Antarctic ice – more accurate estimates

Guest post by Verity Jones @ Digging In The Clay

Cracking ice shelves make headlines, but ice loss estimates that are revised downwards don’t.  While there is great hand wringing over coastal ice loss in Greenland and the West Antarctic Peninsula, East Antarctica has more than eight times the ice mass of either.

East Antarctica lies to the right of the Transantarctic Mountains which cut the continent in half at its 'waist' between the two largest ice shelves. (Location Map from T. Scambos and J. Bohlander. "Images of Antarctic Ice Shelves". National Snow and Ice Data Center: http://nsidc.org/data/iceshelves_images/index_modis.html)

Last week’s Science magazine had a News Focus article on estimates of ice loss in Antarctica. It quietly discussed a paper published in May by two NASA scientists:
H. Jay Zwally & Mario B. Giovinetto (2011) Overview and Assessment of Antarctic Ice-Sheet Mass Balance Estimates: 1992– 2009. Surv Geophys DOI 10.1007/s10712-011-9123-5  (note this is Open Access)

Estimates of Antarctic ice net variation vary widely.  This is in part due to the different methods used, but the magnitude of the change might surprise you.

“Mass balance estimates for the Antarctic Ice Sheet (AIS) in the 2007 report by the Intergovernmental Panel on Climate Change and in more recent reports lie between approximately +50 to -250 Gt/year for 1992 to 2009. The 300 Gt/year range is approximately 15% of the annual mass input and 0.8 mm/year Sea Level Equivalent (SLE).”

The paper set out to investigate the various estimates, assessing previously published results that used three standard methods which the Science article conveniently summarises:

“Each of the three methods has its foibles.

  • In the first, altimetry, researchers bounce laser or radar signals off the ice to measure its height and thus its volume. The method has been used to survey most of the continent, but converting changes in volume to changes in mass raises major uncertainties.
  • The second technique, gravity, employs the two satellites of the Gravity Recovery and Climate Experiment flying over the ice in tight formation to measure mass directly. But that record goes back to only 2002, and data analysis is tricky.
  • Finally, the input-minus-output method (IOM) works by subtracting ice flow into the sea from total snowfall. Both numbers are huge, however, and the mass of snow falling on East Antarctica is especially hard to gauge.”

Zwally and Giovinetto’s reassessment also included a challenge to some assumptions, substituting field measurements and making ‘preferred estimates’. These took account of the uncertainties inherent in the various techniques. Their reanalysis provides much lower estimates of net change in ice, ranging from +27 to -40 billion tons per year.  For 1992 – 2001 they are prepared to go even further, estimating a loss of only 31 billion tons per year.  These still sound like huge numbers, but to put it in perspective, 2400 billion tons of snow falls in Antarctica each year, so we’re dealing with a gain or loss in the range +1.1 to -1.7%.

The same techniques applied by the authors in a previous paper (Zwally HJ et al (2011) Greenland ice sheet mass balance: distribution of increased mass loss with climate warming. J Glaciol 57(201):88–102) brought a significant convergence to estimates produced by ICESat altimetry and the GRACE gravity signal in Greenland, however, while the Greenland ice sheet continues to grow inland and thin at the margins, overall recently it has been losing mass.

What I find most refreshing is the revision and quantification of uncertainty. What is shocking is not the magnitude of the possible loss, but the short timescale on which the estimates are based and the lack of knowledge of historical data.

“The new analysis is a “perfectly reasonable reinterpretation,” glaciologist Andrew Shepherd of the University of Leeds, U.K., says. “The paper’s main contribution is a very convincing argument that one needs to account for uncertainties in a consistent way.” “

The Science article ends by hinting that this may make no more than tiny ripples in the consensus.

“Getting more than a feeling for what Antarctic ice is doing to sea level will take more than one group’s reassessment of the published literature, researchers agree. The Intergovernmental Panel on Climate Change (IPCC) is already working on an ice balance assessment for its report due in mid-September 2013, but researchers say more must be done to focus scientists’ attention on the problem.”

Does that sound like a dismissal? It does to me.

About these ads

35 thoughts on “Antarctic ice – more accurate estimates

  1. “What is shocking is not the magnitude of the possible loss, but the short timescale on which the estimates are based and the lack of knowledge of historical data.”

    Excellent article. Changes in Arctic ice cover are routine, natural, and have been going on for many thousands of years. Estimating and projecting trends from 1979 is simplistic, and leads to erroneous conclusions.

  2. Since an ice shelf is already floating, breaking off of a piece does nothing to change sea level. Ice that is in an ice shelf is, or should already be considered, “lost” ice. In fact, an expanding ice shelf will raise sea levels, a contracting ice shelf won’t change sea levels at all.

  3. Note to Author – missing word:

    “Estimates of Antarctic ice net variation vary widely. This is in part due to the different methods used, but the magnitude of the might surprise you.”

    …magnitude of the — might surprise you.

    [Word inserted. Not sure if it's correct or not. ~dbs, mod.]

  4. So can we afford to shelve the plans to spend Trillions on “decarbonising” for a couple of years, until the science is REALLY settled?

    Just askin’?

  5. “Getting more than a feeling for what Antarctic ice is doing to sea level will take more than one group’s reassessment of the published literature, researchers agree. The Intergovernmental Panel on Climate Change (IPCC) is already working on an ice balance assessment for its report due in mid-September 2013, but researchers say more must be done to focus scientists’ attention on the problem.”

    Anyone else suspect that if the slant of the conclusions had been a bit different this paper would be being ballyhooed as the new canonical work in the field?

    • Dave Wendt says:
      July 27, 2011 at 11:10 pm

      Anyone else suspect that if the slant of the conclusions had been a bit different this paper would be being ballyhooed as the new canonical work in the field?

      The conclusion you quote is from the Science article covering the original paper. The paper itself (hit the DOI link for the PDF – it’s free) has unbiased conclusions – just as we should expect from scientists!

  6. So, as with most climate-related issues, we don’t really know what is happening. Doom-mongers please note, and pass it on to the politicians.

  7. “…for its report due in mid-September 2013, but researchers say more must be done to focus scientists’ attention on the problem.”

    Who are these unnamed researchers?

    How many of them are there?

    Do they think the ‘science is settled’?

    Are doubts creeping in?

    Maybe they read WUWT…

  8. The Andrew Shepherd quoted above is the one who attracted some ridicule last year when his University published a press release headed “Melting icebergs causing sea level rise” which turned out to be “equivalent to 49 micrometers per year spread across the global oceans”.

  9. “What is shocking is not the magnitude of the possible loss, but the short timescale on which the estimates are based and the lack of knowledge of historical data.”

    I posted something along these very exact lines at Non-Skeptical Science. If the current presumed rate of loss is extrapolated back across the previous century, a century of warming, it should be the case that much more ice existed back then than now. Given that submarines were poking their heads through the arctic in the middle of the century I’d expect at least anecdotal observations to support such a view.

  10. “The Science article ends by hinting that this may make no more than tiny ripples in the consensus.” No surprise there then, somebody probably told them not to “interpret” their findings in too greater detail:-)sarc off.

    V interseting article & hones in on many “uncertaintiy” issues. One of the classics is the hackenyed expression “if it continues at this rate by 20…………whenever it will all be gone”. As to headlines to sell papers, which would many of us jump at……”Man of Cloth in teenage temptress scandal!!!!!” or “Vicar opens Girls High School Fete”? The former drags you in, the latter turns you off, to hell with accuracy & detail. I’m in the wrong job, I should have been a journalist, it’s so easy!

  11. It sounds like a non problem to me. Based on so many estimates and measurements of variables that are difficult to quantify, another estimate, pushes the whole thing into the realms of fantacy.

    So an interesting exercise but the end product could use some refinement.

    Since sea levels are fairly stable at the moment why worry?

  12. What would be useful with either Arctic or Antarctic ice shelves is the maximum age of such ice? It would give an indication of the rate of turnover or when the last time the ice shelves broke up. Perhaps ice shelves have a ‘shelf life’ (apologies for pun) and after many years of flexing with tides will naturally break up and then start the whole process of reforming. After all they are not going to be thousands of years old. However, there are also other ‘disturbances’ to the structural integrity of these fields, such as the activities around studying them: driving on them; landing planes on them; etc.

  13. Verity Jones says:
    July 28, 2011 at 12:16 am
    Dave Wendt says:
    July 27, 2011 at 11:10 pm

    Anyone else suspect that if the slant of the conclusions had been a bit different this paper would be being ballyhooed as the new canonical work in the field?

    The conclusion you quote is from the Science article covering the original paper. The paper itself (hit the DOI link for the PDF – it’s free) has unbiased conclusions – just as we should expect from scientists!

    Slant was a poor choice of words. I meant to indicate that if the folks at Science had found the results more aligned with their position, there great reticence about it being merely “one group’s reassessment of the published literature” would likely be entirely reversed.

  14. The big reduction in the estimates from IOM methods from -136Gt to just 13Gt is the result of replacing some of the field data with estimates from computer models.

    Some here have pointed out the ‘lukewarm’ response this seems to get from the ‘Team’.
    But if it had shown results favoring a BIGGER mass loss I suspect response here would have been positively frigid – and the issue of model use highlighted!

  15. Dave Wendt says:
    July 28, 2011 at 3:14 am
    “Slant was a poor choice of words. I meant to indicate…”
    I realised what you meant (and agree) – I just wanted to make it clear for others. There was also the danger of a troll flinging mud along the lines of “if you had bothered to read the actual paper…” which I was keen to circumvent.

    tallbloke
    They also quoted Richard Alley and a couple of others, but none of the quotes they printed support that conclusion.

    It really is poor for a publication like Science to resort to this kind of behaviour. I would have thought an appropriate conclusion would have been upbeat – how this kind of striving for increased accuracy is laudable in a field where there is so much uncertainty.

  16. I met Jay Zwally in the late 1990s; he had fairly close ties to Al Gore at the time, and as a younger scientist then, seemed to me to be sort of ga-ga about how intimately involved Gore was in the science of global warming. My impression was that, at the time, he was completely into both Gore the politician and global warming the cause.

    So it is quite nice for me to see that Zwally’s desire to do accurate science appears to trump whatever political instincts he might, 13 years after I met him, still harbor.

  17. I have to agree to disagree that should I agree I would be disagreeing with the agreement with which no disagreement could be agreed which then no further agreement could my disagreements ever agree.

    How long until a carbon distrator takes that +1.1/-1.7% net accumlation and applies it to gross mass?

  18. From the conclusions of the paper…
    “Considering the state-of-art for determining the input and output fluxes, it is difficult to see
    how the IOM can achieve the required accuracy even for a snapshot in time. Furthermore,
    the problems with the IOM are even more difficult for determining trends.”
    Doesn’t sound like settled science to me. Some skeptic science happening though and they are hopeful that improvements in GRACE estimates will help with the uncertainty.

  19. izen says:
    July 28, 2011 at 5:05 am

    The big reduction in the estimates from IOM methods from -136Gt to just 13Gt is the result of replacing some of the field data with estimates from computer models.

    Some here have pointed out the ‘lukewarm’ response this seems to get from the ‘Team’.
    But if it had shown results favoring a BIGGER mass loss I suspect response here would have been positively frigid – and the issue of model use highlighted!

    The difference being the Team roots for their CAGW ideology, while climate skeptics/realists root for science.

  20. Annual precipitation over the planet’s surface is estimated at a little over 1000 mm, while the sea-level rise is 2 – 3 mm/yr (depending on your warmist/skeptic data source). Some studies have indicated a drop in atmospheric humidity, but I’ve never seen the volume of this drop quantified.
    At the same time specific sea-level contributions are given for alpine glaciers (0.25? mm/yr).

    How much moisture does the atmosphere hold, and so how could this influence sea-levels, I wonder.

  21. YOU: “Goshy, them gorebal wammists are dumb bumbs. Ha ha. *I* said that. Me! It was me said it. We mo’ better now. Look mommy! Mommy? Uh…hey! I’m here. How ’bout this? Gah! Oops! I fall down.”

    RESULT: The thundering machines sputtered and stopped. A firestorm of fear. Men began to feed on men.

  22. “The Intergovernmental Panel on Climate Change (IPCC) is already working on an ice balance assessment for its report due in mid-September 2013, but researchers say more must be done to focus scientists’ attention on the problem.”
    Yes, they need to get their crack “research” teams from GreenPeace on it pronto.

  23. charles nelson says:
    July 28, 2011 at 12:55 am
    Has anyone heard of the Piri Reis Map?

    Yes, why do you ask? Rhetorically?

    Long ago I read a book by a retired Navy captain who took the map literally. He couldn’t read the Turkish which described a warm southern coast which the author took for ice. The (15th century)mapmaker repeated a section of the Atlantic coast of South America which he had apparently acquired from different sources and not understood that they were duplicates. This helped extend it to the southern terra incognita, which is to say, the map isn’t very reliable even in the Turkish–it predates the data obtained by Magellan.

    But sailors in 1900 explored southern oceans that had been mapped as land only a hundred years before. So yes, the ice has been receding for a long time. –AGF

  24. What say you RGates? After all, you are a self proclaimed expert with supposedly expansive knowledge of this topic, who is far superior to us plebes.

  25. I dont know what the stress is with ice loss in West Antarctic with the discovery in 2005 of a very large underwater volcano that managed to spring up in a very short time frame. Which means lots of hot la[]va.

  26. Smokey says:
    July 28, 2011 at 9:25 am
    Piri Reis link:

    http://www.world-mysteries.com/sar_1.htm

    At least one boat has been found preserved in peat, 8 thousand years old that contains a mast step. The reason why we think humans have only been in North America for 20 thousand years is due to circular reasoning. They can’t exist therefore we don’t look for them. In reality, sea level rise over the past 20 thousand years has wiped out most evidence of human settlements.

    The simplest means to move heavy objects is by water. Load something on a floating log when the wind is blowing in the right direction and you have invented a sail boat. From then on it is simply a matter of making it more efficient.

    If the log is a tree that has washed out to sea by a storm swollen river, all the better. If it is a palm tree the nuts will keep you alive for months until you reach land. In this fashion early humans could well have crossed oceans.

    In the tropics it is not unusual to see floating islands, where whole sections of jungle have washed down a river out to sea. The vines and roots tie everything together. Fish almost immediately start to colonize the underside of the island, making for a ready food supply.

    6 months of the year in the tropics rain is plentiful. With the prevailing winds and currents, you travel 30 miles per day without a sail, no matter how inefficient your raft. That is 1000 miles per month. Few humans on foot can match this pace. When conditions are right, It is enough to cross oceans. With a sail you can do even better.

    The monsoon in the Indian ocean is perhaps the greatest sailing route on earth. Half the year the wind blows from the Solomon Islands to Egypt. The other half the year it blows from Egypt to the Solomons. One third of the distance around the globe. A log with a branch sticking up to catch the wind can make the trip.

  27. Brian says:
    July 30, 2011 at 1:33 pm
    It looks like 2011 could be headed for a record Artic melt:

    http://news.yahoo.com/2011-headed-record-arctic-melt-214206330.html

    The fact that CT’s Greenland Sea sub region graph is the only one of fourteen that has been showing a positive anomaly for more than a month now, suggests to me that whatever may be occurring with regard to Arctic Sea Ice is likely based once again on factors not related to AGW.

    http://arctic.atmos.uiuc.edu/cryosphere/IMAGES/recent365.anom.region.5.html

    BTW, I just scanned the graphs on the Sea Ice Reference Page and it would appear that most are showing this year’s decline curve flattening in relation to 2007, with some significantly above 2007 at this point. As the old song goes I guess we’ll “see you in September”.

  28. I am poor at html and posting graphics, so – regrettably – I must apologize in advance, but will let others more skilled apply their talents and post the original photos, comparing today’s images of the same ice shelf with what was claimed to be a disastrous collapse only 9 years ago.

    Antarctic ice, 2002, “massive ice island” breaks off. But now?

    Source pdf: http://www.iac.ethz.ch/education/bachelor/climate_systems/notizen/The-Cryosphere.pdf
    See Pages 17 and 18, Larsen Ice Shelf Collapse:

    “When Larsen B collapsed, a shelf area of about 3,250
    km2 and 720 billion tons of ice disintegrated in a 35-day period
    beginning on 31 January 2002″

Comments are closed.