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.
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?
The Wilkins continues to break up.
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?
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.
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.
Steve Salmony — nice editorial letter. Thank you for posting.
nice animation from esa at
http://www.esa.int/esaEO/SEMG58VG3HF_planet_0.html
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.”
https://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.
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
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?
# 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
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?
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?
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.
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
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.
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.
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’.
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?
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?
“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.
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
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?
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]
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]
Richard Wakefield writes:
Wherever you’re getting your information from is wrong. Sea level rise is now up to 3.3 mm/yr.
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.
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.
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
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?
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]
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.
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]
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
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?
[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.
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:
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
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
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:
https://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.
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.”
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]
George Tobin writes:
Google “ice-albedo feedback”
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.
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.
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?
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?
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.
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)?
#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.