AR4 SPM sea level proves more interesting than expected

Posted by William M. Connolley on February 8, 2007
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Its probably a measure of how accepted the bulk of the AR4 SPM is, that the most interesting discussion about it seems to center around the sea level rise uncertainty ranges. There does indeed seem to be some confusion here… RP Jr explores this, and points out that it would have been nice had the IPCC made a comparison easier, with which I agree (and complains about the take on this from the most authoritative source, of course RC; though as far as I can see he is wrong to say that we assert that the range isn’t lower).

But (before venturing onto the minutiae) I’ll say that as far as I can tell the main message is For each scenario, the midpoint of the range in Table SPM-2 is within 10% of the TAR model average for 2090-2099. So whatever disagreement is small; and likely to be due to estimates of the range rather than the central value.

The other point to make is what we’re discussing here is *not* what the septics have been pushing, which is based on a simple misreading of the report (thus RPs Thus, I conclude that the top end estimate has indeed come down from the TAR to the AR4, and those making this observation are accurately representing the AR4 is a touch disingenuous, because there are two schools here: those correctly noting a lowing of the range and those incorrectly touting nonsense that the value is halved: the latter sadly including the WSJ).

The AR4 itself says The ranges are narrower than in the TAR mainly because of improved information about some uncertainties in the projected contributions (which on reflection probably suggests that the narrowing of the range is real, unless of course they are wrong :-). And the converse of this is that the *minimum* rise has increased, from 0.09 TAR to 0.18 AR4 (I don’t see the skeptics trumpeting this; nor anyone else really as its too small to be exciting).

I can’t claim to fully understand the differences between the two sets of numbers. I agree it would be ratther nice for them to be comparable, but making things back-comparable can be tedious and (from the point of view of the science) largely pointless.

But there are two other points of tedia that may affect the comparison: As Gavin points out to RP in a comment (and which RP uses in his picture, but not within his text)they change to 2090-2099 from 2100 for the headline changes, and the reduction to 90% from 95% in the uncertainties. Switching to a 2090-2100 average knocks off about 5 y of SLR off the top end, which is about 0.04m (although there is a smaller opposite adjustment for the other end). And the change to a 90% range reduces the values somewhat (but by how much?).

Previously I’d written that the 0.59 (AR4) should be compared to 0.7 (TAR) based on the grey envelope but I’m no longer sure thats true: it could well be that 0.88 is the value to compare.

So… bottom lines: This is a storm in a teacup. But we should get it right. Comparison isn’t easy. But since IPCC themselves say the ranges have narrowed, they probably have.

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Comments

  1. #1 Eli Rabett
    2007/02/08

    Strikes me you have to wait for the WGI report or break the embargo. In the meantime, many electrons will go to the anode.

  2. #2 Lab Lemming
    2007/02/09

    As an analyst, I’d say this:
    The two values are both within error of each other.
    What more could you want?

  3. #3 Lubos Motl
    2007/02/09

    Dear William,

    I can’t really speak on behalf of all septics, but I actually believe that others would agree, too. The reason why we don’t talk about the minimum estimate is because this minimum estimate is completely irrelevant, essentially zero, and it has never played any role in the media, policymaking, and other places.

    The maximum estimates in particular – and the average – is what has actually determined the atmosphere in the society. The previous estimates have been corrected from mediocre values to negligible, unspectacular values, too.

    In human terms, there won’t be any sea level rise that anyone should ever care about unless he is a specialist in that field.

    The numerous numerical errors in the SPM show how careless researchers the IPCC people are, especially the SPM “elite”, and how a large number of people in a body doesn’t improve anything about the quality of the material they publish.

    Best
    Lubos

  4. #4 Chris O'Neill
    2007/02/10

    “The reason why we don’t talk about the minimum estimate is because this minimum estimate is completely irrelevant”

    Yeah, sure, that’s the reason. The fact that it’s related to a narrowing of the forecast range is a pure coincidence.

    “In human terms, there won’t be any sea level rise that anyone should ever care about”

    Presumably that’s because in the couple of hundred years it’ll take for Greenland ice flow to really get under way there won’t be any humans left.

  5. #5 Luboš Motl
    2007/02/10

    Dear Chris, you write:

    “The fact that it’s related to a narrowing of the forecast range is a pure coincidence.”

    Sure that it is related. It’s because narrowing of the forecast range is irrelevant, too. The sea levels were rising by 2 millimeters a year in the first half of the 20th century and 1.5 millimeters a year in the second half. It is not hard to see that they will probably continue to do the same thing which gives 20 centimeters per 100 years or less.

    This is an obvious guess and there exists no magic that could predict the sea levels in 2100 much more accurately than that. At any rate, the only relevant fact for policymaking is that the sea levels are not going to change in any foreseeable future by any amount that would deserve any discussion, and whoever says otherwise is a liar.

    Your scenario about dying humans because of some obscure Greenland dynamics you described is so silly that it doesn’t deserve my answer.

  6. #6 guthrie
    2007/02/10

    I was under the distinct impression that sea level rise was up at 3mm a year now, and has been for a few years.

  7. #7 Chris O'Neill
    2007/02/11

    “It is not hard to see that they will probably continue to do the same thing which gives 20 centimeters per 100 years or less.”

    Who is the IPCC to argue with Motl when they say the observed rate of sea level rise from 1993-2003 was 3.1 +/-0.07 mm/year.

    “the only relevant fact for policymaking is that the sea levels ARE NOT GOING TO CHANGE in any foreseeable future by any amount that would deserve any discussion”

    Certainty is bliss.

    “some obscure Greenland dynamics”

    Yes we’ve been told a big lie that most of Greenland’s ice cap didn’t exist 127,000 years ago and that the missing ice caused sea level to be 4-6 metres higher than it is now.