New climate study is frenemy of climate science driven policy

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There is a new study out in Nature that is liable to be misinterpreted, or that may be flawed in a way that lends itself to misuse, in the context of climate science driven policy.

The study is “Northern Hemisphere hydroclimate variability over the past twelve centuries” by Fredrik Charpentier Ljungqvist, Paul J. Krusic, Hanna S. Sundqvist, Eduardo Zorita, Gudrun Brattström & David Frank

I’ll make just a few comments here, but mainly, I want to point you to comments by climate scientist Michael Mann (author of The Hockey Stick and the Climate Wars: Dispatches from the Front Lines and Dire Predictions, 2nd Edition: Understanding Climate Change).

The main question related to policy is this. Do warmer conditions such as we are experiencing now as a result of human greenhouse gas pollution change the hydrology of the planet? The answer, based on various research projects, is yes. Two main things seem to pertain. First, there is more moisture in the air owing to the air being warmer and sea surface temperatures being higher. More moisture holding capacity and more evaporation (movement of water into the air as vapor) result in this effect. At the same time, changes in weather patterns can clump a good amount of this moisture up, so even a modest amount of increase in atmospheric moisture can (and does) result in major precipitation events, causing flooding and other untoward events. This clumping can also serve to deprive some areas of moisture for extended periods of time, and major droughts such as in the Middle East and California are attributed at least in part to this effect.

The study seems to show that this is not likely. The study looks at paleo data over thousands of years, testing and extending a model to apply to present and future climates. The result seems to show that the more extreme changes in hydrology, either wetter or dryer, are not likely. However, Mann makes the point that the kind of data used in this study, such as tree rings, do not reliably show extreme events. In other words, extreme events in the past likely happened without leaving much of a signal.

Mann’s comments are in a facebook post partly reproduced here:

…The study represents a laudable effort to document past changes in extreme rainfall and drought using paleoclimate proxy data, but there are some shortcoming with the study, and especially with the way it is being billed by some of the study’s authors and certain organizations.

A press release from the international paleoclimate organization ?#?PAGES? is accompanied by the rather bold headline “Climate models overestimate twentieth century wet and dry climate extremes”. The lead author Fredrik Ljungqvist is quoted in the press release stating that the discrepancy between the smaller hydroclimatic variations shown by their paleoclimate proxy reconstruction and the greater variations shown by climate models imply that “Climate models strongly overestimate the intensification of wet and dry extremes in the twentieth century”.

Does this study in fact meet the burden of establishing that models are overestimating extremes in rainfall and drought?

Almost certainly *not*.

The discrepancy could arise, of course, from the opposite problem: that the paleoclimate proxy data are *underestimating* hydroclimatic extremes. In my view, that is a far more likely explanation.

Our own extensive work analyzing paleoclimate proxy data has shown has demonstrated they are not well suited for reconstructing past climate *extremes*. Tree rings and many other chemical and biological climate proxy records, by their nature, tend not to record very large short-term fluctuations, and for this reason they are likely to show muted extremes, i.e. less extreme variation than actually exists in the climate record. We published several articles demonstrating this problem over the past several years:

  • Schurer, A., Hegerl, G., Mann, M.E., Tett, S.F.B., Separating forced from chaotic climate variability over the past millennium, J. Climate, 26, 6954-6973, 2013.
  • Mann, M.E., Rutherford, S., Schurer, A., Tett, S.F.B.,Fuentes, J.D., Discrepancies between the modeled and proxy-reconstructed response to volcanic forcing over the past millennium: Implications and possible mechanisms, J. Geophys. Res. 118, 7617-7627, doi:10.1002/jgrd.50609, 2013.
  • Mann, M.E., Fuentes, J.D., Rutherford, S., Underestimation of Volcanic Cooling in Tree-Ring Based Reconstructions of Hemispheric Temperatures, Nature Geoscience, 5, 202-205, 2012.
  • (all available here)

    So, in conclusion, it would be rather dangerous to extrapolate from this one potentially flawed new paleoclimate study any sweeping conclusions about climate models and human-caused climate change. Such over-interpretations of paleoclimate data poorly serve the critical public discourse over the impacts of climate change, and can in fact do harm to the paleoclimate discipline by publicizing bold but unsubstantiated claims that are very likely to be refuted by further work.

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    14 thoughts on “New climate study is frenemy of climate science driven policy

    1. Given that the rise in carbon dioxide we are experiencing is at a level and certainly a rate that is unprecedented, does this not make the problem of hidden extreme events in the paleoclimate even more problematic? I.e. we are not observing a stead rise from 280ppm to 560 ppm, say, over several 10,000 years, but in the blink of a geological eye (not to mention a biological, evolutionary one).

    2. If it is true that extreme events don’t leave much trace in the paleoclimate proxy data, than one can really draw no conclusion from such data about extreme events – one way or the other.

      Therefore, Dr. Mann’s assertion that:

      “The discrepancy could arise, of course, from the opposite problem: that the paleoclimate proxy data are “underestimating* hydroclimatic extremes. In my view, that is a far more likely explanation.”

      is no better grounded than its opposite.

      We cannot know (from paleoclimate proxy data) whether extreme events are underestimated OR overestimated.

      Or am I missing something?

      This criticism seems like an appeal to authority to me (Dr. Mann appealing to his own authority, and therefore just personal opinion – NOT based paleoclimate proxy data).

      Perhaps that is why he published on Facebook?

      Perhaps Dr. Mann will write a letter to Nature?

    3. RickA,

      Are you serious?

      What you seem to be missing is comprehension of the English language.

      How is saying “in my view” different from saying “this is just my personal opinion”?

    4. While ignoring RickA, I’m just wondering the reason behind why tree rings are unreliable for measuring extreme moisture content events. Is it that there is an extinction coefficient in the tree ring growth? Is there only so much growth (or non-growth) one can observe in a tree ring based on rainfall in a given year?

    5. You ask “How is saying “in my view” different from saying “this is just my personal opinion”?”.

      It is not.

      That was my point.

    6. Perhaps that is why he published on Facebook?

      More likely: this paper was just published? This comment was a first response to it. One cannot do the same thing on a publication’s website.

      he discrepancy could arise, of course, from the opposite problem: that the paleoclimate proxy data are *underestimating* hydroclimatic extremes. In my view, that is a far more likely explanation.

      If that were all he said your comment about the alternate interpretation being just as likely would make a little sense, but you leave out the reason for his comment: everything else after the word “explanation”. The comment is hardly an idle statement made in a vacuum.

    7. George #4,

      The most obvious limiting factor would be soil saturation/runoff.

      How would you detect rainfall beyond that point through tree-rings?

      But there are also going to be structural limiting factors, like available nutrients, operating in the longer term.

    8. Expert reactions:

      Professor Steven Sherwood Director of the Climate Change Research Centre at The University of New South Wales:

      “Previous studies, based on models, have shown that warming-induced trends in regional precipitation have not yet emerged from natural variability (“noise”). This seems inconsistent with the paper’s claim that the changes predicted by these same models are unrealistic, since it should not yet be possible to tell even according to the models themselves.”

      Dr Markus Donat:
      “It seems to me that the researchers’ claim of discrepancies between climate models and proxies during the most recent century is based on an “apple-to-oranges-comparison”. They use proxies of water availability (that is affected by both precipitation and evaporation) but compare against modelled precipitation only.”

      Dr James Renwick:
      “I am not too surprised that there is disagreement for the 20th century as there is a strong component of random variability evident in the observational record. The picture of the “wet getting wetter and the dry getting drier” is one that is very likely to emerge over the course of this century but has not been evident, or expected, during the 20th century.”

    9. “There is a new study out in Nature that is liable to be misinterpreted, or that may be flawed in a way that lends itself to misuse, in the context of climate science driven policy.”

      It’s already happened, Greg. Yahoo runs this headline:

      Climate forecasts may be flawed, says study

      “Predictions of unprecedented rainfall extremes in the 20th century driven by global warming turned out wrong, a study said Wednesday, casting doubt on methods used to project future trends.”

      Turned out to be wrong. Wow. Who writes this stuff?

    10. For the twentieth century there must be direct precipitation measurements available. How doe they compare??

      1. That is a good question, but it might be harder to answer than apparent. Looking at long term trends, the records are not as good as one might like for the entire hemisphere.

    11. RickA, not everyone spins falsehoods to intentionally mislead others so that they can feel less insecure about their own concerns.

      Don’t kid yourself. You do this, so it’s just natural for you. Hence, you project this onto everyone else. That is human nature.

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