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Environnement écologie

Perspectives Ocean Science: Ice Sheet Stability and Sea Level
mai 2009

Science 15 May 2009:
Vol. 324. no. 5929, pp. 888 - 889
DOI: 10.1126/science.1173958
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA.

     Volume changes in the Antarctic Ice Sheet are poorly understood, despite the importance of the ice sheet to sea-level and climate variability. Over both millennial and shorter time scales, net water influx to the ice sheet (mainly snow accumulation) nearly balances water loss through ice calving and basal ice shelf melting at the ice sheet margins (1). However, there may be times when parts of the West Antarctic Ice Sheet (WAIS) are lost to the oceans, thus raising sea levels. On page 901 of this issue, Bamber et al. (2) calculate the total ice volume lost to the oceans from an unstable retreat of WAIS, which may occur if the part of the ice sheet that overlies submarine basins is ungrounded and moves to a new position down the negative slope (see the figure).

Host – Robert Frederick
     For decades, scientists have hypothesized that the western Antarctic Ice Sheet is inherently unstable due to its geography, including its underlying bedrock, which is mostly well below sea level. That makes the western Antarctic Ice Sheet a marine ice sheet, and so, different from the larger ice sheets of the eastern Antarctic and Greenland that are supported by land above sea level. Were the western Antarctic Ice Sheet to collapse, past estimates put the global sea level rise at about 5 to 6 meters. But in a paper in this week's Science, Jonathan Bamber and colleagues reassess the potential sea level rise and come up with a lower estimate of about 3.3 meters globally, but with regional variation that would make it almost a meter higher along the coasts of the United States.
     I spoke with Bamber from the University of Colorado, Boulder, where he is a visiting CIRES Fellow. Bamber is a professor of physical geography at the University of Bristol in England.
Interviewee – Jonathan Bamber
     It's been hypothesized for over 30 years now that the West Antarctic Ice Sheet is inherently unstable. I should say there are three ice sheets on the planet – there's Greenland, East Antarctica, and West Antarctica. And only one of those do glaciologists believe it is potentially inherently unstable, and that's West Antarctica. And the point there is that this instability means that the ice sheet could potentially rapidly collapse or rapidly put a lot of ice into the oceans. And, about 30 years ago a scientist—someone called John Mercer—published results, he suggested that if that happened the global sea level rise from it would be 5 meters. And that number – 5 to 6 meters – is the number that people have used, and that's the number that is in current currency, you know, in other words, that's the value that everybody quotes when they talk about collapse of West Antarctica. Why is that important? Well, I mean, a one-and-a-half meter sea level rise would displace 17 million people in Bangladesh alone. Sea level rise is considered to be one of possibly the most serious consequence of climate change. So, it's of utmost importance to understand what the potential threats to the coastlines and people living in coastal areas is. And West Antarctica is potentially the biggest or one of the most serious threats. And really nobody's looked at what the number, since the Mercer study, nobody's really looked at what that number is, and we've got a lot of new data sets and improved understanding about some of the processes involved.
     So, the hypothesis that West Antarctica was inherently unstable was suggested actually over 30 years ago now. And then, during the 1980s people developed numerical models of ice sheets, and these models didn't show any instability, but we now realize why that is. But, during 1980s and 90s the hypothesis kind of went out of fashion, because these numerical models didn't show any kind of instability at all. In fact, they evolved very, very slowly through time, over thousands of years. But then, in about the last decade, through a new suite of satellite observations, we've seen some dramatic changes taking place in West Antarctica that strongly indicate that this hypothesis is actually correct. We've also had some new developments in our theoretical understanding of the hypothesis, which also suggests that what people proposed 30 years ago is, in fact, correct. So, this hypothesis has now reemerged to something that is very likely real and a major concern.

Robert Frederick
And this is the marine ice sheet instability hypothesis?
Jonathan Bamber
     Exactly. So, the data... Well, the original study, back in 1978, was based on some very basic ice thickness data collected in Antarctica, which I think this data gives you information about what the depth of the bedrock is underneath the ice sheet. And it can't be collected by satellite – it either has to be done on the ground or by an airplane, so it's expensive and very labor intensive to get these data. And over the last 30 years we've acquired very much more ice thickness data over the whole of Antarctica, but particularly over West Antarctica. We also have much better surface topography over West Antarctica. And those two data sets are critical in determining two things: first, the volume of ice that potentially could contribute to sea level rise, and secondly, the proportion of West Antarctica which is potentially susceptible to this marine instability.

Robert Frederick
     What's your team's assessment, then, for the potential sea level rise if the western Antarctica Ice Sheet were to collapse?
Jonathan Bamber
     So, our reassessment of the contribution of West Antarctica to sea level rise, if it were to collapse, is about 3.3 meter, and we believe that that's an upper bound to its contribution because of the various assumptions that we've made in our calculations, which have been quite liberal. And that's around about half the value that has been quoted up until now, which has typically have been between 5 and 6 meters global sea level rise. But, the second important result of our analysis is that – and this is something that's been known before, but we've modeled this is in a more accurate way, if you like, than previously – is that the sea level rise is not uniform across the world, across the world's oceans. And it turns out that the peak, the maximum increase in sea level rise, is centered at about a latitude of 40 degrees along the Pacific and Atlantic seaboards of North America. And that peak increase is about one-and-a-quarter times the global value. So, to put that in perspective – and that's true even for a partial collapse of the ice sheet – so, say, West Antarctica lost a meter of sea level rise globally, the increase around the North American coastline would be about 1.25 meters.

Robert Frederick
     Why wouldn't it affect the western side of the Pacific and the eastern side of the Atlantic as well?
Jonathan Bamber
     Right. So, this is where it gets kind of a little bit technical. But, if you take a large lump of ice that's centered on West Antarctica, and you dump it in the ocean, you change the
     Earth's gravity field, because you're taking a large mass from one place, and you're distributing it throughout the oceans. And, basically, the world's oceans they're, the shape of them matches the gravity field. And so, what happens is you make the gravity a bit weaker around West Antarctica and a bit stronger in the Northern Hemisphere. And so, you actually get a a sea level lowering in the Southern ocean, and you get a kind of buildup—a pileup of water—in the Northern Hemisphere. So, that's one effect. But there is a second effect, as well, which is that it's a concept that physicists call conservation of angular momentum. And what that is is if you imagine you have a gyroscope spinning on its axis, and you stick a little bit of blue tack on it – not on its axis of rotation but somewhere else – well then, what happens is the axis of rotation changes a little bit, you know, your gyroscope shifts over and tilts a bit more to one side because of that mass that you stuck there. And, that's exactly what happens to the Earth when you take a mass from one place, and you stick it somewhere else – the axis of rotation changes very slightly. That effect is called true polar wander. It's, if you like, it's the wander of the axis of rotation of the Earth. And what that means is - what happens then is that you get water shifting across the lines of longitude, in other words in the east-west direction, as a consequence of that. And, that's why you get water piling up in the western Atlantic and the eastern Pacific.

Robert Frederick
     Now, you mentioned that this prediction of 3 to 4 meter rise would occur with a collapse of the western Antarctic Ice Sheet – how long would this potential sea rise occur if it were to be gradual – say, in the way that the western Antarctic Ice Sheet is losing ice currently?
Jonathan Bamber
     Okay. So, our best estimate of the mass loss in West Antarctica at the minute is about 0.5 millimeter of sea level rise a year, and that's, relatively speaking, that's sort of small rate compared to the total volume that we're talking about, which is, you know, 3.5 meters or whatever. So, you could work that out – if it's half a millimeter a year, and you want 3.3 meters to be input then that's, what, 7,000 years or something. But, the whole idea about this marine ice sheet instability is that it's a very rapid process. It's something that could happen over centuries, rather than thousands of years. And so, the early theories on this have suggested time scales of just a few hundred years for a near-complete collapse of the ice sheet. Now, the question is about rates, is a good one. And it is rates of sea level rise that are critical, not the absolute number. Now, this is just part of a bigger study. I mean, ideally, what we'd do here is we'd take our estimate of what the total contribution is, and then we do a subsequent study—which we hope to do, by the way—which is an estimate of how rapidly this could happen. And you stick the two together and then we've got some idea about rates of sea level rise. But, the second part is a pretty challenging study, which is trying to model how quickly the ice sheet could lose that mass.

Study Halves Prediction of Rising Seas

Conséquences de la fonte de la glace (Antarctique et Groenland)

     Une fonte de la calotte glaciaire de l'Antarctique ferait monter le niveau des eaux océaniques de manière moins spectaculaire qu'on ne le pensait jusqu'ici, mais avec des effets tout aussi dramatiques, selon une étude publiée jeudi aux Etats-Unis.
     S'appuyant sur de nouvelles mesures de la géométrie de la calotte glaciaire de l'Antarctique, des chercheurs britanniques et néerlandais estiment désormais que si elle disparaissait, l'élévation du niveau des océans serait de 3,2 mètres et non pas de cinq à sept mètres comme le prévoyaient de précédents travaux.
     Mais, selon cette étude publiée jeudi par le magazine Science, même une hausse d'un mètre du niveau des océans serait suffisamment importante pour affecter le champ de gravité terrestre dans l'hémisphère sud et modifier la rotation de la planète.
     Ce changement de rotation entraînerait une accumulation de l'eau océanique dans l'hémisphère nord et pourrait se traduire par des différences importantes dans le niveau des différents océans, la plus forte élévation étant alors à attendre sur les côtes est et ouest des Etats-Unis.
     "Le schéma d'élévation du niveau des océans est indépendant de la rapidité et de la quantité de fonte de la calotte glaciaire de l'Ouest Antarctique", met en garde le principal auteur de la recherche, Jonathan Bamber, de l'université de Bristol en Grande-Bretagne.
     "Même si la calotte glaciaire de l'Ouest Antarctique ne contribuait qu'à une élévation d'un mètre du niveau des océans étalée sur de nombreuses années, le niveau des mers le long des côtes nord-américaines connaîtrait une élévation 25% supérieure à la moyenne", écrit-il.

Comme 2000 comètes de Halley
     L'Antarctique renferme environ neuf fois la quantité de glace du Groenland et est considéré comme une bombe à retardement pour le niveau des océans. La calotte glaciaire de l'Ouest Antarctique suscite des craintes particulières car elle est formée en grande partie de glace reposant sur des soubassements rocheux à l'intérieur des terres qui se trouvent en-dessous du niveau des océans.
     De grandes surfaces de banquise flottante empêchent actuellement que cette glace n'atteigne l'océan, mais les scientifiques craignent que cela ne finisse par arriver si la banquise se détache.
     Les chercheurs ne savent pas à quelle vitesse la calotte glaciaire antarctique risque de disparaître, mais si elle fond à un rythme constant pendant 500 ans, le niveau des océans augmentera de 6,5 millimètres par an, soit deux fois plus vite que le rythme actuel.
     "Même si elle est moins importante que dans les prévisions antérieures, l'échelle d'instabilité est énorme", avertit Erik Ivins, de l'Institut californien de technologie, dans un article accompagnant l'étude.
     "La masse totale gagnée par les océans (...) sera à peu près équivalente à la masse qui serait projetée sur terre par l'impact de 2.000 comètes de Halley", ajoute-t-il.
     La situation est compliquée par le fait que le Groenland semble perdre encore plus de glace que l'Antarctique. Or, "il suffit que le Groenland perde la moitié de la masse perdue par l'Antarctique pour que l'effet soit équivalent sur les mouvements polaires, en raison de sa position plus éloignée du pôle", explique Erik Ivins.
     L'accélération du mouvement de la glace dans la mer d'Amundsen, qui borde le continent antarctique et est en grande partie recouverte de banquise, est encore "plus inquiétante".
     "Si la ligne côtière de la calotte glaciaire est repoussée plus loin vers l'intérieur des terres, la glace qui repose sur des soubassements rocheux situés profondément en-dessous du niveau de l'océan pourrait devenir instable", écrit-il.