Ice Shelf Instability
Guest contribution from Mauri S. Pelto
Ice shelves are floating platforms of ice fed by mountain glaciers and ice sheets flowing from the land onto the ocean. The ice flows from the grounding line where it becomes floating to the seaward front, where icebergs calve. For a typical glacier when the climate warms the glacier merely retreats, reducing its low elevation, high melting area by increasing its mean elevation. An ice shelf is nearly flat and cannot retreat in this fashion. Ice shelves cannot persist unless the entire ice shelf is an accumulation zone, where snowpack does not completely melt even in the summer.
Ice shelves have long been recognized as keys in buttressing Antarctic Ice Sheets. In turn ice shelves rely on pinning points for buttressing. The pinning point are where the floating ice shelf meets solid ground, either at lateral margins or a subglacial rise meets the bottom of the ice shelf causing an ice rise on the shelf surface.
The recent collapse of Wordie Ice Shelf, Mueller Ice Shelf, Jones Ice Shelf, Larsen-A and Larsen-B Ice Shelf on the Antarctic Peninsula has made us aware of how dynamic ice shelve systems are. After their loss the reduced buttressing of feeder glaciers has allowed the expected speed-up of inland ice masses after shelf ice break-up. (Rignot and others, 2004).
Several recent papers examine the causes of breakup of both Larsen B and Wilkins Ice Shelf, which prompts a closer look at the role of surface melting, structural weakness development and ice shelf thinning in this process.
In 1995 a substantial section of the northern Larsen Ice Shelf broke up in a few days. This was the first glimpse at a rapid ice shelf collapse. The breakup followed a period of warming and ice shelf front retreat, prompting (Rott and others, 1996) to observe that “after an ice shelf retreats beyond a critical limit, it may collapse rapidly as a result of perturbated mass balance”.
During the austral summer of 2001/02, melting at the surface of Larsen Ice Shelf in the Antarctic Peninsula was three times in excess of the mean. This exceptional melt event was followed by the collapse of Larsen B Ice Shelf, during which 3,200 km2 of ice shelf surface was lost. That meltwater was playing a key role in collapse was underscored by the unusual number of melt ponds that existed that summer and that the new ice front after collapse close to the limit of surface meltponds seen in images leading up to the March event (Scambos and others, 2003).
The ice shelves actually collapse via rapid calving, and the physics connecting meltwater to calving is its ability to enhance crevasse propagation. When filled 90% or more with meltwater a sufficiently deep crevasse can overcome the overburden pressure that tends to close the crevasse at depth (Scambos and others, 2000). Days before the final Larsen break-up, it is evident that the crevasses cut through the entire ice shelf. It also appeared that large meltponds contracted indicating that they were beginning to drain though the crevasses to the sea (Scambos and others, 2003).
As scientists it would have been easy to close the book on the issue after identifying the meltwater process. However, detailed examinations have continued identifying other key elements in the tale of collapse. The decade prior to collapse the Larsen-B Ice Shelf had thinned primarily by melting of the ice shelf bottom by 18 m (Shepard and others, 2003). This preconditions the ice shelf to failure by weakening its connection to pinning points as the shelf becomes more buoyant. This goes back to the critical limit mentioned by Rott (1996).
Glasser and Scambos (2008) reexamined the Larsen Ice Shelf breakup for structural weaknesses and observed the following. They noted that the rifting and crevasses parallel to the ice front crosscut the meltwater channels and ponds, hence, post dating them. The number and length of the rifts increased markedly in the year before collapse. Substantial rifts also existed between tributary glaciers feeding the ice shelf as far as 40 km behind the ice front. Enlargement of and development of new rifts in these regions occurred in the year prior to collapse. Downstream of the tributary glacier junction there are no evidence of relict rifts, illustrating that these rifts are a feature of the last 20 years. After ice shelf collapse the ice front receded to the pre-existing rifts, and the pre-existing rifts defined the area of collapse. In this case the structural weaknesses preconditioned the ice to rapid breakup. Rift formation occurred in areas of velocity differences and natural weaknesses Velocity differences are largest between tributaries and near the ice front.
The latest example of a collapsing ice shelf is Wilkins Ice Shelf (WIS), which lost 425 km2 in late February and early March 2008. The dynamic nature of the WIS is examined by Braun, Humbert and Moll (2008), their findings are summarized below. WIS is buttressed by Alexander, Latady, Charcot and Rothschild Island and by numerous small ice rises, indicating subglacial contact. Recent history indicates that WIS experiences no continuous ordinary calving, but single break-up events of various magnitudes. They further show that drainage of melt ponds into crevasses were of no relevance for the break-up at WIS. On WIS the evolution of failure zones is associated with ice rises. Analysis of rifting indicated that in 1990 the central area of WIS did not have any substantial rifts. In 1993/94, rift formation started to expand at the northern ice front. Today, the central part of WIS is intersected by long rifts that formed in and around ice rises. The rifts can cover tens of kilometers. The evolution and coalescing of the rifts are followed by break-up events at the ice front. Hence, the connection of rift systems seems to be the trigger for collapse. The recent break-up has left a narrow 6 km wide; already fractured connection to Charcot Island in a sensitive area that is stabilizing the northern part of the ice shelf. A new rift connection formed between already existing fractures, crosses almost the entire northern shelf, which makes WIS even more fragile and vulnerable. This area of interconnected rifts is 2100 km2. An additional 3000 km2 of the 13 000km2 of WIS, is at risk if this connection to Charcot Island is lost as rifts around the Petrie Ice Rise indicate an area of weakness. The conclusion for WIS is pre-conditioning of the ice shelf by failure zones occurring at ice rises and triggered by break-up events are leading to a sequence cascade of failure.
Below you can see the evident rifts near Charcot Island in this March MODIS image and the narrow connection of the ice shelf to this pinning point. The lack of sea ice on the north facing ice front is also noteworthy.
It appears that ice shelf thinning is the key pre-conditioning factor for collapse. The mechanisms for ice shelf thinning include basal melting, meltwater production and rift development. These are interrelated mechanisms that pre-condition the ice shelves to collapse. This will be a key area of continued investigation to understand this critical process for the Antarctic Ice Sheet. At the moment it seems that the key process to rapid calving events is the rift development. Rift development is observed to begin at points of natural weakness. For both ice shelves prior to collapse an expansion of the area where rifts exists has been observed. In both cases this seems to result from pre-conditioning via thinning due to basal melt and surface melt. Rifts development is accentuated by water filling crevasses. A new study will be looking at the impact of reduced sea ice at the front as well (Scambos and Massom, 2008). It is obvious that the glaciologic community will be watching the Wilkins Ice Shelf next Austral summer.
References:
Rignot, E., Casassa, G., Gogineni, P., Krabill, W., Rivera, A., and Thomas, R. (2004). Accelerated ice discharge from the Antarctic Peninsula following the collapse of Larsen B Ice Shelf. Geophysical Research Letters 31: L18401, doi:10.1029/2004GL020697.
Scambos, T., Hulbe, C., Fahnestock, M. and Bohlander, J. (2000). The link between climate warming and break-up of ice shelves in the Antarctic Peninsula. Journal of Glaciology 46: 516–530.
Scambos, T., C. Hulbe, and M. Fahnestock (2003). Climate-induced ice shelf disintegration in the Antarctic Peninsula. In: Domack, E., Leventer, A. Burnett, A., R. Bindschadler, R., P.
Vaughan, D. G., Marshall, G. J., Connolley, W.M., Parkinson, C., Mulvaney, R., D., Hodgson, D.A., King, J.C., Pudsey, C.J. and Turner, J. (2003). Recent rapid regional climate warming on the Antarctic Peninsula. Climate Change 60: 243-274, 2003.
12 June 2008 at 7:17 AM
Is it possible to to estimate a probability density function of the likelihood of the west antarctic ice shelf melting as a function of global temperature increase from climate change? Is it possible to do the same thing for Greenland? I suspect that this would have a significant impact on the expected social cost of carbon.
12 June 2008 at 7:29 AM
I’m confused. It’s an interesting subject, but this article could do with proof-reading - at several points there seem to be missing clauses and similar problems - and some of the things written seem unclear. For example, is WIS exceptional? If not, why say that “They further show that drainage of melt ponds into crevasses were of no relevance for the break-up at WIS. On WIS the evolution of failure zones is associated with ice rises” [but why?], but also “The mechanisms for ice shelf thinning include basal melting, meltwater production and rift development”? Don’t these seem inconsistent? Sorry to be so critical in this comment:(
12 June 2008 at 8:02 AM
letter to the editor
Chapel Hill (NC) Newspaper
June 11, 2008
Solutions exist if we apply the science.
Humankind is surely experiencing the fulfillment of a Chinese proverb: “We live in interesting times.” Many of our brilliant scientists report that God is a delusion. On the other hand, intuitive and gifted believers regularly tell us that these scientists themselves suffer from a form of delusional atheism. No one knows, I suppose, which of these groups is correct.
I am one of those people who believes the family of humanity can use God’s gift of science to take the measure of any global challenge and find solutions that are consonant with universal values. But, before we can move forward to reasonably address and sensibly overcome a challenge to human wellbeing and environmental health such as global warming, that challenge needs to be openly acknowledged and widely discussed. I suppose it is a function of my life experience to suggest that we accurately “diagnose” whatever the challenge is before proceeding to implement “treatment” options.
If great spiritual and scientific leaders are somehow on the right track when realizing, “The Earth has a human-induced fever and could overheat,” then at least one available treatment option is to carefully and skillfully examine the extant scientific evidence related to global warming and to make necessary changes in human behavior, both individually and collectively.
All of the above serves to set the stage for our consideration of a question. How can politicians and economic powerbrokers in the human community be empowered to muster the “political will” necessary for addressing human-driven climate change as well as for providing the substantial economic incentives and financial capital necessary to overcome this potential global threat to life as we know it and the integrity of Earth? — Steven Earl Salmony, Chapel Hill
12 June 2008 at 8:32 AM
I am completely uninterested in how the ice breaks up when melting. Much more important is how much heat is being absorbed when this ice changes to water. Any glacial experts out there care to total up this energy absorbed (terawatts/year) and relate it to what the earths’ temp. would be without this cooling provided by melting ice?
12 June 2008 at 9:25 AM
Bad new came from the North Pole recently: http://www.ucar.edu/news/releases/2008/permafrost.jsp
less ice means more rapid warming of permafrost than GCMs are projecting. Anybody surprised?
12 June 2008 at 10:20 AM
I am curious about the relationship between Antarctic sea ice and ice shelves. There has been a recent increase in sea ice. What is causing the increase? And will it help stabilize the ice shelves?
12 June 2008 at 10:53 AM
Very informative article!
I wonder what kind of effect high-latitude ocean warming is having on the ice shelves. One study from a few years ago looked at the causes of the “rapid” growth of Antarctic ice sheets 14 million years ago. The press release is at
http://www.sciencedaily.com/releases/2004/09/040920070738.htm
As we rapidly increase the ocean temperatures, will we also see a rapid warming of the Southern Oceans? Unfortunately there is not much Southern Ocean temperature data - but predictions are that rising temperatures will have widespread impacts. ( http://www.pnas.org/cgi/content/abstract/0712031105v1 ). Antarctic sea ice extent remains mostly unchanged:
http://www.windows.ucar.edu/tour/link=/earth/polar/sea_ice/sea_ice_compare_south.html
The thickness of the sea ice is also important. The thinning of the Arctic sea ice cover began at least two decades before the decrease in actual coverage did (as recorded by U.S. submarines). A nice discussion of the Antarctic sea ice thickness is at http://www.wmo.ch/pages/prog/wcrp/documents/CliC_SeaIceWorkshopFlyer2007.pdf
If you want to see some ridiculous coverage of this issue, check out Pielke Sr.’s March 27 2008 post. Their choice of references? ICECAP, the latest version of fossil fuel investor-funder anti-science PR.
12 June 2008 at 11:06 AM
Last southern hemisphere winter, the sea ice extent was the greatest since the invention of sattelite mounted cameras. This year, this date, the southern hemisphere sea ice extent is 1-million square kilometers ahead of last year (about 6 percent of last years record). http://arctic.atmos.uiuc.edu/cryosphere/
Is there some global climate model that predicts anything like that occuring?
[Response: The overall SH trends are not significant notwithstanding some big years recently. Trends in the different sectors are though. There has been a decrease to the west of the Antarctic peninsula, and an increase to the east - conceivably associated with a long term positive trend in the Southern annular mode (SAM). That increase is suggested by models as a response to the Antarctic ozone hole and increasing greenhouse gases. Significant decreases in SH sea ice are not expected yet - but I haven’t examined the ensemble of model runs to see what their variability is like in SH sea ice. - gavin]
12 June 2008 at 11:38 AM
Thanx for the column. I was just reading the paper by Wiens et al.,”Simultaenous teleseismic and geodetic observations of the stick-slip motion of an Antarctic ice stream”, Nature, v453, p770, 5 June 2008. I recall that similar icequakes have been recorded in NE Greenland, and I was wondering if there had been similar studies there (or elsewhere). There was another nice recent paper about a set of interconnected subglacial Antarctic lakes filling and draining within days, but i do not remember that any seismic signature was recorded.
Another question occurred to me: from the Wiens paper, the total ice moved into the Ross ice shelf by a single ice stream seems to be on the order of several 1000 km^3/yr. The estimate from GRAVIS for net ice mass loss from all of WAIS is on the order of several hundred km^3. So the difference must be made up by thickening of the ice shelf and snow accumulation. But it seems that the net ice mass loss is the difference of two much larger numbers: the total snow accumulation less the total melt and calving loss. Each of these seems to be on the order of 1e5 km^3. So the net mass loss is on the order of 1% of total snow accumulation or total mass loss.
Does this not indicate that even a 1% increase in melting/calving rate will double net mass loss ?
On a related note: do any ice sheet models predict increase in sea ice as the ice shelves collapse and float away ?
sidd
12 June 2008 at 11:43 AM
RE #4 “Any glacial experts out there care to total up this energy absorbed (terawatts/year) and relate it to what the earths’ temp. would be without this cooling provided by melting ice?”
I’ve also been looking for an answer to this question. In the GISS land-ocean temperature data (http://data.giss.nasa.gov/gistemp/tabledata/GLB.Ts+dSST.txt) the first quarter of 2008 was the coolest since 4Q 2000. Is it reasonable to look for a relationship between this cool trough and the recent Wilkins Ice Shelf event?
There’s an entanglement between this technical issue and the subject of human perceptions: if ice-sheet failure can be expected to postpone effects of climate-change (such as temperature rise), how long will this reprieve last, and how will it affect our response?
An example of such entanglement can be seen just above, in posts 6 and 7. It can be a mistake to rely on common sense where complex systems are involved, but it seems natural to expect that when ice-sheets break up, the former sheet-ice becomes sea-ice (for a few months, anyhow). Where else would it go?
12 June 2008 at 11:45 AM
Many comments have noted the sea ice extent and its role. The sea ice itself has not too date been identified as a factor in ice shelf breakup, in general the ice sheves see little open water at their front. The potential problem would be increased wave action at the ice shelf front. At this point it would be pure conjecture to consider its impact. That is the point of the upcoming study proposed by Scambos and Massom. At this point it is hard to anticipate an impact on the larger ice shelves such as the Ross Ice Shelf, but on the thinner ice shelves on the Peninsula it could be a final straw that helps accentuate the near ice front rifting. #2 I am sure you are correct, that the article could you use more proof reading, not my strongest suit. The mechanims noted basal melt, surface melt and rifting are not unrelated nor inconsistent. They clearly work in concert, the question is the details behind the relationships. In the case of WIS there seems to be less of a role of surface melt. The rifting does require some thinning mechasism to pre-condition to prime the ice shelf for rifting. #4 The role in the earth’s heat budget of the breakup of the ice shelves, I do not know. It is definetly of much less importance than the potential for ice shelf breakup and consequence for accelerating feeder glaciers to raise sea level. You should care as much about the ice shelves as any aspect of global warming.
12 June 2008 at 12:23 PM
The link to the University of Texas’s (Austin) Secrets of the West Antarctic Ice Sheet is still good. This symposium can be viewed on the web. The British Antarctic Survey does a presentation that discusses the role of increased current velocity on the melting of ice shelf bases. They note that it is increased wind speed, driving ocean currents that are causing the increase in shelf base melt. I guess that the wind increase is a result of the ozone hole versus global warming. But, global warming should more than take up the slack as the hole begins to close in 30 or so years.
I enjoy your grammar as it reminds me fondly of my father in law’s speech (Peruvian).
12 June 2008 at 12:39 PM
“Any glacial experts out there care to total up this energy absorbed (terawatts/year) and relate it to what the earths’ temp. would be without this cooling provided by melting ice?”
What you should really be asking, given that sea ice coverage has been growing globally, is how much cooler the global temperatures would be without this effect of latent heat being introduced into the environment as H2O moves to a more ordered state ie ice.
12 June 2008 at 1:47 PM
Nice summary syntax.
May we conclude that possibly the Arctic could be free of shelves and platforms this summer? I heard the year 2013 predicted. And surely that does not mean floating ice free, but rather just sheets and platforms of a certain size. How will that remaining ice be counted?
Checking http://nsidc.org/arcticseaicenews/
12 June 2008 at 2:17 PM
Gavin says
“Significant decreases in SH sea ice are not expected yet - but I haven’t examined the ensemble of model runs to see what their variability is like in SH sea ice. - gavin”
Are you talking about sea ice extent or sea ice thickness? More interestingly, what might be some of the features that caused climate models to miss the rapid retreat of Arctic sea ice?
12 June 2008 at 2:42 PM
Gavin said “The overall SH trends are not significant notwithstanding some big years recently. Trends in the different sectors are though”
Why would total SH sea ice not be significant amongst other trends? Would not sea ice be a measure of global warming which would lead to increased sea levels? If the SH sea ice is growing, one would presume temps are colder or below freezing for a longer period of time, thus reducing melting and increases in sea levels. Also, considering the recent increases in global temps, why wouldn’t record winter 07 SH sea ice, followed by winter 08 SH on record pace, not be noteworthy? Is it because it runs against consensus? Clearly there is some importance behind SH sea ice and last year’s record and this years pace.
12 June 2008 at 2:45 PM
#5 Alexander Ač
Surprised? Sadly Not.
If the Arctic goes rapidly (I think it will), then much of the projection and impacts work of the IPCC becomes irrelevant. Which is not a good thing.
Mauri Pelto.
Thank You.
12 June 2008 at 3:48 PM
#17
Not necessarily. The Arctic has historically experienced more extremes than the rest of the planet. Even if the sea ice reaches another record low, that doesn’t necessarily mean global warming is exceeding the IPCC projections…note that the extremely low Arctic sea ice of last fall still led into one of the colder northern hemisphere winters in recent memory.
So the weather at the poles does not necessarily indicate global weather trends.
12 June 2008 at 3:53 PM
Re: #16 (floodguy)
I don’t think Gavin meant “significant” in the sense of “important” or “meaningful,” but rather in the sense of “statistically significant,” i.e., distinguishable from random noise.
However, the change in southern hemisphere sea ice extent is statistically significant — just barely. This is mainly due to the high values so far in the 1st half of 2008; if they persist, the trend will remain significant but if they don’t the trend will probably not remain significant. Using data from late 1987 (the beginning of satellite observations) through May 2008, SH sea ice extent shows a positive trend rate of +12,000 km^2/yr. NH sea ice extent currently shows a negative trend rate of -131,000 km^2/yr.
[Response: Thanks for the update. Might be worth doing as a proper post… - gavin]
12 June 2008 at 4:32 PM
[#4] - You can find your answer in the Technical Summary of the AR4 IPCC report:
http://ipcc-wg1.ucar.edu/wg1/wg1-report.html
(See Figure TS.15)
The short answer is that heat absorbed by the oceans dominates the energy content of the earth climate system - eg by a factor of 100:1 for global oceans:Arctic sea ice
12 June 2008 at 4:39 PM
Insert the word STATISTICALLY before the word SIGNIFICANT in Gavin’s comment. He’s assuming that readers know what “significant” means when speaking of trends.
Well, no, actually.
12 June 2008 at 4:55 PM
#16 Floodguy,
Southern Hemisphere Anomaly
http://arctic.atmos.uiuc.edu/cryosphere/IMAGES/current.anom.south.jpg
Northern Hemisphere Anomaly
http://arctic.atmos.uiuc.edu/cryosphere/IMAGES/current.anom.jpg
And to get some idea of the perspective of those wiggles:
Southern Hemisphere Anomaly
http://arctic.atmos.uiuc.edu/cryosphere/IMAGES/current.area.south.jpg
Northern Hemisphere Area
http://arctic.atmos.uiuc.edu/cryosphere/IMAGES/current.area.jpg
Last year’s Arctic drop was 25%, that’s together with a whole range of observations that show a substantial change in the Arctic that outweighs the significance of the Antarctic sea-ice changes. In terms of sea ice they are very diferent issue; nobody is seriously talking about losing the Antarctic Ice Cap within a decade.
12 June 2008 at 5:07 PM
I’m intrigued by Ike’s post (currently #7). An increase in the westerlies that circle the globe around Antarctica is a pretty robust projection in most models (IIRC), and has already been observed, at least to some extent. Those westerlies help to drive the circumpolar current, so as they intensify, we should see the current become stronger. The wind belts are also projected to move towards higher latitudes.
The paper that Ike quotes from suggests that this may have been the mechanism for rapid glaciation 14 million years ago. If that is true, then there is the possibility that as warming progresses there might be further Antarctic cooling (irrespective of ozone recovery). I suppose there’s also the possibility that increased storminess around the fringes of Antarctica could provide a mechanism to get moisture into the interior, and thus grow more ice.
If the Arctic goes rapidly (I think it will), then we may see a world with only one cold pole. and I doubt that’s been modelled anywhere…
12 June 2008 at 5:54 PM
Floodguy, what Gavin probably means is that the recent SH sea ice extent variations are not statistically significant, unlike Arctic sea ice.
See this:
http://tamino.wordpress.com/2007/10/08/sea-ice-north-and-south-then-and-now/
Remember that sea ice is frozen sea water floating on the ocean, hence its melting does not affect sea level like accelerated flow/melt of ice sheets.
12 June 2008 at 6:52 PM
I think the point here is that sea ice volume is a the critical measure, which is a function of both thickness and extent.
In the Arctic, sea ice thinning preceded the loss of summer sea ice by decades. In the Antarctic, due to a lack of historical data (it seems unlikely that U.S. submarines would have collected datasets on Antarctic sea ice thinning - but maybe?), we don’t know what the sea ice thickness trend is.
If you want to look at the actual data for the past decades on sea ice extent, see
http://www.windows.ucar.edu/tour/link=/earth/polar/sea_ice/sea_ice_compare_south.html
The pattern so far is that thinning sea ice results in an ice pack that is far more sensitive to wind and current forcing than a thicker ice pack. Yet we simply have no idea of what the thickness trend in Antartic sea ice is. Using sea ice extent to predict future trends is nonsensical - If you had looked at Arctic sea ice extent up to 2005, you would have not predicted the sudden summertime collapse, right?
The real topic here, however, is how fast mass will be lost from the West Antarctice Ice Sheet. If we get accelerated ice loss from Greenland and the WAIS, that would mean that IPCC predictions of 21st century sea level rise are underestimates.
12 June 2008 at 7:29 PM
Let’s remember that all icebergs come from the calving of ice shelves or the calving of glaciers at the coast.
If someone could show the number of icebergs has increased recently, then one could show the break-up of ice-shelves is unusual in an historical context.
12 June 2008 at 7:47 PM
Floodguy, you seem to be having trouble parsing the sentence. OK first find the subject: SH trends. This means that the way things are changing, and since this is “realclimate” and not “realweather”, a trend is of order decades. Now let’s find the verb–there it is: are. And it is negated by the “not” and then we have the predicate: significant. So basically, what tat means is that you have a couple of years, and that’s weather. Hence THE TREND is not significant. That help?
12 June 2008 at 8:06 PM
You can do the math on the effect of latent heat. It takes 333.55 kJ/kg to melt water, so 1 cubic km of water takes about 3 x 10^17 joules to melt. Given the difference in albedo between ice and water, this will be made up in about 5 years due to increased solar absorption once the ice is gone.
12 June 2008 at 8:32 PM
“station temperature records for the past 50 years and report statistically insignificant temperature fluctuations over continental Antarctica excluding the Antarctic Peninsula, with the exception of Amundsen-Scott South Pole Station, which cooled by -0.17 K decade-1 for 1958–2000″
Monaghan, A. J., D. H. Bromwich, W. Chapman, and J. C. Comiso (2008), Recent variability and trends of Antarctic near-surface temperature, Journal of Geophysical Research, 113, D04105, doi:10.1029/2007JD009094.
Mackintosh, A., White, D., Fink, D., Gore, D.B., Pickard, J. and Fanning, P.C. 2007. Exposure ages from mountain dipsticks in Mac. Robertson Land, East Antarctica, indicate little change in ice-sheet thickness since the Last Glacial Maximum. Geology 35: 551-554.
The mass of Antarctica’s grounded ice sheet has been steadily growing over the past quarter-century.
Van de Berg, W.J., van den Broeke, M.R., Reijmer, C.H. and van Meijgaard, E. 2006. Reassessment of the Antarctic surface mass balance using calibrated output of a regional atmospheric climate model. Journal of Geophysical Research 111: 10.1029/2005JD006495.
“The advent of satellite imagery since the mid-1970s enabled scientists to remotely ‘view’ Antarctica and measure the extent of sea ice. This data shows little or no change, or possibly even an increase in sea ice extent since the 1970s. So what is happening? Is Antarctic sea ice extent increasing or decreasing? The answer is both! The effect seen depends on the timescale we are considering.”
http://www.aad.gov.au/default.asp?casid=14700
44.1% greater ice extent and 30% greater ice concentration in January 2008 compared to 1980.
http://nsidc.org/cgi-bin/bist/bist.pl?annot=1&legend=1&scale=75&tab_cols=2&tab_rows=2&config=seaice_extent_trends&submit=Refresh&hemis0=S&img0=trnd&hemis1=S&img1=plot&mo0=05&year0=2008&mo1=06&year1=2007&.cgifields=no_panel
12 June 2008 at 9:00 PM
http://www.agu.org/eos_elec/2008/Tedesco_89_13.html
12 June 2008 at 9:01 PM
If there are significant calving events over several years then surely we must expect the extent of sea ice to increase by a sort of ice-cube in your drink effect. The surrounding ocean will lose heat to the melting of the shelves and so be more prone to seasonal freezing.
12 June 2008 at 9:19 PM
Could the meltwater/crevasse phenomenon affect glaciers as well? If so, and meltwater eats through the bottom of glacial crevasses to reach ground level, could this lead to the feared effect of glaciers becoming much more lubricated and moving into the sea significantly faster?
12 June 2008 at 10:27 PM
Here’s the link to the Secrets of the West Antarctic Ice Sheet panel discussion. I’d forget my head if it weren’t attached. http://www.esi.utexas.edu/walse/ The British Antarctic Survey presentation discusses the hypothesis that increased wind speed and thus increased currents are raising air and water temperatures on and under the ice shelves.
13 June 2008 at 2:27 AM
Gavin - Floodguy’s questions about SH ice would sure seem relevant to global conditions. Are you suggesting that the SH anomaly is statistically insignificant or climatologically insignificant or both?
13 June 2008 at 5:22 AM
Hi climate people,
Is “Cryosphere today” http://arctic.atmos.uiuc.edu/cryosphere/ a good source of scientific information ?
The time of year right now is at the precipice of last years massive anomaly : http://arctic.atmos.uiuc.edu/cryosphere/IMAGES/current.365.jpg.
Do people think this was just a bad year or will there be an even deeper anomaly this year as all that single year ice is rapidly melted, before multiyear ice manages to slow the melt.
Looking at the anomaly in 2006, it looks like the anomaly in 2007 starts from where the 2006 anomaly left off.
If that is what happens I think we can expect to quickly get back to -3 million square km and then reach a minimum of around -4 million square km (or a remaining ice area of 2 million km) this year.
Is there any more thorough analysis of this data to predict the coming year or do we have to wait till the data becomes statistically significant by which time there will likely be 0 square kilometers of ice summer.
Alex
13 June 2008 at 5:22 AM
[29] - Looking at the anomaly graphs posted in [22] by CobblyWorlds, it is clear that 1980 was a fluke negative outlier. Using it as a comparison for the 2008 positive spike is misleading.
You should compare to a longer period mean. When you do so the difference between the two poles is clearly apparent. There is a very consistent negative trend in NH sea ice, which is accelerating. SH sea ice is mostly stable, with much greater noise and some recent positive spikes.
The difference in no way undermines the Arctic signal as an important sign of global warming happening now.
13 June 2008 at 6:14 AM
It is an odd phenomenon a discussion starts on ice shelves, and yet sea ice becomes the topic, hmmmmm. There seems to be several point of confusion. First a numberof posts indicate that the impact on the energy balance of the region would be impacted by the WIS breakup. Well no, compare the volume of ice involved to the the volume of ice that comprises the sea ice or even the ice on a large lake like Lake Erie in the winter, and you will be reminded that ice melt has a limited role in cooling already cold water or in cooling the air overhead. Second the amount of icebergs by areal extent compared to the Antarctic sea ice is tiny and is not an important contribution to the formation of further sea ice or to the overall heat balance of the system. The sea ice in the southern hemisphere also lacks the potential impact of the ice sheet. The role of meltwater in crevasse formation and exploitation is minor on most glaciers, however, on thin sections of ice shelves, the post notes that it does have a role, a thermal role. It is a role that was observed on the Larsen, but not on the WIS. The purpose of the post it to illustrate that it is beginning to appear that meltwater was not the key to the breakup of these ice shelves, but did have a significant pre conditioning role. The main pre-conditioner is rifting which is more robust when the ice shelf becomes thinner.
13 June 2008 at 7:26 AM
Dear All,
Tamino has posted a rather sobering (as if we needed more — where is my merlot?) description of new research showing that the permafrost in the north is on its way to meltdown with attendant consequences.
http://tamino.wordpress.com/2008/06/12/the-big-thaw/
13 June 2008 at 7:45 AM
Joeduck–A couple of years worth of data is “weather” not climate. You just can’t hope to wrest signal from noise for such a short period of time. Remember, what we’re talking about here is “GLOBAL CLIMATE CHANGE”. It is important to understand what all three of those words imply.
13 June 2008 at 7:58 AM
#11 Mauri Pelto (whom I thank for the article and apologise to for my ignorant comment) wrote:
“The role in the earth’s heat budget of the breakup of the ice shelves, I do not know.”
Am I guilty of another “insignificant” misreading wrt your “heat budget”? Surely the breakup of the ice shelves is due to anthropogenic global warming?
13 June 2008 at 8:04 AM
RE # 18
Sean, you said:
[So the weather at the poles does not necessarily indicate global weather trends.]
On what reserach, published studies, etc.do you base that comment.
Fact is, we do not know the impact of a massive dark surface at the Arctic Ocean on the convective currents, percip and temp of western NA. The world population and Archer Daniel Midland et.al. depend upon climate stability in Western NA; some for survival and others for profit (i.e., ethanol).
If you have insights we do not yet have, please share them.
John McCormick
13 June 2008 at 8:45 AM
The reasons for the ice sheet breaking of is due to tectonic activity along the Antarctic Peninsula. Undersea volcanic activity is warming that part of the oceans.
http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=221412
Notice in this NASA image of the warmer oceans along that region:
http://rst.gsfc.nasa.gov/Sect16/antarctic_temp-AVH1982-2004.jpg
That’s where the volcanic activity is happening along the plate boundary.
“Scientists Discover Undersea Volcano Off Antarctica
ScienceDaily (May 31, 2004) — ARLINGTON, Va. — Scientists working in the stormy and inhospitable waters off the Antarctic Peninsula have found what they believe is an active and previously unknown volcano on the sea bottom.”
http://www.sciencedaily.com/releases/2004/05/040527235943.htm
The melting ice sheet there has nothing to do with global warming. I ask the question again, how many things that do not support AGW must happen before you at RC give up AGW? What is needed to falsify the theory?
[Response: The question should be what will it take to stop people grasping at the flimsiest straws before they accept that climate is changing? There is no volcano under the Wilkins ice sheet - nor the Larsen B (actual location). Blaming that for the warming seen thousands of miles away on the other side of the Peninsula is like blaming Mt Etna for the European 2003 heat wave. - gavin]
13 June 2008 at 9:46 AM
#19 thank you Tamino and others for the replies.
On a different note, do you foresee any affect to NH ice from a negative PDO, if the PDO has indeed turned negative?
13 June 2008 at 10:07 AM
Joeduck, see Gavin’s inline response to #19, and Tamino’s postings.
“Barely significant statistically” — and it’s the two-dimensional area is what’s watched there.
Reports of melting from underneath are discussed above.
Get a lot of fresh water released from melting underneath, it rises above the denser salt water — and perhaps some of that freezes again around the edges during the winter as the air’s cold? I’d speculate that’s happening. People on the spot will find out.
13 June 2008 at 10:52 AM
About Arctic summer 2007 sea ice please see
Zhang, Jinlun, Ron Lindsay, Mike Steele, and Axel Schweiger, 2008. What drove the dramatic retreat of arctic sea ice during summer 2007? Geophys. Res. Lett., 35, L11505, doi:10.1029/2008GL034005, June 11, 2008
Abstract
A model study has been conducted of the unprecedented retreat of arctic sea ice in the summer of 2007. It is found that preconditioning, anomalous winds, and ice-albedo feedback are mainly responsible for the retreat. Arctic sea ice in 2007 was preconditioned to radical changes after years of shrinking and thinning in a warm climate. During summer 2007 atmospheric changes strengthened the transpolar drift of sea ice, causing more ice to move out of the Pacific sector and the central Arctic Ocean where the reduction in ice thickness due to ice advection is up to 1.5 m more than usual. Some of the ice exited Fram Strait and some piled up in part of the Canada Basin and along the coast of northern Greenland, leaving behind an unusually large area of thin ice and open water. Thin ice and open water allow more surface solar heating because of a much reduced surface albedo, leading to amplified ice melting. The Arctic Ocean lost additional 10% of its total ice mass in which 70% is due directly to the amplified melting and 30% to the unusual ice advection, causing the unprecedented ice retreat. Arctic sea ice has entered a state of being particularly vulnerable to anomalous atmospheric forcing.
About Arctic Ocean please see
Liu, Jiping, Z. Zhang, Y. Hu, L. Chen, Y. Dai, and X. Ren, 2008. Assessment of surface air temperature over the Arctic Ocean in reanalysis and IPCC AR4 model simulations with IABP/POLES observations. J. Geophys. Res. – Atmos., 113, D10105, doi:10.1029/2007JD009380, May 21, 2008
Abstract
The surface air temperature (SAT) over the Arctic Ocean in reanalyses and global climate model simulations was assessed using the International Arctic Buoy Programme/Polar Exchange at the Sea Surface (IABP/POLES) observations for the period 1979–1999. The reanalyses, including the National Centers for Environmental Prediction Reanalysis II (NCEP2) and European Centre for Medium-Range Weather Forecast 40-year Reanalysis (ERA40), show encouraging agreement with the IABP/POLES observations, although some spatiotemporal discrepancies are noteworthy. The reanalyses have warm annual mean biases and underestimate the observed interannual SAT variability in summer. Additionally, NCEP2 shows an excessive warming trend. Most model simulations (coordinated by the International Panel on Climate Change for its Fourth Assessment Report) reproduce the annual mean, seasonal cycle, and trend of the observed SAT reasonably well, particularly the multi-model ensemble mean. However, large discrepancies are found. Some models have the annual mean SAT biases far exceeding the standard deviation of the observed interannul SAT variability and the across-model standard deviation. Spatially, the largest inter-model variance of the annual mean SAT is found over the North Pole, Greenland Sea, Barents Sea and Baffin Bay. Seasonally, a large spread of the simulated SAT among the models is found in winter. The models show interannual variability and decadal trend of various amplitudes, and can not capture the observed dominant SAT mode variability and cooling trend in winter. Further discussions of the possible attributions to the identified SAT errors for some models suggest that the model’s performance in the sea ice simulation is an important factor.
13 June 2008 at 12:27 PM
Richard Wakefield, Want to falsify the hypothesis that humans are behind the current warming epoch? All you have to do is come up with a physical, dynamical model that does as good a job or better of explaining all the trends and doesn’t include an increased greenhouse effect. Of course, then you have to explain why the greenhouse effect should magically stop when CO2 concentration goes above 280 ppmv, but one thing at a time. Go ahead. We’ll wait.
13 June 2008 at 1:56 PM
Volcanoes, Richard? Do they also explain the meltdown of the entire Arctic?
13 June 2008 at 3:19 PM
Image at the top of the thread was:
http://www.realclimate.org/images/ice_shelf.jpg
News: http://www.physorg.com/news132575332.html
http://www.physorg.com/newman/gfx/news/thebreakupof.jpg
“… Wilkins Ice Shelf has experienced further break-up with an area of about 160 km² breaking off from 30 May to 31 May 2008. ESA’s Envisat satellite captured the event – the first ever-documented episode to occur in winter…. animation, comprised of images acquired by Envisat’s Advanced Synthetic Aperture Radar (ASAR) between 30 May and 9 June, highlights the rapidly dwindling strip of ice that is protecting thousands of kilometres of the ice shelf from further break-up.”
13 June 2008 at 3:28 PM
Here’s a working link to the animation from ESA, starting with 5/30 and day by day showing the breakup as far as it’s gone. The press release says further cracks in the shelf indicate the breakup is continuing.
http://yubanet.com/uploads/2/asa_imm_geo_sub_L_0.gif
13 June 2008 at 3:42 PM
Mauri, Thanks for a great summary.
A couple of points. Although ice shelf thinning does indeed appear to precede ice most ice shelf break-ups, no model has been forwarded yet that connects this thinning to the rapid disintegration process we see in the Larsen A, Larsen B, and Wilkins (and several icebergs in late stages of decay; Scambos et al., 2005 GRL). One possible link, however, is that reduced ice thickness leads to reduced connection with pinning points on the flanks of the shelf, and therefore lower internal compressive stress (or lower net tensional stress, to be accurate).
Braun, Humbert, and Moll have done an excellent job characterizing the history of change in the Wilkins Shelf, and have detailed how changes in its perimeter (small break-up events) had a near-immediate effect on internal rifting. However, they make one point that I think will be widely mis-interpreted (and I think they are mis-interpreting it themselves): that surface melt ponding had nothing to do with the recent Wilkins break-up event of March, 2008. That is, sensu stricto, true: the only observed surface ponds in the Wilkins are on the northeastern corner of the shelf, well away from the break-up area. But the Wilkins is known to have an intense melt season, and has been observed to have water-saturated firn. This water-saturated condition may be sufficient to drive the hydro-fracture model we describe (Scambos, Hulbe, Fahnestock, 2003; and, of course, it’s based on early related ideas by Weertman and Robin).
Note that all the ‘disintegration’ events occur in late summer, at the end of large melt seasons. The Larsen A broke up in late January, 1995; the Larsen B in March, 2007, and the Wilkins in March 2008. This seasonality continues to point to surface melting as the key cause of ice shelf break-up.
13 June 2008 at 4:19 PM
Mauri,
Thank you for the excellent article. It’s contributions like this that make Real Climate one of the most valuable websites discussing climate change.
Are there any ongoing monitoring efforts for the large ice sheets (Ross and Ronne)? If so, have they seen any of the early signs like thinning of the bottom of the ice shelf, formation of rifts or meltwater ponds?
13 June 2008 at 4:31 PM
The Wilkins continues to break up.
13 June 2008 at 4:43 PM
P.S. from http://earth.esa.int/ew/planning/pl_wilkinsiceshel-mar08.htm
WASHINGTON, March 26, 2008 (From AFP) - “Antarctica’s massive Wilkins Ice Shelf has begun disintegrating under the effects of global warming, satellite images by the University of Colorado’s National Snow and Ice Data Center showed. … With the Antarctic summer drawing to a close, scientists do not expect the ice shelf to further disintegrate in the next several months….”
Good thing they kept watching anyhow. Mauri, what are the researchers saying to each other this weekend?
13 June 2008 at 4:51 PM
By the way, this isn’t just an Arctic vs. Antarctic issue…it’s Northen Hemisphere vs. Southern Hemisphere. Take a look at this:
http://data.giss.nasa.gov/cgi-bin/gistemp/do_nmap.py?year_last=2008&month_last=5&sat=4&sst=1&type=anoms&mean_gen=0112&year1=1987&year2=2007&base1=1971&base2=2000&radius=1200&pol=reg
The northern hemisphere as a whole has been warming faster than the southern hemisphere, both land and water. I realize the common explanation for this is that it’s due to the southern hemisphere being a greater percentage water, but that doesn’t explain why the northern Atlantic has warmed so much more than the southern Atlantic, for example.
It seems to be an odd phenomenon.
13 June 2008 at 5:30 PM
Could you remind us how ice shelf instability is expected to affect sea level rise? If I understand it correctly, the collapse of the ice shelf and subsequent melting of the ice therein doesn’t directly affect sea level, but its secondary effects on the land based glaciers, to which the shelf is attached, do.
13 June 2008 at 6:33 PM
Steve Salmony — nice editorial letter. Thank you for posting.
13 June 2008 at 8:22 PM
nice animation from esa at
http://www.esa.int/esaEO/SEMG58VG3HF_planet_0.html
13 June 2008 at 9:16 PM
Re: #46- ” Want to falsify the hypothesis that humans are behind the current warming epoch? All you have to do is come up with a physical, dynamical model that does as good a job or better of explaining all the trends and doesn’t include an increased greenhouse effect. Of course, then you have to explain why the greenhouse effect should magically stop when CO2 concentration goes above 280 ppmv, ….”
He might have to do better than that, Ray, as Raypierre says in a post last December ” If somebody comes along and has the bright idea that, say, global warming is caused by phlogiston raining down from the Moon, that does not make everything we know about thermodynamics, infrared absorption, energy balance, and temperature suddenly go away. Rather, it is the job of the phlogiston advocate to quantify the effects of phlogiston on energy balance, and incorporate them in a consistent way beside the existing climate forcings. Virtually all of the attempts to poke holes in the anthropogenic greenhouse theory lose sight of this simple and unassailable principle.”
http://www.realclimate.org/index.php/archives/2007/12/les-chevaliers-de-l%e2%80%99ordre-de-la-terre-plate-part-ii-courtillots-geomagnetic-excursion/langswitch_lang/in#more-504
It’s going to take another Copernicus to make that drastic a change in our worldview.
13 June 2008 at 10:14 PM
If i may repeat some of a post from 9 July 2007:
Mercer, Nature, 1978, v271 pp.321-325
“One warning sign that a dangerous warming is beginning in Antarctica, will be a breakup of ice shelves in the Antarctic Peninsula just south of the recent January 0C isotherm; the ice shelf in the Prince Gustav Channel on the east side of the peninsula, and the Wordie Ice Shelf; the ice shelf in George VI Sound, and the ice shelf in Wilkins Sound on the west side.”
http://www.iitap.iastate.edu/gcp/sealevel/ross.html
Smith et al. Antarctic Science, 19(1), pp131-142 (2007)
http://pubs.usgs.gov/fs/fs17-02/fs017-02.html
sidd
13 June 2008 at 10:26 PM
I didn’t go through all the comments so this might have been posted, but another recent study at least suggests that increased shortwave flux because of less cloud cover did not contribute substantially to the anomalous NH sea ice decline in September 2007
A. J. Schweiger et al., Did unusually sunny skies help drive the record sea ice minimum of 2007?, GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L10503, doi:10.1029/2008GL033463, 2008
I do have a question for anyone here who may have played with GIS data on this, particularly from here (http://nsidc.org/data/gis/data.html and linked inside ftp://sidads.colorado.edu/DATASETS/NOAA/G02135/shapefiles ). Monthly sea extent shapefiles are here, and I put them into ArcGIS and used the tool to sum up the extent of the polygons and from GIS, September 2007 extent appears to be far less than 4 x 10^6 sq. kilometers (it was in the mid to high 3’s), as opposed to the literature value of ~4.28 x 10^6 sq. kilometers. Sea ice extent far less than the literature value seems consistent, so I was wondering if anyone had any insight into the shapefiles here?
14 June 2008 at 12:27 AM
# 44: Comment by Hank Roberts, 13 June 2008 10:07 AM
“Get a lot of fresh water released from melting underneath, it rises above the denser salt water — and perhaps some of that freezes again around the edges during the winter as the air’s cold? I’d speculate that’s happening. People on the spot will find out.”
If this is correct, we ought to see a salinity signature toward the ocean surface. I am playing with the data at
http://www.antarctica.ac.uk/met/SCAR_ssg_ps/OceanREADER/Salinity.html#saline%20circumpolar
but more qualified opinions are probably available here
sidd
14 June 2008 at 8:51 AM
Re 46: There is far too much reliance on models. The real world has a habit of not wanting to play by your rules. What is going to falsify AGW is the planet itself. It’s just going to take a few more years, as in a previous post that RC staff have admitted it would take of continued no increase in temp for the next 10 years.
The fact remains that there are serious problems with AGW predictions; the most serious of all is that sea level rise has not accelerated from the 1.7mm/yr it has been for the past 110 years of measurements (If AGW were true, depending on who’s making the prediction, sea level rates would have to rise from 3 to 40 TIMES the current rate, IPCC and Gore respectively).
If all these glaciers and ice is melting, why has this not shown up in the rate of sea level rise? If the excuse is that it will take a few more years before the change happens, then the current rate of rise is NOT due to global warming as the media claims. They are wrong to make the claim that current sea level rise is due to global warming. If there is a lag then that claim MUST be false.
Models must be tested against reality, and so far your models of sea level rise have failed completely. How many more years of no acceleration are required before AGW is rejected?
14 June 2008 at 8:58 AM
Here is a decent news article on the ice shelves:
http://www.csmonitor.com/2008/0328/p25s10-wogi.html
What is really odd is that even though the evidence is in (both poles are melting, as are mountain glaciers, endless numbers of American reporters and editors still feel compelled to place the statement “Some skeptics believe global warming is a hoax” at the bottom of every single news article they write on climate and weather. I actually called one of them up, and politely asked who they were quoting - “official NWS and NASA spokespeople” is the response I got. I then asked if they had read the news stories about political suppression of scientists at NASA, NOAA and the NWS - and then I asked why they didn’t feel compelled to place “Some skeptics believe AIDS is a hoax” on stories about that epidemic? The reporter got very upset and asked why I was “harrassing her.” I replied that she was the one who wrote the story, wasn’t she?
14 June 2008 at 9:50 AM
The continuing WIS disintegration is expected, just not yet, so glaciologists thought we could relax through the Austral winter, but does not appear so. The rifts seen on May 30 near the front for the WIS make it clear that not much was holding it together. Compare to the rifts in March in the original post. This is a dynamic process that we are beginning to understand and it is fascinating and frightening to observe. Until recently most of the glaciologic research has focussed on the larger ice streams of West Antarctic feeding the Ross and Ronne-Filchner ice Shelves. In the case of the Ronne-Filchner ice shelf, there has been a bit of a thickening of the ice streams near the transition to the floating ice shelf. This and the relatively consistent velocity of the ice streams feeding this complex indicates the comparative stability of this system. http://www.agu.org/pubs/crossref/2005/2005GL023844.shtml
A view of changes in the front of the Ross ice Shlef prepared by the USGS is seen at http://www.agu.org/pubs/crossref/2005/2005GL023844.shtml
This view shows little coherent change, instead it is the occassional massive iceberg breakoffs that control the ice front changes.
#55 You are exactly right. Just picture, you are in a kayak with the nose of your boat pressed against a large flat floating piece of ice in your way, you can move it but it slows you down. Now a change in the current moves the ice out of your way and all of a sudden forward progess is easier. Same thing for a glacier that feeds an ice shelf.
14 June 2008 at 10:11 AM
RE: #42
Richard,
I’ve looked at your links and although there have been some fairly recent discoveries of geologic activity and structures in the Antarctic little or none of it seems to be new in the geologic sense. In fact, one article suggests that some of the geologic structures are being observed for the first time because of recent ice retreat/melt. No argument is made in the article that geologic activity is the cause of the recent melting.
If this activity isn’t new and ice sheets and glaciers are now melting, how could these ice structures have been formed in the first place? Given this and Gavin’s point regarding the proximity of the volcanoes to the areas where most of the melting/breakup is being observed your argument doesn’t have a lot of merit.
Peter
14 June 2008 at 4:32 PM
Just my 2c… On today’s page, http://www.esa.int/esaCP/SEMG58VG3HF_index_0.html, ESA’s title is “Even the Antarctic winter cannot protect Wilkins Ice Shelf”. Well, yes, it’s full winter down there… Wilkins further disintegration was supposed to continue in next january or february, not in June. It’s just like seeing Greenland thawing in December…
For Alex #35 and others, many scientists monitor Artic this summer. Their outlooks are summarized here: http://www.arcus.org/search/seaiceoutlook/index.php. May’s Outlook has been issued recently - but sure to read the “Full report” tab. And, this will continue all summer long.
14 June 2008 at 5:20 PM
Richard (62), you’re simply wrong. Sea level rise was 3.1 +/- 0.7 mm per year from 1993 to 2003. It was 1.7 mm per year from 1.7 (+/- 0.5) mm per year over the 20th century, and slightly more from 1961 to 2003.
14 June 2008 at 5:34 PM
Richard Wakefield (62) — Regarding observed and predicted sea level rise, see the section of the Technical Summary from
http://ipcc-wg1.ucar.edu/wg1/wg1-report.html
Should help you to discover these have little to do with ‘models’.
14 June 2008 at 5:53 PM
sorry for the typing error in #67
I’d also add that sea level rise is not unique to “A”GW. I wonder why Richard signled out CO2, as opposed to the sun or cosmic rays?
14 June 2008 at 6:23 PM
Richard Wakefield says: “There is far too much reliance on models.”
Would you prefer Ouija boards? Psychics? Tea leaves? Sorry, Richard, that is what scientists do–take measurements and come up with models that explain them.
In the time you have been posting demonstrated denialist drivel, you could have made a start at learning the actual science of climate. No one says you have to believe it, but at the very least it would come closer to informed skepticism, rather than ignorant rants. So, let’s start with lesson 1–the oceans are really, really big. Climate changes on scales of decades. Also remember that as sea level continues to rise, it takes more water to make up the same rise.
In any case, I’d say it’s probably too early to say whether sea level rise rate is not increasing. Do you have hard data otherwise or is this another of your volcano stories?
14 June 2008 at 7:33 PM
“How many more years of no acceleration are required before AGW is rejected?”
Keep repeating your mantra, Richard, it keeps you busy and out of the way.
14 June 2008 at 10:09 PM
Re: 62 Richard states, “that sea level rise has not accelerated from the 1.7mm/yr it has been for the past 110 years of measurements”
Sea level rise from 1993 to the present has been 3.3 mm/year, about double the past century’s rate of rise. See this link:
http://www.cmar.csiro.au/sealevel/sl_hist_last_15.html
14 June 2008 at 11:43 PM
I do not see any discussion of the history of the ice shelf being discussed. According to an article in CS Monitor (http://www.csmonitor.com/2008/0328/p25s10-wogi.html), the Wilkins ice shelf is relatively modern, having only formed in the past thousand years or two, and may have formed with the onset of the “Little Ice Age.” If true, then leaving this sort of information out of the discussion may make the current break up of the shelf seem a little more serious than it actually is. If the Wilkins ice shelf is transient, then why shouldn’t it break up … what’s the big deal?
15 June 2008 at 2:45 AM
Re # 62 Richard, about sea level rise you are correctly informed about the latest study which is:
Berge-Nguyen, M., A. Cazenave, A. Lombard, W. Llovel, J. Viarre, and J.F. Cretaux. 2008. Reconstruction of past decades sea level using thermosteric sea level, tide gauge, satellite altimetry and ocean reanalysis data. Global and Planetary Change Vol. 62, No 1-2, pp. 1–13, May 2008
The IPCC and debaters here are outdated. We see a rise in sea level that is below the estimate of the IPCC and we see no acceleration through the past five decades. Basically, nothing seems to be happening with sea level that is remotely out of the ordinary.
[Response: People ought to read this article and not just take Timo’s summary of it at face value. –raypierre]
15 June 2008 at 4:36 AM
Comment by The Tuatara — 12 June 2008 @ 5:07 PM
quote if the Arctic goes rapidly (I think it will), then we may see a world with only one cold pole. and I doubt that’s been modelled anywhere… unquote
http://researchnews.osu.edu/archive/nowarm.htm
Not modelled, perhaps, but thought about.
JF
[Response: Actually, people working on initiation of Antarctic glaciation work on this regime all the time. There aren’t many of us, but it happens. Rob DeConto has some of the most complete work published in this area, but I’m doing modelling of this climate state myself for various reasons. There’s a lot to learn, and it is true that doubling or quadrupling CO2 puts the climate on track to look something like the Miocene. I guess Hansen would say we’re at risk of going all the way back too the Eocene but evaluating how realistic that is is one of the things that has gotten a lot of us interested in the time when Antarctic glaciation was just setting in. –raypierre]
15 June 2008 at 5:59 AM
Richard Wakefield writes:
Wherever you’re getting your information from is wrong. Sea level rise is now up to 3.3 mm/yr.
15 June 2008 at 6:34 AM
R.Wakefield makes the point we rely on models too much. The beauty of the papers cited here are that they are completely based on observations not models, repeat not models. #73 Rob it is not clear when the ice shelf formed. After more of it is lost a look at the sediments beneath it will identify that. If it is young the importance for global warming maybe reduced. However, I never mention global warming, I see the importance as observing the mechanisms of ice shelf collapse which we must understand.
15 June 2008 at 8:55 AM
Re #4, #10:
Assuming roughly 10′000 km^3 arctic sea ice loss during the past 20 years, about 15′000 metric tons are melting per second (averaged over the whole year), consuming 4.9 TW (equivalent to a radiative forcing of 10 mW/m^2) or about 1/3 of the world’s primary energy consumption. So it doesn’t even compensate for the heat emitted directly into the atmosphere by human activity. (The data from comment #20 translates to 2.2 TW for the 1993-2003 mean.)
The lost pieces of Wilkins Ice Shelf (including the recent break-up) have a volume of 120 km^3, assuming they were about 200 m thick. That would be 1/4 of the arctic sea ice loss during 1 year.
15 June 2008 at 9:10 AM
Actually, sea level has fallen over the past two years.
Data uncorrected for changes in average pressure.
http://sealevel.colorado.edu/current/sl_noib_ns_global.jpg
Data corrected with an inverted barometre.
http://sealevel.colorado.edu/current/sl_ib_ns_global.jpg
15 June 2008 at 9:26 AM
Is there a quantifiable lag time / memory factor involved in ice sheet melt? Global temperatures have not changed for almost a decade so it would seem odd that major changes in ice mass would be underway now unless it were an accumulated response to the previous warmer decades. So doesn’t the present decade presage a leveling off or decrease in melt?
Also, don’t AGW modes predict that the response at the poles follow rather than precede significant warming at the warmer latitudes? Given that net warming for the half-century is still in the range of half a degree and not accelerating, why would anyone expect any significant AGW response at the poles?
15 June 2008 at 11:15 AM
Re 67: What is your references to back of that claim?
“http://drs.nio.org/drs/handle/2264/646
Abstract: Mean-sea-level data from coastal tide gauges in the north Indian Ocean wereare used to show that low-frequency variability is consistent among the stations in the basin. Statistically significant trends obtained from records longer than 40 years yielded sea-level-rise estimates between 1.06–1.75 mm/ yrear-1 , with a regional average of 1.29 mm yr-1, when corrected for global isostatic adjustment (GIA) using model data, with a regional average of 1.29 mm-1.. These estimates are consistent with the 1–2 mm /year-1 global sea-level-rise estimates reported by the Intergovernmental Panel on Climate Change (Church et al., 2001). ”
Wunsch, C., Ponte, R.M. and Heimbach, P. 2007. Decadal trends in sea level patterns: 1993-2004. Journal of Climate 20: 5889-5911.
“a global mean of about 1.6 mm/year, or about 60% of the pure altimetric estimate, of which about 70% is from the addition of freshwater.”
“http://www.agu.org/pubs/crossref/2007/2006GL028492.shtml
Abstract
Nine long and nearly continuous sea level records were chosen from around the world to explore rates of change in sea level for 1904–2003. These records were found to capture the variability found in a larger number of stations over the last half century studied previously. Extending the sea level record back over the entire century suggests that the high variability in the rates of sea level change observed over the past 20 years were not particularly unusual. The rate of sea level change was found to be larger in the early part of last century (2.03 ± 0.35 mm/yr 1904–1953), in comparison with the latter part (1.45 ± 0.34 mm/yr 1954–2003). The highest decadal rate of rise occurred in the decade centred on 1980 (5.31 mm/yr) with the lowest rate of rise occurring in the decade centred on 1964 (−1.49 mm/yr). Over the entire century the mean rate of change was 1.74 ± 0.16 mm/yr. ”
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VF0-4GCX0FY-1&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=a545eb0c7b7a2e8ecccea0e4ed38425a
Abstract
In this paper we compare sea level trends observed at a few selected tide gauges of good quality records with thermosteric (i.e., due to ocean temperature change) sea level trends over 1950–1998 using different gridded ocean temperature data sets from Levitus et al. (2000) [Levitus, S., Stephens, C., Antonov, J.I., Boyer, T.P., 2000. Yearly and Year-Season Upper Ocean Temperature Anomaly Fields, 1948–1998. U.S. Gov. Printing Office, Washington, D.C. pp. 23.], Ishii et al. (2003) [Ishii, M., Kimoto, M., Kachi, M., 2003. Historical ocean subsurface temperature analysis with error estimates, Mon. Weather Rev., 131, 51–73.] and Levitus et al. (2005) [Levitus S., Antonov, J.I., Boyer, T.P., 2005. Warming of the world ocean, 1955–2003. Geophys. Res. Lett. 32, L02604. doi:10.1029/2004GL021592.]. When using the Levitus data, we observe very high thermosteric rates at sites located along the northeast coast of the US, north of 37°N. Such high rates are not observed with the Ishii data. Elsewhere, thermosteric rates agree reasonably well whatever the data set. Excluding the northeast US coastline sites north of 37°N, we compare tide gauge-based sea level trends with thermosteric trends and note that, in spite of a significant correlation, the latter are too small to explain the observed trends. After correcting for thermosteric sea level trends, residual (observed minus thermosteric) trends have an average value of 1.4 ± 0.5 mm/year, which should have an eustatic (i.e., due to ocean mass change) origin. This result supports the recent investigation by Miller and Douglas (2004) [Miller, L., Douglas, B.C., 2004. Mass and volume contributions to 20th century global sea level rise. Nature 428, 406–408.] which suggests that a dominant eustatic contribution is needed to explain the rate of sea level rise of the last decades observed by tide gauges, and shows that Cabanes et al. (2001) [Cabanes, C., Cazenave, A., Le Provost, C., 2001. Sea level rise during past 40 years determined from satellite and in situ observations. Science 294, 840–842.] arrived at an incorrect conclusion due to peculiarities in the gridded Levitus et al. (2000) [Levitus, S., Stephens, C., Antonov, J.I., and Boyer, T.P., 2000. Yearly and Year-Season Upper Ocean Temperature Anomaly Fields, 1948–1998. U.S. Gov. Printing Office, Washington, D.C. pp. 23.] data set.”
[Response: Final line edited out. No need for gratuitous and inflammatory name-calling. The information is fine and people should work it into their discussion of sea level rise. Thanks for that. –raypierre]
15 June 2008 at 11:27 AM
Regarding sea level rise,it’s important to remember that meltwater from floating sea ice and ice shelves don’t contribute to a rise. If you were to measure the surface level of a glass of water containing ice cubes before and after the cubes melt, the level remains the same.
In other words the diminishment of the extent of the Arctic sea ice and the calving of ice shelves connected to continents don’t cause a rise in sea level, only melting ice that comes off of the land will do to this.
15 June 2008 at 11:42 AM
Re 72:
I went to your link, seen it before. This is rather revealing:
http://www.cmar.csiro.au/sealevel/sl_hist_few_hundred.html
“We have used a combination of historical tide-gauge data and satellite-altimeter data to estimate global averaged sea level change from 1870 to 2004. During this period, global-averaged sea level rose almost 20 cm, with an average rate of rise of about 1.7 mm/yr over the 20th Century. The sea level record indicates a statistically significant increase in the rate of rise between 1870 to 2004.”
AGW claims the most increase was the past 50 years (since that is when the vast majority of FF use and human population growth occured), why would the change in sea level rise occur long before our CO2 emissions were significant? Also note the average. What is also interesting in that link is this:
“This data has shown a more-or-less steady increase in Global Mean Sea Level (GMSL) of around 3.3 ± 0.4 mm/year over that period. This is more than 50% larger than the average value over the 20th century. Whether or not this represent a further increase in the rate of sea level rise is not yet certain.”
This site is not linking any current small change to anything, including AGW. It is still a far cry from the rate needed by the alarmist positions. You need to at least double that rate just to make the lower end of the IPCC predictions.
Re 70: Ray, you have no clue who I am or what I understand, but instead of dealing with the evidence, you instead fall back on the default position of anyone who has their dogma questioned — ad hominem attack. Models are not theories. Models are imperfect attempts to represent what is known and used to make predictions. All the hype about the alarmist future of the world due to CO2 emissions is from predictions based on these models, there is no guarantee these predictions will come true, but the holders of the polemic of AGW say it certainly is the future. Those people are flat wrong, and the future will show that as the planet does not behave as the “models” expect. In fact, the AGW community had to scramble to adjust their models because of the current no-warming trend since 1998, that was NOT predicted by the models.
Thus I ask again, what will it take the planet to do to falsefy AGW theory?
[Response: Your claim about the “AGW community” having to “scramble” etc. shows such a disconnect between the actual situation in the modelling community — to say nothing of the actual “trend” data — and your perception of it, that I can see why nobody here is taking you seriously. There is a real issue on the table with regard to the extent to which decadal fluctuations in the rate of sea level rise can be used to evaluate and improve the models, and the issues are there both with regard to modelling and to the nature of the data. I don’t see that your tendentious spin on this is contributing much to the discussion –raypierre]
15 June 2008 at 11:58 AM
Mr. Pelto, thanks for the explanations. The second link in your comment #64, 14 June 2008, 0950, is the same as the first one to the Joughin and Bamber paper in GRL, about the Ronne-Filchner shelf. Should it not point to USGS data for the Ross shelf ?
sidd
15 June 2008 at 12:26 PM
Rob, how much of the CS Monitor article you cite did you actually read?
Did you get as far as these two paragraphs, a ways down the page?
—-excerpt—-
“In 1993, we predicted that this was going to be a vulnerable ice shelf,” says David Vaughan of the British Antarctic Survey. “But we got the time scales completely wrong. We were saying 30 years at that time, and now it’s happened within 15.”
Glaciologists are concerned about Antarctica’s ice shelves because most of them represent brakes of solid ice that slow the glaciers’ flow to the sea. Without those brakes, the glaciers would surge, calve into icebergs, and significantly raise the sea level.
—–enc excerpt—-
You read that, then ask “what’s the big deal?” — how?
15 June 2008 at 12:48 PM
[54] - There are suggestions that the Atlantic Meridional Overturning Circulation is in a strong phase. This is sometimes called the Atlantic Multi-Decadal Oscillation, which has been seen in long thousand year model runs, but the observations aren’t long enough to pin down its existence in the actual oceans.
That would explain the north Atlantic warming faster than the south Atlantic.
Also, some areas will warm faster than others simply due to noise. What is definitely not the case is that the northern hemisphere is warming up because the southern hemisphere is cooling down - ie a shift of heat from one to the other.
However, the medieval warm period and little ice age look stronger [to my eye] in northern hemisphere only reconstructions than in global reconstructions, suggesting that could be a contributory factor explaining their prominence in the historical record.
15 June 2008 at 12:54 PM
Re Rob’s “what’s the big deal” question in
73: If the breakup of the Wilkins Ice Shelf were an isolated incident, we might be able to dismiss it.
The reality is that the entire cryosphere is melting at an accelerating rate. The story Rob posted clearly puts the Wilkins breakup into context:
15 June 2008 at 1:20 PM
Rob Huber (73) — According to orbital forcing theory, the globe should continue to slowly cool towards the next attempt at a stade (massive ice sheets) in about 20,000 more years. Instead, there is ample evidence that the warming in past 100+ years has undone the previous 7000 years or so of natural cooling. If Wilkins ice shelf is indeed relatively modern, this just supplied more evidence for that.
Here is some of the other evidence.
90–7000 years ago:
http://www.npr.org/templates/story/story.php?storyId=914542
http://www.physorg.com/news112982907.html
5200 years ago:
http://researchnews.osu.edu/archive/quelcoro.htm
http://news.softpedia.com/news/Fast-Melting-Glaciers-Expose-7-000-Years-Old-Fossil-Forest-69719.shtm
http://en.wikipedia.org/wiki/%C3%96tzi_the_Iceman
15 June 2008 at 4:39 PM
Richard Wakefield writes:
> you have no clue about who I am or what I understand
Without IP numbers no one can say one ‘Richard Wakefield’ is the same as another, but many by that name from Ontario post much alike, e.g.
http://tierneylab.blogs.nytimes.com/2008/01/10/a-spot-check-of-global-warming/index.html?hp#comment-76752
http://www.creditablecarbon.com/forum/viewtopic.php?p=22&sid=3943f9f06969a57085c7a06f5db641b6
15 June 2008 at 5:10 PM
Re Comments 80 and 82- Richard, there’s a Biblical expression, I believe, that states “Seek and Ye shall find.” Wan’t some evidence of the consistency of models with what’s really happening in the world? Then check this contributory post given(by Gavin) about James Hansens projections two decades ago and the match between observations and the model projections:
http://www.realclimate.org/index.php/archives/2007/05/hansens-1988-projections/langswitch_lang/ra#more-447
There have been other tests of the validation of model consistency by running models forward from the past,which sucessfully reproduce the present day climate.and and the incorporation of Mt. Pinatubo into models, and the models,ensembles of them, have performed successfully in describing the global impact of this event, regarding temporary cooling.The models are also able to accurately show the vertical temperature changes in the atmosphere.
If you look, you’ll find ample evidence of the validation of climate models.
15 June 2008 at 5:16 PM
On sea level rise, Richard likes to cite: Berge-Nguyen, M., A. Cazenave, A. Lombard, W. Llovel, J. Viarre, and J.F. Cretaux. 2008. Reconstruction of past decades sea level using thermosteric sea level, tide gauge, satellite altimetry and ocean reanalysis data. Global and Planetary Change Vol. 62, No 1-2, pp. 1–13, May 2008
The full paper is available here (if you have a subscription):
http://www.science-direct.com/science?_ob=ArticleURL&_udi=B6VF0-4RC2NPR-1&_user=10&_coverDate=05%2F31%2F2008&_rdoc=1&_fmt=high&_orig=browse&_srch=doc-info(%23toc%235996%232008%23999379998%23687592%23FLA%23display%23Volume)&_cdi=5996&_sort=d&_docanchor=&_ct=13&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=2116d200a8f96f26d3a37d50865c3061
From the abstract:
“reconstructed spatial trends over 1993–2003 agree well with the regional sea level trends observed by Topex/Poseidon.”
The links provided to the Topex/Poseidon data clearly show a sea level rise of 3.3 mm/year, almost double the rate from the 20th century.
Here are a few other articles on sea level rise during the satellite era:
http://www.sciencemag.org/cgi/content/abstract/294/5543/840?tdate=11%2F30%2F2001&HITS=10&hits=10&fdate=10%2F1%2F2000&stored_search=&maxtoshow=&FIRSTINDEX=0&titleabstract=Sea+Level&searchid=1007046006605_34779&RESULTFORMAT=
“Sea Level Rise During Past 40 Years Determined from Satellite and in Situ Observations
Cecile Cabanes, Anny Cazenave, Christian Le Provost
The 3.2 ± 0.2 millimeter per year global mean sea level rise observed by the Topex/Poseidon satellite over 1993-98 is fully explained by thermal expansion of the oceans. For the period 1955-96, sea level rise derived from tide gauge data agrees well with thermal expansion computed at the same locations. However, we find that subsampling the thermosteric sea level at usual tide gauge positions leads to a thermosteric sea level rise twice as large as the “true” global mean. As a possible consequence, the 20th century sea level rise estimated from tide gauge records may have been overestimated.” Science 26 October 2001
“A 20th century acceleration in global sea-level rise
John A. Church1,2 and Neil J. White1,2
Received 6 October 2005; revised 22 November 2005; accepted 1 December 2005; published 6 January 2006.
[1] Multi-century sea-level records and climate models
indicate an acceleration of sea-level rise, but no 20th
century acceleration has previously been detected. A
reconstruction of global sea level using tide-gauge data
from 1950 to 2000 indicates a larger rate of rise after 1993
and other periods of rapid sea-level rise but no significant
acceleration over this period. Here, we extend the
reconstruction of global mean sea level back to 1870 and
find a sea-level rise from January 1870 to December 2004
of 195 mm, a 20th century rate of sea-level rise of 1.7 ±
0.3 mm yr1 and a significant acceleration of sea-level rise
of 0.013 ± 0.006 mm yr2. This acceleration is an important
confirmation of climate change simulations which show an
acceleration not previously observed. If this acceleration
remained constant then the 1990 to 2100 rise would range
from 280 to 340 mm, consistent with projections in the
IPCC TAR. Citation: Church, J. A., and N. J. White (2006), A
20th century acceleration in global sea-level rise, Geophys. Res.
Lett., 33, L01602, doi:10.1029/2005GL024826.”
“Evidence for enhanced coastal sea level rise during the 1990s
S. J. Holgate and P. L. Woodworth
Proudman Oceanographic Laboratory, Bidston, UK
Received 2 February 2004; accepted 11 March 2004; published 9 April 2004.
[1] Sea level rise over the last 55 years is estimated to have
been 1.7 ± 0.2 mm yr1, based upon 177 tide gauges divided
into 13 regions with near global coverage and using a Glacial
Isostatic Adjustment (GIA) model to correct for land
movements. We present evidence from altimeter data that
the rate of sea level rise around the global coastline was
significantly in excess of the global average over the period
1993–2002.We also show that the globally-averaged rate of
coastal sea level rise for the decade centered on 1955 was
significantly larger than any other decade during the past
55 years. In some models of sea level rise, enhanced coastal
rise is a pre-cursor of global average rise. It remains to be seen
whether the models are correct and whether global-average
rates in the future reflect the high rates of coastal rise
observed during the 1990s. INDEX TERMS: 1635 Global
Change: Oceans (4203); 1223 Geodesy and Gravity: Ocean/
Earth/atmosphere interactions (3339); 4215 Oceanography:
General: Climate and interannual variability (3309); 4556
Oceanography: Physical: Sea level variations. Citation: Holgate,
S. J., and P. L. Woodworth (2004), Evidence for enhanced coastal
sea level rise during the 1990s, Geophys. Res. Lett., 31, L07305,
doi:10.1029/2004GL019626.
1. Introduction
[2] Global sea level is believed to have risen at a rate
of 1–2 mm yr1 during the past 100 years, based on
evidence from the sparse global tide gauge data set [Church
et al., 2001]. On the other hand, analysis of near-global,
precise radar altimetry has suggested a rate of rise nearer to
3 mm yr1 for the past decade [Cabanes et al., 2001; Nerem
and Mitchum, 2002; Leuliette et al., 2004]. At face value,
this suggests a recent acceleration of the global sea level
secular trend.”
15 June 2008 at 5:34 PM
Richard Wakefield,
the reference for my numbers is directly from IPCC AR4 Chapter 5. I’d also add that the 3+ mm/yr is specifically for 1993-2003, which is distinct from “second half of the 20th century” in some of your papers.
You have also not answered my question on why your sea level argument is constricted to the anthropogenic component in climate change. Does increasing the sun or decreasing cosmic rays or increasing martian galactic beams not raise temperature which raise sea levels? There are plenty of other metrics (instrumental record, glacier mass balance, etc) that demonstrate the globe is warming, and any causal agent which results in ice melt (which we know is happening) or thermal expansion should theoretically result in eustatic sea level rise.
As for your insistence that models are the only thing we have to assess the future of climate change, you seem to be ignoring the radiative physics which extends back over a century and the quantification of 2x CO2 beginning with Arrhenius (which GCM did he have?) as well as the paleoclimatic record which unequivocally demonstrates the role of greenhouse gases in planetary climate.
Of more importance than this quibbling, is the rapid loss of arctic sea ice, and breakup of various parts in Antarctica (Larson B, and now Wilkins) and as for sea-level contributing ice, the potential implications for eustatic sea level rise which may be greater than AR4 estimates, see
Glaciers Dominate Eustatic Sea-Level Rise in the 21st Century, Mark F. Meier, Mark B. Dyurgerov, Ursula K. Rick, Shad O’Neel, W. Tad Pfeffer, Robert S. Anderson, Suzanne P. Anderson, and Andrey F. Glazovsky (24 August 2007), Science 317 (5841), 1064. [DOI: 10.1126/science.1143906]
16 June 2008 at 6:20 AM
George Tobin writes:
Google “ice-albedo feedback”
16 June 2008 at 6:24 AM
Richard Wakefield posts:
No, they are not. Global warming theory long predates computer models of its effects. Neither Arrhenius in 1896 nor Challenger in 1938 used computer models. From the increase in temperature expected from doubling CO2 and paleoclimate data about such changes in the past, the “alarmist future of the world” is logically implied.
16 June 2008 at 7:04 AM
Wakefield is great bait. #80 asks a good question about melt rates. What we are observing in terms of breakup is both long term dynamics from progressive thinning due to long term warming, and short term melt. We have continued to see examples of very high melt in regions such as sw Greenland during the summer of 2007 and in regions of the Antarctic Peninsula. The hazard is that glacier dynamics pay attention to the long term, surface melt today is not that important to their current behavior. Hence, one a collapse begins, it is hard to slow down even if there is not additional warming.
16 June 2008 at 11:09 AM
Has anyone published an estimate of what the rate of sea level rise would look like if there were not so much precipitation over Antarctica?
16 June 2008 at 2:40 PM
An interesting thread. However, it seems to this observer that most, if not all, of the discussion is based on an assumption that increased break-up of the iceshelf is due to “warming”.
However, it seems to me that the process could be much more complicated than that. For example, I can see scenarios where increased precipitation in the Anarctic creates increased snow loads, which causes the glaciation process to accelerate with more ice being delivered onto the surrounding oceans per unit time.
Ice floating on seawater (by definition above the melting point of ice) experiences mechanical stresses leading to breakup. It may not necessarily be due to a simple conflation of warming = increased ice break-up.
I am surprised in a whole thread that no one seems to consider these mechanical effects, although the lead post hints at it. Are they not a factor?
16 June 2008 at 3:53 PM
Fair and Balanced, I don’t think your theory passes muster. First, we wouldn’t have increased precipitation if we didn’t have warming and more evaporation. Second, the precipitation is mostly taking place far inland, and so is exerting little direct pressure on the ice shelf.
16 June 2008 at 5:06 PM
Fair and Balanced, if the issue were increased glacial flow due to increased snowfall, Why would the leading edge of the ice sheets be retreating rather than simply melting faster to stay in place (if not advancing)?
16 June 2008 at 11:34 PM
#96 Tenney,
Davis et al (2005) in Science suggest that Antarctic gain from 1992 to 2003 may have slowed sea level rise by roughly 0.12 mm/yr. This is roughly the same time period (1993-2003) that IPCC says increased 3.1 mm/yr.
17 June 2008 at 1:15 AM
Re 97-99, please see
Thomas, Elisabeth R., Gareth J. Marshall, and Joseph R. McConnell, 2008. A doubling in snow accumulation in the western Antarctic Peninsula since 1850. Geophys. Res. Lett., 35, L01706, doi:10.1029/2007GL032529, January 12, 2008
Abstract
We present results from a new medium depth (136 metres) ice core drilled in a high accumulation site (73.59°S, 70.36°W) on the south-western Antarctic Peninsula during 2007. The Gomez record reveals a doubling of accumulation since the 1850s, from a decadal average of 0.49 mweq y−1 in 1855–1864 to 1.10 mweq y−1 in 1997–2006, with acceleration in recent decades. Comparison with published accumulation records indicates that this rapid increase is the largest observed across the region. Evaluation of the relationships between Gomez accumulation and the primary modes of atmospheric circulation variability reveals a strong, temporally stable and positive relationship with the Southern Annular Mode (SAM). Furthermore, the SAM is demonstrated to be a primary factor in governing decadal variability of accumulation at the core site (r = 0.66). The association between Gomez accumulation and ENSO is complex: while sometimes statistically significant, the relationship is not temporally stable. Thus, at decadal scales we can utilise the Gomez accumulation as a suitable proxy for SAM variability but not for ENSO.
Further, the IPCC estimates “Current global model studies project that the Antarctic ice sheet will remain too cold for widespread surface melting and is expected to gain in mass due to increased snowfall.”