Greenland and Antarctic ice sheet decay, continued

In a previous post on the worrying pace of Greenland and Antarctic melting, I mentioned that Greenland’s melt rate had been the subject of a bit of discussion. There was evidence of rapid melting from Greenland’s southeast glaciers from the late 90s to the mid 2000s, but this “galloping” (as Kerr put it) pace seemed to have ended in 2005. Of course, the denialists jumped all over this apparent reprieve, using it to snipe at Al Gore and imply that further mention of accelerated Greenland melting was naught but greenie scaremongering.

As noted in the prior post, Pritchard et al. found that while this rapid acceleration did seem to peak around 2005 in the southeast, elsewhere in Greenland melt rates continued to accelerate. These findings, along with similar from Antarctica, have been given further support from GRACE (Gravity Recovery And Climate Experiment) satellite data, as published by Isabella Velicogna in a new GRL study: Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE.

GRACE data from Greenland and Antarctic ice mass balance from April 2002 through February 2009 confirm that not only has significant melting continued on both ice sheets, their respective rates are actually best fit by quadratic rather than linear trends- indicating that the melting of each is continuing to accelerate.

Those concerned about rapid ice sheet decay would do well to avoid the embarrassment of the denialists, and refrain from senselessly extrapolating such a short trend into the future- although there is certainly no harm in mocking how swiftly their talking points are continually refuted by reality. Likewise, those crowing about the minor “recovery” [still the third largest melt recorded] of Arctic sea ice should probably keep this paper in mind as well.

Snark aside, the results of the GRACE survey are troubling. Unfortunately- contra the denialists and to paraphrase Samuel Clemens- reports of the demise of accelerating melt have been greatly exaggerated. Velicogna finds that after applying a seasonal filter and a quadratic fit, the rate of Greenland ice loss doubled during the observed period while the rate in Antarctica increased by roughly 140%. This is not a matter of academic interest or rhetorical point scoring- such dynamic melting is precisely the kind of behavior that paleoclimatic data imply for the ice sheets, and why sea level rise will assuredly be much worse than what the base AR4 numbers reflect.

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14 responses to “Greenland and Antarctic ice sheet decay, continued

  1. I saw this post on WUWT citing a study that Antarctic snow melt (misquoted in the post as “ice melt”) reached a 30 year minimum in 2008-09.

    http://wattsupwiththat.com/2009/10/08/antarcticas-ice-story-has-been-put-on-ice/

    Obviously one measurement from one part of the world doesn’t disprove global warming, but I was hoping this layman could get some enlightenment.

    First, the abstract of the study (the full text is behind a pay wall) seems to say that declining snow melt in the last few years is due to an unusual synchronization between ENSO and Southern Hemisphere Annular Mode (SAM). I’d interpret that as meaning this is an example of short-term effects temporarily outweighing long-term trends. Is this accurate?

    Second, could declining recorded snow melt be due to increased precipitation due to warmer waters? (e.g. could an increase in melting snow be masked by an increase in falling snow? Not sure how this is measured).

    Third, is there a difference between snow melt and ice melt? Which is more important for AGW?

    Last, does any of that actually matter? If it turned out that less snow WAS melting in the Antarctic, what conclusions would you be able to draw about AGW?

    Thanks!

    • 1. Yes, but it’s more complicated than just the temporary synchronization of two naturally-occurring phenomena “outweighing” anthropogenic warming. The positive SAM is likely also at least partially anthropogenic in nature- driven by ozone depletion (and to a lesser extent GHGs). As ozone levels recover we expect to see increased snowmelt resume as the positive SAM subsides.
      2. Certainly plausible, but I don’t know of any studies explicitly trying to quantify the relative contributions of the two, or even that you’d need to invoke such to explain the current observations. That’s an excellent question for Bob Grumbine.
      3. The snowmelt index is defined as the number of days of melt multiplied by the extent of melt (duration times area) rather than an actual “amount” of melting. The latter would be determined by Antarctica’s total mass balance over time. Obviously, the two can be related (and I believe are mostly so on the coasts and would be in the interior if not for confounding factors like ozone-depletion- but don’t quote me on that), but we’re much more concerned with how much ice sheet decay contributes to sea level rise, and of course the more dire possibility of ice sheet collapse.
      4. The study in question does not contradict what we know about Antarctica, its response to anthropogenic perturbations, or what we expect of it in the future. Antarctica is not only continuing to contribute to sea level rise, its contribution- as noted in this post- has dramatically accelerated over this same period. In short, no. Conflating the findings of that paper with sea level rise is completely dishonest and typical denialist nonsense.

      Don’t take my word for it, read the paper itself. And to anyone else reading this- for heaven’s sake, don’t take anything you read at WUWT or World Climate Report at face value.

  2. Thanks for the info. Much appreciated. And you don’t have to worry about me taking anything I read at WUWT at face value :)

  3. It seems that we were overestimating the gravity signal from the rock in Antarctica, and, although still losing significant amounts of ice, the loss may be slightly slower than estimated. “WAGN researchers do not yet know how large the overestimation was. A more definitive correction will be conducted by other researchers who specialize in interpreting GRACE data”:

    The press
    The paper

    Not a great impact I suppose, but still interesting. Cheers.

    • We’ll see how it affects the total estimates of ice loss (and thus relative contribution to SLR), but there are already wide error bars associated with these, so I doubt it will make much of a difference if their conclusions are accurate. Additionally, in terms of the most concerning aspect of this post- i.e. the accelerating rate of change- the alleged biases are irrelevant as the authors themselves acknowledge:

      However, any sudden increase in the rate of ice loss will be resolved unambiguously by GRACE since the mass rates associated with PGR do not change significantly over several years.

      I’ll look through the methodology of the Veliconga paper and see if/how its other conclusions might be affected.

  4. I hope this isn’t silly, but I’m trying to understand the meaning of the vertical axis on the graphs above-which have both positive and negative values for ice mass. How do I read what this is telling us about mass change from year to year?

  5. Marco Tedesco was interviewed the 2.nd of May 2009 about snow and ice melting on Greenland and in Antarctirca, see http://www.abc.net.au/rn/scienceshow/stories/2009/2558510.htm.

    He was asked about the connection between this melting and climate change / sea level rise. He said: ”Well, for Antarctica we are navigating very dangerous waters I think because the trends that we get from Antarctica is not what we call statistically significant, which means that whatever number you’re getting in terms of increasing or decreasing melting at most of the places in Antarctica, if you add one year or subject one year, that result might change. …… On Greenland we have instead statistically significant trends, so we know that there is an increase in melting.“
    Do you think it is correct to state as I do in 1 ?
    1. Marco Tedesco warns against using the Antarctica snow melting graph as an argument against anthropogenic global warming.

    Earlier in the interview he said that most of the melting in Antarctica occures over the ice shelves, protruding blocks of ice into the ocean, and that they separate the warmer air coming from the ocean with the ice sheet.
    Do you think it is correct to state as I do in 2 and 3 ?
    2. The main ice loss in Antarctica occures when the ice blocks melt in the seawater, and is therefore not observed as snow melting on the surface.
    3. The ice blocks in the water protect the ice shelves from further melting by cooling the sea water. This is a negative feedback.

    Some clima deniers arranged a seminar in Oslo Norway some weeks ago. A Professor Emeritus presented Marco Tedesco’s graph and used it as an argument against anthropogenic global warming. The seminar was covered on both the front page and on six (!) pages within Norway’s leading technical paper, with the snow melting graph as one of the main clues. I prepare a comment, and this is the background for my questions. Question 3 is not soo important in this context; it is more for my own curiousity.

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