That East Antarctic mass loss, in full

Posted by William M. Connolley on November 27, 2009
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Ah, a fertile theme for post names. Excellent.

So, much excitement over a new GRACE study in Nurture (Accelerated Antarctic ice loss from satellite gravity measurements, J. L. Chen et al.) indicating that Antarctica as a whole was losing mass: In agreement with an independent earlier assessment, we estimate a total loss of 190plusminus77 Gt yr-1, with 132plusminus26 Gt yr-1 coming from West Antarctica. However, in contrast with previous GRACE estimates, our data suggest that East Antarctica is losing mass, mostly in coastal regions, at a rate of -57plusminus52 Gt yr-1, apparently caused by increased ice loss since the year 2006.

But as you’re aware, I’m obliged to be bitter and cynical about all this kind of stuff, so you must be wondering “how is he going to get out of that“. Well maybe I won’t, but I’ll try. First off though, can we please all ignore the septic dribble about Ant Mass Gain Disproves GW. We all know that is tosh, so leave it aside.

Let’s begin by quibbling their use of “long-term”: according to fig 3, 2006-9 is “long term” (and 2009 isn’t even over!). Clearly this is an abuse of the term. we may be looking at no more than a few years fluctuation. Only time will tell on that one.

Continuing, I find papers that talk only in terms oh Gigatonnes (ooh scary) annoying; I’d expect a conversion into mm of SLR at some point. I don’t see them providing that, so I’ll have to: 190 Gt/yr ~ 0.6 mm/yr. Also known as 6 cm/century. So without further acceleration this is not going to be exciting (that is why they need “acceleration” in the title).

Looking back, I see I talked about GRACE in 2006 when they only had 3 years data. That paper said “152 ± 80 km3/year of ice, equivalent to 0.4 ± 0.2 mm/year of global sea level rise”; as far as I can see the new results are within the error bounds of the old (and vice versa) so the main result of this new paper is the change, in ~2006, of the slope, rather than any vast new mass loss. As before, the PGR term is a problem: PGR model errors are probably the dominant limitation to Antarctic mass rate estimates as the paper wisely says; they can vary their answer substantially by choosing different PGR models.

That will have to do I think.

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Comments

  1. #1 Douglas Watts
    2009/11/27

    gigatons arescary … but not as scary as teracarat.

  2. #2 Eli Rabett
    2009/11/27

    Grace is a game changer, and Eli suspects that with time folk will figure out creative ways of getting more out of it.

  3. #3 bigcitylib
    2009/11/27

    Heretic!

  4. #4 dhogaza
    2009/11/27

    Is “in full” your new mantra? :)

  5. #5 carrot eater
    2009/11/27

    57 +/- 52. Those are some nice error bars.

  6. #6 lumpy
    2009/11/28

    “But as you’re aware, I’m obliged to be bitter and cynical about all this kind of stuff, so you must be wondering “how is he going to get out of that”. Well maybe I won’t, but I’ll try. First off though, can we please all ignore the septic dribble about Ant Mass Gain Disproves GW. We all know that is tosh, so leave it aside.”

    We all know it is tosh, but even proving that to the denialosphere is impossible to date. Surely we have to get to that stage before we can move on.

  7. #7 Adam
    2009/11/28

    “…in full” == “That will have to do…” ?

    The sloppiness of IT professionals!
    It’s no wonder joe six-whatever can’t programme the video. How can we base our entire economy on anything computer people do, etc….degenerate into half coherent rant…etc., etc…

  8. #8 PeterPan
    2009/11/28

    Thanks for this.

    So, to sum it up, Velicogna 2009 (143 ± 73 Gt/yr) reported on the acceleration but confirmed the previous linear trend (Velicogna 2006 – 152 ± 80 km3/year). Is Km3/yr equivalent to Gt/yr? Because it sounds odd to me that there is an acceleration but the linear trend (central value) is now lower than the previous one (152 initially and now 143). And Chen et al 2009′s results are within the error bounds (190 ± 77 Gt/yr). And in any case, these linear trends aren’t very important in terms of sea level rise (unless the acceleration continues): 6 cm/century.

    *How do we convert Gt/Km3 /yr into sea level rise? Is just a linear relationship? (32 Gt/yr -> 1 cm/century?)

    [Just divide the volumne by the area of the ocean (pedantically, note that SLR won't be uniform) -W]

    A further issue is that PGR is overestmiated (Bevis et al 2009 – press) and would substract around 33 Gt/yr from GRACE estimates. That would leave 110 Gt/yr as the central value.

    [I don't think you can be certain enough to say it is overestimated; just uncertain -W]

    Related to this, the IPCC 2007 blamed the West Antarctic ice loss on ongoing grounding line retreat since the Last Glacial Maximum:

    “They found [...] a trend in antarctic shrinkage of about 90 Gt yr–1, primarily because of retreat of the West Antarctic grounding line in response to the end of the last ice age”
    http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-chapter4.pdf (p 366)

    The references are Huybrechts 2002 (abstract) and Huybrechts et al 2004 (full pdf): “quite large thinning rates up to 30 cm/year over the West Antarctic ice sheet, related to ongoing grounding line retreat since the Last Glacial Maximum”.

    I’m not really sure they are speaking about the same thing, but this would leave just around 20 Gt/yr that might be AGW-related (110 (PGR-corrected) minus 90 Gt/yr related to the grounding line retrat).

    Cheers!

  9. #9 Douglas Watts
    2009/11/28

    Are current post-glacial rebound estimates as cloudy for the Northern Hemisphere, ie. Canada, New England, Scandinavia, as for Antarctica?

    [Ha, you could just read the paper :-) PGR model errors are probably the dominant limitation to Antarctic mass rate estimates6, 23. PGR models in Antarctica suffer from a lack of fundamental data available in Northern Hemisphere regions, including contemporary rates of vertical motion, and geomorphological evidence constraining ice-load history. The result is variability among PGR models. -W]

  10. #10 PeterPan
    2009/11/28

    Thanks for your inline comment and the answer about turning Gt/Km3 into sea level rise. I think I’ve got it, but I’m note really sure since I get 0.5 mm/yr instead of 0.6 for 190 Gt/yr. I’ve seen in Wikipedia that the area of the ocean is ~3.61×10^14 m2, i.e., 3.61×10^8 Km2. Then 190 Km3 would mean a SLR of:

    190/(3.61×10^8) -> 1.9/3.61 x 10^-6 -> 0.52 mm/yr

    Are we using different estimates for the area of the ocean? Thanks (I will perfectly understand if you don’t have time for this).

    [Don't rely on my 0.6; as I recall, I saw on wiki that 50 Gt corresponds to 0.13 mm so translated 190 (~4*50) into 0.6 (~4*0.13) which was close enough for my purposes -W]

  11. #11 Douglas Watts
    2009/11/28

    Thanks, W. Not long after I posted the question, I realized the comparative lack of PGR data for the Antarctic was most likely due to the paucity of data, due to it being Antarctica.

  12. #12 Alastair McDonald
    2009/11/28

    “But as you’re aware, I’m obliged to be bitter and cynical about all this kind of stuff,…”

    I didn’t realise that you thought it was your duty. I thought that you really believed that the world does not change. On the 150th anniversary of the publication of Darwin’s Origin of the Species isn’t it time that you accepted that the world evolves, and that ice sheets form and melt away no matter whether they are based on land or sea. It is just that those on the sea disappear faster. And it requires higher CO2 levels to melt those at higher altitudes.

  13. #13 Hank Roberts
    2009/11/28

    ftp://ftp.ifm.uni-hamburg.de/outgoing/scharffe/BACKUP/paper/2009_GRL_Leuliette.pdf

    … Using corrected and verified Jason-1 and Envisat altimetry
    observations of total sea level, upper ocean steric sea level
    from the Argo array, and ocean mass variations inferred
    from GRACE gravity mission observations, we find that
    the sum of steric sea level and the ocean mass component
    has a trend of 1.5 ± 1.0 mm/a over the period, in agreement
    with the total sea level rise observed by either Jason-1 (2.4 ±1.1 mm/a) or Envisat (2.7 ± 1.5 mm/a) within a 95%
    confidence interval.
    Citation: Leuliette, E. W., and L. Miller
    (2009), Closing the sea level rise budget with altimetry, Argo, and GRACE, Geophys. Res. Lett., 36, L04608, doi:10.1029/
    2008GL036010

  14. #14 PeterPan
    2009/11/29

    What I find interesting is that the IPCC thought that the ice sheets would be in mass balance during the 20th century (in terms of sea level, something like +10 cm from the Greenland ice loss and -10 cm from the Antarctic gain). However, we are already seeing an imabalance in both sheets that would give +11 cm (linear trend; i.e. without acceleration). In Velicogna’s own words:

    “One option is to fit a linear trend, as done in most prior studies. For the period April 2002–February 2009, we obtain a trend of -230 ± 33 Gt/yr for the Greenland Ice sheet and of -143 ± 73 Gt/yr for the Antarctica Ice sheet. This corresponds to a total 1.1 ± 0.2 mm/yr sea level rise from the two ice sheets”

    And I think that further acceleration is consistent with the semi-empirical aproaches from Rahmstorf 2007, Grinsted et al. 2009 or Pfeffer et al 2008, that give results close to 1 m. for the end of the 21st century.

  15. #15 Hank Roberts
    2009/11/30

    http://news.bbc.co.uk/2/hi/science/nature/8387137.stm

    The story is attributed to
    http://www.scar.org/

    I don’t find a press release there, but there are several meetings, perhaps the story is from one:

    3rd IBCSO Meeting
    30 November 2009, AWI Bremerhaven, Germany
    This is a closed meeting, with attendance by invitation only

    Antarctic Treaty Summit: Science-Policy Interactions in International Governance
    30 November – 3 December 2009
    Smithsonian Museum, Washington DC

    December 2009
    4th International conference
    Earth From Space: The most effective solutions
    1 – 3 December 2009, Moscow, Russia

    [That is all a bit weird. I need to have a look -W]

  16. #16 Andy Wickert
    2009/12/01

    I’m a late commenter due to Thanksgiving, during which I tried to stay away from my computer.

    But I love ze PGR so I must tack something on.

    FY(plural)I, Peltier, Argus, and the rest of their crew have worked up an ICE-6G model (see their AGU abstract entitled “A Next Model of Northern Hemisphere Glaciation History: ICE-6G (VM5a)”), and they have a paper submitted to GJI). It should be a pretty big improvement over ICE-5G, because we now have lots of space geodetic data over time periods to remove lots of measurement noise and short-period variability. What’s really great is that GPS excel at measuring in the horizontal, and horizontal PGR motions are very sensitive to the detailed geometry of the ice sheet.

    However, the focus of their abstract is Canada. Sorry Antarctica: you’ll get fixed up at some point.

    [Thanks. I know David Vaughn and the BAS folk were very keen to put GPS int Antarctica. But we can only afford to do a small bit of it. Maybe later -W]

  17. #17 Adam
    2009/12/01

    Hank, they may have been slow at updating their website, as the report & press release are linked from their front page, now:

    http://www.scar.org/publications/occasionals/acce.html

  18. #18 Adam
    2009/12/01

    Also at BAS:

    http://www.antarctica.ac.uk/press/press_releases/press_release.php?id=1065

    (Note publication date: 1st Dec.)

  19. #19 Nicolas Nierenberg
    2009/12/03

    PeterPan,

    “the IPCC thought that the ice sheets would be in mass balance during the 20th century.”

    I can’t find any statement like that in AR4. What is your reference for that statement? Table 10.7 certainly doesn’t back up that assertion. For example for A1B the sum of Greenland and Antarctica is a contribution to rise on both the low and high end.

    Also you really have to add their G&IC figure in to get a true picture as that includes Glaciers and Ice Caps. West Antarctica for example is in that figure see 4.51.

    Sorry about the late response, I am following a link from W’s current post on sea level.

    [Do you mean 20th? 20th would be small, come what may. The TAR says Gr and Ant ~balance to 2100: http://www.grida.no/publications/other/ipcc_tar/?src=/climate/ipcc_tar/wg1/427.htm#tab1112 -W]

  20. #20 PeterPan
    2009/12/06

    Nicolas Nierenberg,

    Sorry, I meatn 21st century and the numbers came from the IPCC 2001:
    http://www.grida.no/publications/other/ipcc_tar/?src=/climate/ipcc_tar/wg1/427.htm#tab1112

    However, I think that sea level projections in IPCC 2007 don’t take ice sheets dynamics into account, did they? So I think I still can say that the IPCC sea level projections consider that the net contributions of both ice sheets are around zero and this results move observations away from the the IPCC model projections towards semi-empirical aproeaches projections.

    Cheers.