Were This Year's Tornadoes Caused By Global Warming?

I have been on the road this entire week giving weather risk mitigation seminars. As recently as yesterday, I was asked if this year's unusually intense tornado season was due to global warming.

I have already discussed that I believe the answer is "no." That posting also cites Dr. Judy Curry's belief the answer is no.  The National Oceanic and Atmospheric Administration concurs and says,

 So far, we have not been able to link any of the major causes of the tornado outbreak to global warming. Barring a detection of change, a claim of attribution (to human impacts) is thus problematic...

But, still, the question persists.

So, I would like to cite some preliminary work done by Drs. Roger Pielke, Jr., Kevin Simmons, and Daniel Sutter comparing the tornado season of 2011 to records since 1950 when the Weather Bureau (now National Weather Service) began to keep comprehensive records of tornado occurrences.

The graph below is from Simmons and Sutter and it shows the year to year levels of tornado damage, adjusted for inflation, to December 31, 2010. You see spikes in 1953 (the season I have compared 2011's to), 1966 (Palm Sunday tornado outbreak) and 1974 ("Superoutbreak").
click to enlarge
Over the last two decades, the trend in damage is clearly down when compared to the first half of the reporting period.

If global temperatures were correlated to tornadoes, you would see a spike in the graph at 1998 and the last decade would be much higher than the high level of damages in the 1966-1978 period when world temperatures were much cooler.
World temperatures (click to enlarge) from 1950 to 2010 to match the period of the
tornado graph. From Hadley Climate Research Unit, Great Britain
A quick review of the two graphs indicates there is no trend in tornado damage (which is a good proxy for tornado numbers and strength) related to world temperatures.

However, Dr. Pielke wondered how 2011's horrible tornado season would look, if added. Roger went on to say,

The Simmons/Sutter dataset provides one means to answer this question. Regular readers will know that it is not enough to look at disaster data over time, even if it has been adjusted for inflation, for the simple reason that society is always changing. The exact same weather extreme occurring years apart will cause more damage if people build more property and accumulate more wealth in the area affected. Simmons figure shown above has no such “normalization” applied to it.

What Roger means is that inflation does not tell the whole story. For example, a tornado in 1974 could not possibly have destroyed a "home theatre" because such a thing did not exist. Roger often normalizes damage figures for hurricanes and other storms to gross national product and other measures.  So, he did a back of the envelope calculation for 1950 and 2011 to date represented in the figure below:
click to enlarge
Keeping in mind that 2011 is only just past the halfway mark, we see that it will likely be the third worst year since 1950. The level grows for 1953 (now #2) which seems accurate given the many similarities to 2011.

Interestingly, the downward tornado trend since about 1980 is even more pronounced until this year.

While this data is very much preliminary, it pretty well puts the metaphoric nail in the coffin of the hypothesis "tornadoes are getting worse due to global warming."


  1. If warm temperatures alone were the trigger for tornadoes, July in the South would be the most active period anywhere for tornadoes and insane outbreaks would be a part of summertime down there and increasing. Yet they are rare down there in July, and when they do happen, 90% of them are very weak or come off tropical cyclones.

    Most July tornadoes in the US happen in the northern Plains and Great Lakes, which are certainly not the hottest places.

  2. Looks to me though that the 3 of the 4 highest peaks correspond to la nina years. There are only 4 periods with ONI's below -1.0 for May through August since 1950, and three of those years are the previous three highest peaks on the chart.

    1988 is the only below -1.0 ONI drop that does not have high tornado damage.
    Be interesting to see what the ONI is for this summer, because it is looking like the La Nina is over.


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