Claim: "Strong to violent tornatoes have actually been decreasing for the past 58 years, and it is possible that the explanation lies with global warming"

What the Science Says: Richard Muller wrote this in an op-ed piece to the New York Times in November 2013. Muller, a retired physics professor formerly from Berkeley has migrated from astronom, with the help from the David Koch oil fortune, to commenting on climate (while in astronomy he was author of the now-discredited "Nemesis" hypothesis about the sun having a distant binary star companion which caused periodic disturbances to the comet clouds). Unfortunately, he has shown a record of "shoot from the hip" slandering good climate scientists and their work, only then to have to admit he was wrong. This is another example. After this piece was published and got a lot of public splash (after all, he's a former professor at Berkeley) numerous actual climate scientists have thoroughly refuted this idea as it relies simplistically on data which cannot be taken at face value (e.g. Brooks and Markowski here). Tornado intensity and damage scales have changed over the years. The meteorology community is well familiar with the fact that tornadoes in the '80's and '90's were rated stronger than they would have been today. There was no correction for changing building standards of construction, and - "The evidence for the overrating of earlier tornadoes includes the fact that environments and damage paths of many strong tornadoes in that retrospective era shared characteristics with weaker tornadoes from later years." (from 6 tornado experts responding to Muller's claims here). Also, in 2003 a new law went into effect requiring a panel of experts to assess the damage of the most damaging twisters, and there was a tendency to de-rate in order to avoid the hassle of going out and doing this work. Contrary to Muller, actual experts in tornadoes maintain that the data is simply not good enough over the 58 year period in question to say whether tornadoes have gotten stronger or weaker.

These tornado data calibration issues do not affect hurricane records, and they in fact show, in agreement with theory, that maximum hurricane winds have been increasing as global warming and ocean temperatures rise

In Short: The evidence is rather that Muller seems to enjoy trying to take credit for splashy ideas which he has not sufficiently thought through, only to be shown he's wrong. Changing tornado intensity and damage during the ~60 year period of rapid global warming cannot be ascertained from the available data at this time. It may well turn out to be trending more intense, or less, or neither.


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