Shifting or stretching the climate bell curve?

Just a quick follow-up to the previous post on attributing extreme weather events to climate change.  Joe over at It’s Okay to Be Smart has posted a nice little gif illustrating the trend in summer temperature extremes over the last half-century, the same type of analysis used in the recent Hansen, Sato & Ruedy PNAS paper:

This animation reminded me of a particularly harsh criticism of the Hansen paper I read a few weeks ago.  The author of that post stridently argues that the PNAS paper is misleading, and that when the mean of a temperature distribution increases but the standard deviation stays the same, only a very small fraction of any new temperature extremes can be blamed on the underlying warming signal (see their figure below).  Fair enough, except that the Hansen paper clearly argues just the opposite, that both mean temperature and temperature variability have increased over the last half-century.    This is clearly illustrated in the animation above and most of the figures in the paper: the bell curve flattens out though remains more-or-less grounded on the left-hand side, with an associated increase in the mean temperature and dramatic increase in the number of 3, 4, and even 5-sigma high temperature events.  Now I’ll readily admit that this is not my area of expertise, and that I cannot vouch for the quality of the datasets or the methodologies in play here.  However, if you’re using the figure below as a staw-man for the observed trends illustrated above, it’s hard to imagine that you’re making an honest argument…

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4 Responses to Shifting or stretching the climate bell curve?

  1. Paul says:

    that graphic is pretty striking. So it seems like they are not denying climate change is happening, but just that the mean shifts back and forth? Seems like there is lots of evidence that it is shifting in one direction, no?

  2. John says:

    No, the post that I link to isn’t denying climate change or anything like that, but rather arguing against the language surround the attribution of the high-temperature extremes to climate change. If I am understanding correctly, the argument associated with that figure is a) same number of extreme hot and cold events are occurring now as in the past, but b) the entire distribution is shifted to the right a bit. Therefore, the author argues that it’s more correct to say that a given extreme event is ~10% more extreme now than it would have been before, rather than saying (as Hansen et al. do) that a 3-standard-deviation high temperature extreme is now ~10x more likely to happen, and thus any such event is ~90% attributable to climate change. Even if this were the case, it seems like semantics to me; either way, the number of extreme high-temperature events is increasing at a worrying pace. But that interpretation is flat-out incorrect if you acknowledge that the distribution is a) stretching out (as is very clearly indicated by the ~25% drop in the magnitude of the peak) in addition to b) shifting rightward.

  3. John says:

    UPDATE: the highly misleading shifted-not-stretched climate bell curve evidently makes an appearance in a While House Council of Economic Advisers report (char #1 in this post from Wonkblog):
    http://www.washingtonpost.com/blogs/wonkblog/wp/2013/03/15/how-the-white-house-thinks-about-climate-change-in-7-charts/

    Science-policy interface FAIL!

  4. John says:

    Here’s a clearer description of the difference between increasing mean and increasing variance in extreme event probability distribution functions from a couple of scientists at the Potsdam Institute for Climate Impact Research:
    http://www.realclimate.org/index.php/archives/2012/03/extremely-hot/

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