Attributing individual extreme weather events to climate change?

From Hansen, Sato & Ruedy 2012 (PNAS)

The conventional wisdom has been that you can’t do that.  However, a new study in PNAS by Dr. James Hansen and colleagues argues that several dramatic heat waves in different parts of the world over the last decade are statistically so far away from the historical norm that they are virtually inconceivable in a world not experiencing anthropogenic climate change.  According to their analysis of large surface temperature datasets, not only have mean temperatures increased, but their standard deviation has increased as well, with so many examples of high-temperature events exceeding three standard deviations relative to a historical baseline that a new normal distribution must be drawn (shifted towards higher temperatures AND spread out more widely).  In the author’s own words:

This is not a climate model or a prediction but actual observations of weather events and temperatures that have happened….  The deadly European heat wave of 2003, the fiery Russian heat wave of 2010 and catastrophic droughts in Texas and Oklahoma last year can each be attributed to climate change….  These weather events are not simply an example of what climate change could bring. They are caused by climate change. The odds that natural variability created these extremes are minuscule, vanishingly small.

James E. Hansen, ‘Climate change is here- and worse that we thought’, Washington Post, 8/3/2012

This represents a clear challenge to the existing mantra that individual weather events cannot be directly attributed to climate change, and it is likely that this viewpoint will face a lot of skepticism (as any new scientific theory should) both from within and outside of the scientific establishment.  It is however consistent with the approach of some other recent high-profile studies that use pure observational statistics (completely independent of physical process models and their associated predictive uncertainties) in order to describe the occurrence of anthropogenic climate change and its negative effects on agriculture.  It will interesting to see whether or not this study will contribute to forming a new consensus on the issue.

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4 Responses to Attributing individual extreme weather events to climate change?

  1. ST says:

    I find it funny that I run the same numbers over a base line of 100 years and I get little to no change. In fact I find that the Northeastern and Northwestern US is falling on the high, slightly up on the low. I would like to know how that is.

    • John says:

      Hmmm… do you use the same GHCN dataset described in the paper, and if so, do you observe the same trends in standard deviation using the 10-year baseline sequences from 1951-1961 to 200-2011 that the authors observe? I certainly have not run this analysis myself, and am not in a position to debate the finer points of the results. For what it’s worth, the affiliation and contact info given in the paper is:
      James Hansen(a), Makiko Sato(a), and Reto Ruedy(b)
      (a) National Aeronautics and Space Administration Goddard Institute for Space Studies and Columbia University Earth Institute, New York, NY 10025
      (b) Trinnovim Limited Liability Company, New York, NY 10025
      To whom correspondence should be addressed. E-mail:

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