeastern Weddell Sea is with a delay of 50 years,respectively(Fig.3c).Owing to our assumption caused locally by an interplay between several climate components. of fixed ice-shelf thicknesses,we cannot accurately predict basal During the twenty-first century,a continuous atmospheric surface mass losses for long periods of high melting.However,if we assume warming (up to 4C per century)decreases the loss of sensible heat that grounding lines retreat into deeper basins29,our melt rates have to by the ocean.Together with an increase in long-wave downward radi- be considered as lower bounds.In addition,numerical experiments ation (up to 10 Wm per century)this reduces the thickness and show that ice shelves adjust to perturbations in ocean temperature on concentration of the sea ice,allowing its drift speed to increase and, timescales ranging from several decades to a few centuries thus,a more efficient momentum transfer to the ocean surface off the As a consequence of the increased input of fresh water due to ice- Luitpold Coast(Fig.3a,b).The enhanced surface stress,which is not shelfbasal melting,the Weddell Sea surface layer and the water masses related to an increase in atmospheric wind stress,directs the coastal on the whole southern and western continental shelves freshen rapidly current southwards towards the Filchner-Ronne Ice Shelf front,as it Today the high-salinity shelf water of these areas is one ingredient for approaches the 700-m-deep sill ofthe Filchner Trough.The importance the formation of deep and bottom waters of the Weddell Sea".These of the different atmospheric forcing variables to the redirection of the water masses change their characteristics as the shelf water freshens coastal current and,thus,the increase in melting at the base of the Given the differences among the climate scenarios and the model Filchner-Ronne Ice Shelf is investigated by means of additional realizations,we do not intend to predict the exact date of the changes in sensitivity experiments (Supplementary Information).Because about the circulation of the southern Weddell Sea.Instead,we emphasize the 80%of the changes occur in the twenty-first century,these experiments sensitivity of a small Antarctic coastal region to climate change with are confined to the period 2000-2099.The first simulation applies potentially severe consequences for the mass balance of a large detrended atmospheric forcing variables only,followed by runs in which Antarctic ice shelf.Determining the extent to which this influences the trends of 2-m temperature(the air temperature at an altitude of2m) the dynamics of the East Antarctic Ice Sheet will require further simu- and/or long-wave downward radiation were consecutively added. lation,forcing a coupled ice-sheet-ice-shelf model with the predicted 0.3 0.2 ah 860 1900 1940 1980 2020 2060 2100 2140 2180 6 12 1860 1900 1940 1980 2020 2060 2100 2140 2180 2,000 1,600 B/A1B 1.200 A/A1B :B/51 800 B/A1B(RIS) 400 1860 1900 1940 1980 2020 2060 2100 2140 2180 Year Figure 3 Modelled time series(1860-2199)for the southeastern Weddell provided because of the dominance of the long-term variability.c,Basal mass Sea.a,Area-integrated (Fig.1)sea-ice volume for BRIOS forced with the losses(BMLs)in gigatonnes per year.Thin and thick lines represent twentieth-century and AlB atmospheric output of the climate model simulations forced with the atmospheric output of the climate models HadCM3-B.Grey and black lines represent monthly means and 5-year running HadCM3-A and HadCM3-B,respectively.HadCM3-A forcing is available only means,respectively.b,Area-mean ocean-surface stress,for the same model as for the period 1900-2099 and the AlB scenario (Table 1).Solid and dashed in a.Not only is the long-term decrease in the sea-ice volume reflected by an lines represent results from forcing with twentieth-century and either AlB or, increase in the ocean-surface stress,but the coherence also holds for single respectively,El output.Black lines show BML for the Filchner-Ronne Ice Shelf events (for example,around 1940 and 2050).A correlation coefficient is not and the grey line shows that for the Ross Ice Shelf(RIS) 10 MAY 2012 VOL 485 NATURE 227 2012 Macmillan Publishers Limited.All rights reserved

propagation, is also suggested by results of the finite-element model FESOM28 when forced with the HadCM3-B/A1B output (Supplemen￾tary Information). FESOM is a coupled ice–ocean model that also takes ice shelves into account, but it has a different architecture and a reso￾lution that allows the simulation of eddies. Therefore, the model is expected to react more intensely to moderate perturbations in atmo￾sphere and sea ice. Owing to the higher resolution of the marginal seas (,10 km) in FESOM, the warm water pulses reach the interior of the Filchner–Ronne Ice Shelf cavity less diluted (Supplementary Fig. 4) and thus cause earlier significant increases in basal mass loss (Supplementary Fig. 5). The analysis of the forcing fields and the BRIOS output reveals that the redirection of the coastal current in the southeastern Weddell Sea is caused locally by an interplay between several climate components. During the twenty-first century, a continuous atmospheric surface warming (up to 4 uC per century) decreases the loss of sensible heat by the ocean. Together with an increase in long-wave downward radi￾ation (up to 10W m22 per century) this reduces the thickness and concentration of the sea ice, allowing its drift speed to increase and, thus, a more efficient momentum transfer to the ocean surface off the Luitpold Coast (Fig. 3a, b). The enhanced surface stress, which is not related to an increase in atmospheric wind stress, directs the coastal current southwards towards the Filchner–Ronne Ice Shelf front, as it approaches the 700-m-deep sill of the Filchner Trough. The importance of the different atmospheric forcing variables to the redirection of the coastal current and, thus, the increase in melting at the base of the Filchner–Ronne Ice Shelf is investigated by means of additional sensitivity experiments (Supplementary Information). Because about 80% of the changes occur in the twenty-first century, these experiments are confined to the period 2000–2099. The first simulation applies detrended atmosphericforcing variables only,followed by runs inwhich the trends of 2-m temperature (the air temperature at an altitude of 2 m) and/or long-wave downward radiation were consecutively added. The warming of the whole Filchner–Ronne Ice Shelf cavity by more than 2 uC boosts average basal melting from 0.2 m yr21 to 4 m yr21 at the end of the twenty-first century, with the maximum exceeding 50 m yr21 near the deep southern grounding line. The values correspond to a jump in basal mass loss from 82 Gt yr21 to ,1,600 Gt yr21 (Fig. 3c), which represents 64% of the simulated circumpolar total. This total increases within two decades from ,1,000 Gt yr21 to ,2,500 Gt yr21 . In contrast, basal mass loss beneath the Ross Ice Shelf remains constant at ,80 Gt yr21 . A similar drastic change in Filchner–Ronne Ice Shelf basal mass loss and circumpolar ice-shelf basal mass loss also happens in the simulations (Table 1) forced with the A1B output of HadCM3-A, but with a delay of 10 years, and the E1 output of HadCM3-B, but with a delay of 50 years, respectively (Fig. 3c). Owing to our assumption of fixed ice-shelf thicknesses, we cannot accurately predict basal mass losses for long periods of high melting. However, if we assume that grounding lines retreat into deeper basins29, our melt rates have to be considered as lower bounds. In addition, numerical experiments show that ice shelves adjust to perturbations in ocean temperature on timescales ranging from several decades to a few centuries30. As a consequence of the increased input of fresh water due to ice￾shelf basal melting, the Weddell Sea surface layer and the water masses on the whole southern and western continental shelves freshen rapidly. Today the high-salinity shelf water of these areas is one ingredient for the formation of deep and bottom waters of the Weddell Sea7,31. These water masses change their characteristics as the shelf water freshens. Given the differences among the climate scenarios and the model realizations, we do not intend to predict the exact date of the changes in the circulation of the southern Weddell Sea. Instead, we emphasize the sensitivity of a small Antarctic coastal region to climate change with potentially severe consequences for the mass balance of a large Antarctic ice shelf. Determining the extent to which this influences the dynamics of the East Antarctic Ice Sheet will require further simu￾lation, forcing a coupled ice-sheet–ice-shelf model with the predicted 1860 1900 1940 1980 2020 2060 2100 2140 2180 0.1 0.2 0.3 0.4 Sea-ice volume (103 km3) 1860 1900 1940 1980 2020 2060 2100 2140 2180 2 4 6 8 10 12 Surface stress (10–3 N m–2) 1860 1900 1940 1980 2020 2060 2100 2140 2180 0 400 800 1,200 1,600 2,000 BML (Gt yr–1) Year B/A1B B/A1B (RIS) B/E1 A/A1B a b c Figure 3 | Modelled time series (1860–2199) for the southeastern Weddell Sea. a, Area-integrated (Fig. 1) sea-ice volume for BRIOS forced with the twentieth-century and A1B atmospheric output of the climate model HadCM3-B. Grey and black lines represent monthly means and 5-year running means, respectively. b, Area-mean ocean-surface stress, for the same model as in a. Not only is the long-term decrease in the sea-ice volume reflected by an increase in the ocean-surface stress, but the coherence also holds for single events (for example, around 1940 and 2050). A correlation coefficient is not provided because of the dominance of the long-term variability. c, Basal mass losses (BMLs) in gigatonnes per year. Thin and thick lines represent simulations forced with the atmospheric output of the climate models HadCM3-A and HadCM3-B, respectively. HadCM3-A forcing is available only for the period 1900–2099 and the A1B scenario (Table 1). Solid and dashed lines represent results from forcing with twentieth-century and either A1B or, respectively, E1 output. Black lines show BML for the Filchner–Ronne Ice Shelf and the grey line shows that for the Ross Ice Shelf (RIS). LETTER RESEARCH 10 MAY 2012 | VOL 485 | NATURE | 227 ©2012 Macmillan Publishers Limited. All rights reserved

RESEARCH LETTER temperature perturbation.The use of the output of two different con- 18.Johns,T.C.et al.Climate change under aggressive mitigation:The ENSEMBLES figurations of HadCM3 in different scenarios and the confirmation of multi-model experiment.Clim.Dyn.37,1975-2004 (2011). 19.Lowe,J.A.et al.New study for climate modelling,analyses,and scenarios.Eos 90, the BRIOS results by FESOM,a coupled ice-ocean model with higher 181-182(2009). resolution and a different model architecture,reduces unavoidable 20.Nakicevovic,N.et al IPCC Special Report on Emissions Scenarios(Cambridge Univ uncertainties when dealing with processes related to climate change. Press,2000). Therefore,we are confident that our proposed mechanism is not a 21.Parkinson,C.L Washington,W.M.A large-scale numerical model of sea ice. J.Geophys.Res.84,311-337(1979). model artefact but quite a realistic mechanism.Consequently,we 22.Hibler,W.D.Ill.A dynamic thermodynamic sea ice model.J.Phys.Oceanogr.9, welcome the effort to monitor the coastal current during the upcoming 815-846(1979) expeditions to the southeastern Weddell Sea. 23.Hellmer,H.H.ImpactofAntarctic ice shelf basal meltingon sea ice and deepocean properties.Geophys.Res.Lett 31,L10307 (2004). Received 7 July 2011;accepted 13 March 2012. 24. Timmermann,R.Beckmann,A.Hellmer,H.H.Simulations of ice-ocean dynamics in the Weddell Sea:1.Model configuration and validation.J.Geophys Res.107,3024(2002) 1.Walker,D.P.etal.Oceanic heat transport onto the Amundsen Sea shelf through a 25.Assmann,K.M.,Hellmer,H.H.Jacobs,S.S.Amundsen Sea ice production and submarine glacial trough.Geophys.Res.Lett 34,L02602(2007). 2.Hellmer.H.H.Jacobs S.S.Jenkins.A.in Ocean.Ice.and Atmosphere:Interactions transport.J.Geophys.Res.110,C12013(2005). 26.Lichey,C.Hellmer,H.H.Modeling giant-iceberg drift under the influence of sea atthe Antarctic Continental Margin(eds Jacobs,S.S.&Weiss,R.F.)83-99(Antarctic ice in the Weddell Sea,Antarctica.J.Glaciol.47,452-460 (2001). Res.Ser.75,American Geophysical Union,1998). 3.Jacobs,S.S.Jenkins,A.,Giulivi,C.Dutrieux,P.Stronger ocean circulation and 27.Kalnay,E M.et aL The NCEP/NCAR 40-year reanalysis project.Bull Am.Meteorol. increased melting under Pine Island Glacier ice shelf.Nature Geosci.4,519-523 Soc.77,437-471(1996). (2011). 28.Timmermann.R.etal Ocean circulation and sea ice distribution in a finite element 4 Payne,A.J.et al.Numerical modeling of ocean-ice interactions under Pine Island global sea ice-ocean model.Ocean Model.27,114-129(2009). Bay's ice shelf.J.Geophys.Res.112,C10019(2007). 29. Timmermann,R.et al.A consistent dataset of Antarctic ice sheet topography, 5. Gordon,C.etal.The simulation of SST,sea ice extents and ocean heat transports in cavity geometry,and global bathymetry.Earth Syst Sci.Data 2,261-273(2010). a version of the Hadley Centre coupled model without flux adjustments.Clim.Dyn. 30.Walker,R.T.Holland,D.M.Atwo-dimensional coupled model for ice shelf-ocean 16,147-168(2000) interaction.Ocean Model.17,123-139 (2007). 6. Rignot,E.et al Acceleration of the contribution of the Greenland and Antarctic ice 31.Gordon,A.L,Visbeck,M.Huber,B.Export of Weddell Sea deep and bottom sheets to sea level.Geophys.Res.Lett.38,L05503(2011). water.J.Geophys.Res.106,9005-9017 (2001). 7. Schroder,M.Fahrbach,E.On the structure and the transport in the eastern Weddell Gyre.Deep-Sea Res.l/46,501-527(1999). Supplementary Information is linked to the online version of the paper at 8. Schroder,M.,Hellmer,H.H.&Absy,J.M.On the near-bottom variability at the tip of www.nature.com/nature. the Antarctic Peninsula.Deep-Sea Res.Il 49,4767-4790(2002). Acknowledgements We thank C.Wubber and W.Cohrs for providing stable computer 9. Nicholls,K.W.Boehme,L. Biuw,M.Fedak,M.A Wintertime ocean conditions facilities at the Alfred-Wegener-Institute for Polar and Marine Research;the Ice2Sea over the southern Weddell Sea continental shelf,Antarctica.Geophys.Res.Lett 35, community for discussions during project meetings;and J.Ridley,M.Martin and L21605(2008). A.Levermann for comments on the manuscript.This work was supported by funding to 10.Foldvik,A.,Gammelsrod,T.Torresen,T.in Oceanology of the Antarctic Continental the Ice2Sea programme from the European Union Seventh Framework Programme, Shelf (ed.Jacobs,S.S.)5-20(Antarctic Res.Ser.43,American Geophysical Union 1985) grant number 226375.This is lce2Sea contribution number 41. 11.Makinson,K.Nicholls,K.W.Modeling tidal currents beneath Filchner-Ronne lce Author Contributions H.H.H.had the idea to force BRIOS with Intergovernmental Panel Shelf and the adjacent continental shelf:their effect on mixing and transport. on Climate Change scenarios,did 50%of the BRIOS simulations,conducted a J.Ge0 ohys.Res..104.13449-13465(1999) significant part of the analysis of model output,wrote the main text of the paper and 12.Nicholls,K.W.Predicted reduction in basal melt rates of an Antarctic ice shelf ina participated in the figure preparation.F.K.did 50%of the BRIOS simulations, warmer climate.Nature 388,460-462(1997). conducted the analysis of the atmospheric forcing and wrote Supplementary 13.Bamber,J.L,Riva,R.E.M.,Vermeersen,B.L.A.LeBrocq,A Reassessment of the Information.R.T.did all FESOM simulations,was involved in the analysis of model potential sea-level rise from a collapse of the West Antarctic loe Sheet.Science 324, output and prepared most of the figures.J.D.provided the glaciological expertise for the 901-903(20091. interpretation of the model results related to basal mass loss.J.R.extracted the 14.Jenkins,A et al.Observations beneath Pine Island Glacier in West Antarctica and atmospheric forcings for all simulations and was involved in the analysis of model implications for its retreat.Nature Geosci.3,468-472(2010) output.All authors participated in the discussion on model results and in drafting the 15.Bamber,J.L,Vaughan,D.G.Joughin,I.Widespread complex flow in the interior paper. of the Antarctic ice sheet.Science 287,1248-1250(2000). 16.Beckmann,A.,Hellmer,H.H.Timmermann,R.Anumerical model of the Weddell Author Information Reprints and permissions information is available at Sea:large-scale circulation and water mass distribution.J.Geophys.Res.104, www.nature.com/reprints.The authors declare no competing financial interests 23375-23391(1999). Readers are welcome to comment on the online version of this article at 17.Collins,M.ef al.Climate model errors,feedbacks and forcings:a comparison of www.nature.com/nature.Correspondence and requests for materials should be perturbed physics and multi-model ensembles.Clim.Dyn.36,1737-1766(2011). addressed to H.H.H.(hartmut.hellmer@awi.de). 228 I NATURE VOL 48510 MAY 2012 2012 Macmillan Publishers Limited.All rights reserved

temperature perturbation. The use of the output of two different con￾figurations of HadCM3 in different scenarios and the confirmation of the BRIOS results by FESOM, a coupled ice–ocean model with higher resolution and a different model architecture, reduces unavoidable uncertainties when dealing with processes related to climate change. Therefore, we are confident that our proposed mechanism is not a model artefact but quite a realistic mechanism. Consequently, we welcome the effort to monitor the coastal current during the upcoming expeditions to the southeastern Weddell Sea. Received 7 July 2011; accepted 13 March 2012. 1. Walker, D. P. et al. Oceanic heat transport onto the Amundsen Sea shelf through a submarine glacial trough. Geophys. Res. Lett. 34, L02602 (2007). 2. Hellmer, H. H., Jacobs, S. S. & Jenkins, A. in Ocean, Ice, and Atmosphere: Interactions at the Antarctic Continental Margin (eds Jacobs, S. S. & Weiss, R. F.) 83–99 (Antarctic Res. Ser. 75, American Geophysical Union, 1998). 3. Jacobs, S. S., Jenkins, A., Giulivi, C. & Dutrieux, P. Stronger ocean circulation and increased melting under Pine Island Glacier ice shelf. Nature Geosci. 4, 519–523 (2011). 4. Payne, A. J. et al. Numerical modeling of ocean-ice interactions under Pine Island Bay’s ice shelf. J. Geophys. Res. 112, C10019 (2007). 5. Gordon, C. et al.The simulation of SST, sea ice extents and ocean heat transports in a version of the Hadley Centre coupled model without flux adjustments. Clim. Dyn. 16, 147–168 (2000). 6. Rignot, E. et al. Acceleration of the contribution of the Greenland and Antarctic ice sheets to sea level. Geophys. Res. Lett. 38, L05503 (2011). 7. Schro¨der, M. & Fahrbach, E. On the structure and the transport in the eastern Weddell Gyre. Deep-Sea Res. II 46, 501–527 (1999). 8. Schro¨der, M., Hellmer, H. H. & Absy, J. M. On the near-bottom variability at the tip of the Antarctic Peninsula. Deep-Sea Res. II 49, 4767–4790 (2002). 9. Nicholls, K. W., Boehme, L., Biuw, M. & Fedak, M. A. Wintertime ocean conditions over the southern Weddell Sea continental shelf, Antarctica. Geophys. Res. Lett. 35, L21605 (2008). 10. Foldvik, A., Gammelsrød, T. & Tørresen, T. in Oceanology of the Antarctic Continental Shelf (ed. Jacobs, S. S.) 5–20 (Antarctic Res. Ser. 43, American Geophysical Union, 1985). 11. Makinson, K. & Nicholls, K. W. Modeling tidal currents beneath Filchner-Ronne Ice Shelf and the adjacent continental shelf: their effect on mixing and transport. J. Geophys. Res. 104, 13449–13465 (1999). 12. Nicholls, K. W. Predicted reduction in basal melt rates of an Antarctic ice shelf in a warmer climate. Nature 388, 460–462 (1997). 13. Bamber, J. L., Riva, R. E. M., Vermeersen, B. L. A. & LeBrocq, A. Reassessment of the potential sea-level rise from a collapse of the West Antarctic Ice Sheet.Science324, 901–903 (2009). 14. Jenkins, A. et al. Observations beneath Pine Island Glacier in West Antarctica and implications for its retreat. Nature Geosci. 3, 468–472 (2010). 15. Bamber, J. L., Vaughan, D. G. & Joughin, I. Widespread complex flow in the interior of the Antarctic ice sheet. Science 287, 1248–1250 (2000). 16. Beckmann, A., Hellmer, H. H. & Timmermann, R. A numerical model of the Weddell Sea: large-scale circulation and water mass distribution. J. Geophys. Res. 104, 23375–23391 (1999). 17. Collins, M. et al. Climate model errors, feedbacks and forcings: a comparison of perturbed physics andmulti-model ensembles.Clim.Dyn.36,1737–1766 (2011). 18. Johns, T. C. et al. Climate change under aggressive mitigation: The ENSEMBLES multi-model experiment. Clim. Dyn. 37, 1975–2004 (2011). 19. Lowe, J. A. et al. New study for climate modelling, analyses, and scenarios. Eos 90, 181–182 (2009). 20. Nakicevovic, N. et al. IPCC Special Report on Emissions Scenarios (Cambridge Univ. Press, 2000). 21. Parkinson, C. L. & Washington, W. M. A large-scale numerical model of sea ice. J. Geophys. Res. 84, 311–337 (1979). 22. Hibler, W. D. III. A dynamic thermodynamic sea ice model. J. Phys. Oceanogr. 9, 815–846 (1979). 23. Hellmer, H. H. Impact of Antarctic ice shelf basal melting on sea ice and deep ocean properties. Geophys. Res. Lett. 31, L10307 (2004). 24. Timmermann, R., Beckmann, A. & Hellmer, H. H. Simulations of ice-ocean dynamics in the Weddell Sea: 1. Model configuration and validation. J. Geophys. Res. 107, 3024 (2002). 25. Assmann, K. M., Hellmer, H. H. & Jacobs, S. S. Amundsen Sea ice production and transport. J. Geophys. Res. 110, C12013 (2005). 26. Lichey, C. & Hellmer, H. H. Modeling giant-iceberg drift under the influence of sea ice in the Weddell Sea, Antarctica. J. Glaciol. 47, 452–460 (2001). 27. Kalnay, E. M. et al. The NCEP/NCAR 40-year reanalysis project. Bull. Am. Meteorol. Soc. 77, 437–471 (1996). 28. Timmermann, R. et al. Ocean circulation and sea ice distribution in a finite element global sea ice–ocean model. Ocean Model. 27, 114–129 (2009). 29. Timmermann, R. et al. A consistent dataset of Antarctic ice sheet topography, cavity geometry, and global bathymetry. Earth Syst. Sci. Data 2, 261–273 (2010). 30. Walker, R. T. & Holland, D. M. A two-dimensional coupled model for ice shelf-ocean interaction. Ocean Model. 17, 123–139 (2007). 31. Gordon, A. L., Visbeck, M. & Huber, B. Export of Weddell Sea deep and bottom water. J. Geophys. Res. 106, 9005–9017 (2001). Supplementary Information is linked to the online version of the paper at www.nature.com/nature. Acknowledgements We thank C. Wu¨bber and W. Cohrs for providing stable computer facilities at the Alfred-Wegener-Institute for Polar and Marine Research; the Ice2Sea community for discussions during project meetings; and J. Ridley, M. Martin and A. Levermann for comments on the manuscript. This work was supported by funding to the Ice2Sea programme from the European Union Seventh Framework Programme, grant number 226375. This is Ice2Sea contribution number 41. Author Contributions H.H.H. had the idea to force BRIOS with Intergovernmental Panel on Climate Change scenarios, did 50% of the BRIOS simulations, conducted a significant part of the analysis of model output, wrote the main text of the paper and participated in the figure preparation. F.K. did 50% of the BRIOS simulations, conducted the analysis of the atmospheric forcing and wrote Supplementary Information. R.T. did all FESOM simulations, was involved in the analysis of model output and preparedmost of the figures. J.D. provided the glaciological expertise for the interpretation of the model results related to basal mass loss. J.R. extracted the atmospheric forcings for all simulations and was involved in the analysis of model output. All authors participated in the discussion on model results and in drafting the paper. Author Information Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests. Readers are welcome to comment on the online version of this article at www.nature.com/nature. Correspondence and requests for materials should be addressed to H.H.H. (hartmut.hellmer@awi.de). RESEARCH LETTER 228 | NATURE | VOL 485 | 10 MAY 2012 ©2012 Macmillan Publishers Limited. All rights reserved