|Science 15 May 2009:
Vol. 324. no. 5929, pp. 888 - 889
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
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
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
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.
And this is the marine ice sheet instability hypothesis?
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
What's your team's assessment, then, for the
potential sea level rise if the western Antarctica Ice Sheet were to collapse?
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.
Why wouldn't it affect the western side of
the Pacific and the eastern side of the Atlantic as well?
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.
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
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
"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