RESEARCH NEWS & VIEWS G L ACIO LO GY
Ice-shelf stability questioned Surface lakes and streams are forming on Antarctica’s ice shelves, making them susceptible to instability and possible collapse. But rivers could mitigate this effect by efficiently exporting meltwater to the ocean. See Letters p.344 & p.349 ALISON BANWELL
T
he hundreds of floating ice shelves surrounding Antarctica have a crucial buttressing effect on inland glaciers1 — if an ice shelf breaks up, the glaciers feeding it will flow more rapidly to the sea2, contributing to global sea-level rise3. Ice shelves are rapidly melting not only from beneath, owing to contact with warming ocean water4, but also from above, because of increasing air temperatures5. Until now, meltwater lakes and streams on Antarctic ice shelves were considered a rarity. But on page 349, Kingslake et al.6 provide evidence that widespread lake and stream networks have been present on top of many Antarctic ice shelves for at least 70 years. Because lakes are thought to be hazardous to ice-shelf stability, these observations suggest that the risk of iceshelf collapse will be amplified if the extent and intensity of surface melting increases. However, on page 344, Bell et al.7 suggest that the increased risk will not be so great if large river networks export a sizeable fraction of the meltwater to the ocean, as observed on Antarctica’s Nansen Ice Shelf. Surface lakes result from the ponding of
a
meltwater in topographic depressions, and can be dangerous to ice shelves because they act as loads that can flex and weaken the ice, causing it to crack8. If a lake suddenly drains through a crevasse to the ocean below, the load deficit from the ice shelf ’s surface can induce more crevasses, potentially triggering a chain reaction of further lake-drainage events9. This process might have been responsible for the large-scale break-up of Antarctica’s Larsen B Ice Shelf in 2002, when more than 2,000 lakes drained in just a few days9. An increase in the coverage of ice-shelf lakes also enhances surface melting, because water absorbs more of the Sun’s radiation than the surrounding, more reflective ice or snow. Kingslake et al. use historical satellite imagery and aerial photography to show that about 700 large-scale lake and stream systems have persisted for decades on Antarctica’s ice shelves (and on some of the glaciers that feed them), often transporting water by as much as 120 kilometres. For example, the authors find that meltwater lakes and streams have existed on the Roi Baudouin Ice Shelf since 1947. A handful of previous studies5,10 have documented surface lakes and streams on
Grounding line
individual ice shelves over a span of a few years. But the authors’ work is the first to extensively map meltwater features and drainage systems on all of Antarctica’s ice shelves, over multiple decades. Streams and rivers, like lakes, act as surface loads on ice shelves, but they also play a crucial part in the movement and distribution of meltwater. As discussed by Kingslake and colleagues, a stream can form when a lake overflows, allowing meltwater to be transported to lower elevations, and perhaps into another lake. Alternatively, Bell et al. show that a large river (or river network) can enable a substantial proportion of an ice shelf ’s total meltwater volume to be exported to the ocean — often by a large waterfall at the ice edge, as observed on the Nansen Ice Shelf by the authors. This process of meltwater export therefore mitigates the risk that meltwater-induced ponding will lead to ice-shelf break-up. Surface meltwater on the Nansen Ice Shelf was first detected in 1909 by Ernest Shackleton and his Nimrod team, who repeatedly had to cross, and navigate around, lakes and streams on their way to the magnetic South Pole11. Now, Bell and colleagues use satellite imagery to show that six of the eight summer melt seasons between 2006 and 2015 were warm enough to allow the formation of a large-scale river network and ice-edge waterfall, facilitating surface-meltwater export to the ocean. Once the waterfall formed, it persisted for 5–25 days, and for longest when air temperatures were highest. Because of rising air temperatures, melt rates on almost all Antarctic ice shelves are expected to increase two- to threefold, on average, by 2050 (ref. 12). However, the extent to which meltwater production will enhance ice-shelf instability is debatable. It was previously suggested9 that the development of hundreds of
b
Ice shelf Crevasse Waterfall
Glacier
Figure 1 | Ice-shelf dynamics. Kingslake et al.6 show that widespread lake and stream networks have existed on many Antarctic ice shelves for decades. These drainage networks are usually thought to be hazardous to ice-shelf stability, but Bell et al.7 suggest that this instability can be mitigated if river networks export a large fraction of surface meltwater to the ocean. a, An ice shelf that has a relatively flat topography is likely to have numerous lakes and small streams. Some of these streams will transport water from the glacier,
. d e v r e s e r s t h g i r l l A . e r u t a N r e g n i r p S f o t r a p , d e t i m i L s r e h s i l b u P n a l l i m c a M 7 1 0 2 ©
3 0 6 | NAT U R E | VO L 5 4 4 | 2 0 A P R I L 2 0 1 7
across the ‘grounding line’, and onto the ice shelf — but few will export water to the ocean. Such an ice shelf could be unstable because of the weight of water on its surface, leading to the formation of crevasses, particularly near the ice front, and enhanced iceberg calving. b, Conversely, an ice shelf that has a steeper slope might have fewer lakes and a large river network, offsetting some of the instability caused by the presence of meltwater by exporting surface water to the ocean through an ice-edge waterfall.
NEWS & VIEWS RESEARCH surface lakes could provide the tipping point, which, once reached, would trigger the breakup of an ice shelf. If this is true, many ice shelves could be exposed to an ever-increasing risk of break-up, given their already extensive lake coverage. However, such a threshold might not be reached if surface water can instead be efficiently exported from the ice shelf to the ocean through large river networks. Various physical factors will determine which of the above processes dominates for individual ice shelves. For example, relatively flat topography and extensive snow coverage will encourage surface-water ponding, which is likely to increase instability (Fig. 1a). Conversely, steeper slopes and bare ice surfaces will encourage water flow and stream development, potentially offsetting some of this increased instability (Fig. 1b). The latter scenario is not currently accounted for by icesheet models, which assume that all meltwater is stored on top of ice shelves, making them increasingly unstable. For example, the results of these models suggest that some of Antarctica’s major ice shelves — such as the Amery, Filchner–Ronne, Larsen C and Ross — will disintegrate after melt rates exceed 1.5 metres per year, in the next century 3. However, Bell and colleagues’ analysis of the surface topography of these four ice shelves puts this prediction into question. These two studies suggest that the surface hydrology on Antarctica’s ice shelves will play a crucial part in deciding their individual fates, and those of the outlet glaciers that feed them. However, the authors do not explicitly address
the likely additional role of increased melting on the undersides of ice shelves caused by ocean warming4. Given that the Antarctic Ice Sheet contains enough ice to raise global sea levels by 60 m (ref. 13), identifying and quantifying the role of all surface and subsurface processes on the potential stability of ice shelves is becoming increasingly important. ■ Alison Banwell is at the Scott Polar Research Institute, University of Cambridge, Cambridge CB2 1ER, UK. e-mail: [email protected] 1. Fürst, J. J. et al. Nature Clim. Change 6, 479–482 (2016). 2. De Rydt, J., Gudmundsson, G. H., Rott, H. & Bamber, J. L. Geophys. Res. Lett. 42, 5355–5363 (2015). 3. DeConto, R. M. & Pollard, D. Nature 531, 591–597 (2016). 4. Pritchard, H. D. et al. Nature 484, 502–505 (2012). 5. Lenaerts, J. T. M. et al. Nature Clim. Change 7, 58–62 (2017). 6. Kingslake, J., Ely, J. C., Das, I. & Bell, R. E. Nature 544, 349–352 (2017). 7. Bell, R. E. et al. Nature 544, 344–348 (2017). 8. Scambos, T. et al. Earth Planet. Sci. Lett. 280, 51–60 (2009). 9. Banwell, A. F., MacAyeal, D. R. & Sergienko, O. V. Geophys. Res. Lett. 40, 5872–5876 (2013). 10. Langley, E. S., Leeson, A. A., Stokes, C. R. & Jamieson, S. S. R. Geophys. Res. Lett. 43, 8563– 8571 (2016). 11. David, T. W. E. & Priestley, R. E. Geological Observations in Antarctica by the British Antarctic Expedition 1907–1909 (Lippincott, 1909). 12. Trusel, L. D. et al. Nature Geosci. 8, 927–932 (2015). 13. Vaughan, D. G. et al. in Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (eds Stocker, T. F. et al.) 317–382 (Cambridge Univ. Press, 2013).
ST RUCTURAL B IO LO GY
A receptor that might block itself The structure of the angiotensin II type 2 receptor reveals a potential mode of self-blocking action. This might explain its lack of signalling, and opens up avenues of investigation into its function and role in disease. See Article p.327 C H R I S T O P H E R G . TAT E
T
hroughout the human body, the coordination of cell activity by hormones and neurotransmitter molecules is mediated by a multitude of cell-membrane proteins known as G-protein-coupled receptors (GPCRs). These form the largest membranereceptor family in humans, and are implicated in a range of disorders and diseases, including high blood pressure, migraine and cancer. It is essential for scientists to understand both the pharmacology and the structures of these
receptors to develop new therapeutics. On page 327, Zhang et al.1 describe the crystal structure of a particularly enigmatic GPCR, the angiotensin II type 2 receptor (AT2R), which is a potential target for the treatment of cardiovascular disease2. Unexpectedly, its structure deviates from the conventional GPCR structure, a finding that might explain its unusual signalling behaviour. The structures of GPCRs are highly evolutionarily conserved, despite having widely divergent amino-acid sequences3. Each GPCR has two main structural states: inactive,
50 Years Ago Some elegantly simple results of Defendi, Ephrussi et al. … have provided strong support for the general hypothesis that cancer induced by virus results from the addition of genetic information. Uncontrolled growth is the characteristic property that distinguishes all cancer cells from normal cells. Cancer cells are unable to regulate cell division and, since this characteristic is inherited, it is highly likely that it results from a genetic change. Before the discovery that DNA and RNA viruses can both transform normal cells into cancer cells, the hypothesis most frequently considered was that cancer is caused by the accumulation of somatic mutations leading to the loss of the function of some essential regulatory gene. With the discovery of cancer inducing or oncogenic viruses, an alternative hypothesis could be considered—that cancer results from the acquisition of genetic information. … it is equally possible that oncogenic viruses either cause a deletion of part of the cell genome or induce recessive mutations. The crucial question then is whether the genome of an oncogenic virus persists in transformed cells, perhaps incorporated into the chromosome as a prophage is in a bacterium. From Nature 22 April 1967
100 Years Ago To gain daylight by adjustment of the clock is a brilliant practical idea, but the present method of realising it by moving the hands of the clock is grossly unscientific, and should, I think, be changed for the alternative one. Let the circular disc of the clockdial be put in place by screws in curved slots. … when changing time, we should rotate the dial backwards and forwards respectively, leaving the hands untouched. From Nature 19 April 1917
. d e v r e s e r s t h g i r l l A . e r u t a N r e g n i r p S f o t r a p , d e t i m i L s r e h s i l b u P n a l l i m c a M 7 1 0 2 ©
2 0 A P R I L 2 0 1 7 | VO L 5 4 4 | NAT U R E | 3 0 7
Ice-shelf stability questioned Surface lakes and streams are forming on Antarctica’s ice shelves, making them susceptible to instability and possible collapse. But rivers could mitigate this effect by efficiently exporting meltwater to the ocean. See Letters p.344 & p.349 ALISON BANWELL
T
he hundreds of floating ice shelves surrounding Antarctica have a crucial buttressing effect on inland glaciers1 — if an ice shelf breaks up, the glaciers feeding it will flow more rapidly to the sea2, contributing to global sea-level rise3. Ice shelves are rapidly melting not only from beneath, owing to contact with warming ocean water4, but also from above, because of increasing air temperatures5. Until now, meltwater lakes and streams on Antarctic ice shelves were considered a rarity. But on page 349, Kingslake et al.6 provide evidence that widespread lake and stream networks have been present on top of many Antarctic ice shelves for at least 70 years. Because lakes are thought to be hazardous to ice-shelf stability, these observations suggest that the risk of iceshelf collapse will be amplified if the extent and intensity of surface melting increases. However, on page 344, Bell et al.7 suggest that the increased risk will not be so great if large river networks export a sizeable fraction of the meltwater to the ocean, as observed on Antarctica’s Nansen Ice Shelf. Surface lakes result from the ponding of
a
meltwater in topographic depressions, and can be dangerous to ice shelves because they act as loads that can flex and weaken the ice, causing it to crack8. If a lake suddenly drains through a crevasse to the ocean below, the load deficit from the ice shelf ’s surface can induce more crevasses, potentially triggering a chain reaction of further lake-drainage events9. This process might have been responsible for the large-scale break-up of Antarctica’s Larsen B Ice Shelf in 2002, when more than 2,000 lakes drained in just a few days9. An increase in the coverage of ice-shelf lakes also enhances surface melting, because water absorbs more of the Sun’s radiation than the surrounding, more reflective ice or snow. Kingslake et al. use historical satellite imagery and aerial photography to show that about 700 large-scale lake and stream systems have persisted for decades on Antarctica’s ice shelves (and on some of the glaciers that feed them), often transporting water by as much as 120 kilometres. For example, the authors find that meltwater lakes and streams have existed on the Roi Baudouin Ice Shelf since 1947. A handful of previous studies5,10 have documented surface lakes and streams on
Grounding line
individual ice shelves over a span of a few years. But the authors’ work is the first to extensively map meltwater features and drainage systems on all of Antarctica’s ice shelves, over multiple decades. Streams and rivers, like lakes, act as surface loads on ice shelves, but they also play a crucial part in the movement and distribution of meltwater. As discussed by Kingslake and colleagues, a stream can form when a lake overflows, allowing meltwater to be transported to lower elevations, and perhaps into another lake. Alternatively, Bell et al. show that a large river (or river network) can enable a substantial proportion of an ice shelf ’s total meltwater volume to be exported to the ocean — often by a large waterfall at the ice edge, as observed on the Nansen Ice Shelf by the authors. This process of meltwater export therefore mitigates the risk that meltwater-induced ponding will lead to ice-shelf break-up. Surface meltwater on the Nansen Ice Shelf was first detected in 1909 by Ernest Shackleton and his Nimrod team, who repeatedly had to cross, and navigate around, lakes and streams on their way to the magnetic South Pole11. Now, Bell and colleagues use satellite imagery to show that six of the eight summer melt seasons between 2006 and 2015 were warm enough to allow the formation of a large-scale river network and ice-edge waterfall, facilitating surface-meltwater export to the ocean. Once the waterfall formed, it persisted for 5–25 days, and for longest when air temperatures were highest. Because of rising air temperatures, melt rates on almost all Antarctic ice shelves are expected to increase two- to threefold, on average, by 2050 (ref. 12). However, the extent to which meltwater production will enhance ice-shelf instability is debatable. It was previously suggested9 that the development of hundreds of
b
Ice shelf Crevasse Waterfall
Glacier
Figure 1 | Ice-shelf dynamics. Kingslake et al.6 show that widespread lake and stream networks have existed on many Antarctic ice shelves for decades. These drainage networks are usually thought to be hazardous to ice-shelf stability, but Bell et al.7 suggest that this instability can be mitigated if river networks export a large fraction of surface meltwater to the ocean. a, An ice shelf that has a relatively flat topography is likely to have numerous lakes and small streams. Some of these streams will transport water from the glacier,
. d e v r e s e r s t h g i r l l A . e r u t a N r e g n i r p S f o t r a p , d e t i m i L s r e h s i l b u P n a l l i m c a M 7 1 0 2 ©
3 0 6 | NAT U R E | VO L 5 4 4 | 2 0 A P R I L 2 0 1 7
across the ‘grounding line’, and onto the ice shelf — but few will export water to the ocean. Such an ice shelf could be unstable because of the weight of water on its surface, leading to the formation of crevasses, particularly near the ice front, and enhanced iceberg calving. b, Conversely, an ice shelf that has a steeper slope might have fewer lakes and a large river network, offsetting some of the instability caused by the presence of meltwater by exporting surface water to the ocean through an ice-edge waterfall.
NEWS & VIEWS RESEARCH surface lakes could provide the tipping point, which, once reached, would trigger the breakup of an ice shelf. If this is true, many ice shelves could be exposed to an ever-increasing risk of break-up, given their already extensive lake coverage. However, such a threshold might not be reached if surface water can instead be efficiently exported from the ice shelf to the ocean through large river networks. Various physical factors will determine which of the above processes dominates for individual ice shelves. For example, relatively flat topography and extensive snow coverage will encourage surface-water ponding, which is likely to increase instability (Fig. 1a). Conversely, steeper slopes and bare ice surfaces will encourage water flow and stream development, potentially offsetting some of this increased instability (Fig. 1b). The latter scenario is not currently accounted for by icesheet models, which assume that all meltwater is stored on top of ice shelves, making them increasingly unstable. For example, the results of these models suggest that some of Antarctica’s major ice shelves — such as the Amery, Filchner–Ronne, Larsen C and Ross — will disintegrate after melt rates exceed 1.5 metres per year, in the next century 3. However, Bell and colleagues’ analysis of the surface topography of these four ice shelves puts this prediction into question. These two studies suggest that the surface hydrology on Antarctica’s ice shelves will play a crucial part in deciding their individual fates, and those of the outlet glaciers that feed them. However, the authors do not explicitly address
the likely additional role of increased melting on the undersides of ice shelves caused by ocean warming4. Given that the Antarctic Ice Sheet contains enough ice to raise global sea levels by 60 m (ref. 13), identifying and quantifying the role of all surface and subsurface processes on the potential stability of ice shelves is becoming increasingly important. ■ Alison Banwell is at the Scott Polar Research Institute, University of Cambridge, Cambridge CB2 1ER, UK. e-mail: [email protected] 1. Fürst, J. J. et al. Nature Clim. Change 6, 479–482 (2016). 2. De Rydt, J., Gudmundsson, G. H., Rott, H. & Bamber, J. L. Geophys. Res. Lett. 42, 5355–5363 (2015). 3. DeConto, R. M. & Pollard, D. Nature 531, 591–597 (2016). 4. Pritchard, H. D. et al. Nature 484, 502–505 (2012). 5. Lenaerts, J. T. M. et al. Nature Clim. Change 7, 58–62 (2017). 6. Kingslake, J., Ely, J. C., Das, I. & Bell, R. E. Nature 544, 349–352 (2017). 7. Bell, R. E. et al. Nature 544, 344–348 (2017). 8. Scambos, T. et al. Earth Planet. Sci. Lett. 280, 51–60 (2009). 9. Banwell, A. F., MacAyeal, D. R. & Sergienko, O. V. Geophys. Res. Lett. 40, 5872–5876 (2013). 10. Langley, E. S., Leeson, A. A., Stokes, C. R. & Jamieson, S. S. R. Geophys. Res. Lett. 43, 8563– 8571 (2016). 11. David, T. W. E. & Priestley, R. E. Geological Observations in Antarctica by the British Antarctic Expedition 1907–1909 (Lippincott, 1909). 12. Trusel, L. D. et al. Nature Geosci. 8, 927–932 (2015). 13. Vaughan, D. G. et al. in Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (eds Stocker, T. F. et al.) 317–382 (Cambridge Univ. Press, 2013).
ST RUCTURAL B IO LO GY
A receptor that might block itself The structure of the angiotensin II type 2 receptor reveals a potential mode of self-blocking action. This might explain its lack of signalling, and opens up avenues of investigation into its function and role in disease. See Article p.327 C H R I S T O P H E R G . TAT E
T
hroughout the human body, the coordination of cell activity by hormones and neurotransmitter molecules is mediated by a multitude of cell-membrane proteins known as G-protein-coupled receptors (GPCRs). These form the largest membranereceptor family in humans, and are implicated in a range of disorders and diseases, including high blood pressure, migraine and cancer. It is essential for scientists to understand both the pharmacology and the structures of these
receptors to develop new therapeutics. On page 327, Zhang et al.1 describe the crystal structure of a particularly enigmatic GPCR, the angiotensin II type 2 receptor (AT2R), which is a potential target for the treatment of cardiovascular disease2. Unexpectedly, its structure deviates from the conventional GPCR structure, a finding that might explain its unusual signalling behaviour. The structures of GPCRs are highly evolutionarily conserved, despite having widely divergent amino-acid sequences3. Each GPCR has two main structural states: inactive,
50 Years Ago Some elegantly simple results of Defendi, Ephrussi et al. … have provided strong support for the general hypothesis that cancer induced by virus results from the addition of genetic information. Uncontrolled growth is the characteristic property that distinguishes all cancer cells from normal cells. Cancer cells are unable to regulate cell division and, since this characteristic is inherited, it is highly likely that it results from a genetic change. Before the discovery that DNA and RNA viruses can both transform normal cells into cancer cells, the hypothesis most frequently considered was that cancer is caused by the accumulation of somatic mutations leading to the loss of the function of some essential regulatory gene. With the discovery of cancer inducing or oncogenic viruses, an alternative hypothesis could be considered—that cancer results from the acquisition of genetic information. … it is equally possible that oncogenic viruses either cause a deletion of part of the cell genome or induce recessive mutations. The crucial question then is whether the genome of an oncogenic virus persists in transformed cells, perhaps incorporated into the chromosome as a prophage is in a bacterium. From Nature 22 April 1967
100 Years Ago To gain daylight by adjustment of the clock is a brilliant practical idea, but the present method of realising it by moving the hands of the clock is grossly unscientific, and should, I think, be changed for the alternative one. Let the circular disc of the clockdial be put in place by screws in curved slots. … when changing time, we should rotate the dial backwards and forwards respectively, leaving the hands untouched. From Nature 19 April 1917
. d e v r e s e r s t h g i r l l A . e r u t a N r e g n i r p S f o t r a p , d e t i m i L s r e h s i l b u P n a l l i m c a M 7 1 0 2 ©
2 0 A P R I L 2 0 1 7 | VO L 5 4 4 | NAT U R E | 3 0 7
