By Paul Homewood
[Note Update Below]
Quantification of long term tornado trends has been hampered by the fact that many more tornadoes are reported today than was the case in the past. NOAA summarise this well:-
With increased national Doppler radar coverage, increasing population, and greater attention to tornado reporting, there has been an increase in the number of tornado reports over the past several decades. This can create a misleading appearance of an increasing trend in tornado frequency. To better understand the variability and trend in tornado frequency in the U.S., the total number EF1 and stronger, as well as strong to violent tornadoes (EF3 to EF5 category on the Enhanced Fujita scale) can be analyzed. These are the tornadoes that would have likely been reported even during the decades before Doppler radar use became widespread and practices resulted in increasing tornado reports.
FSU Professor, James Elsner, has attempted to develop a model to determine the real trends, in the paper “The Decreasing Population Bias in Tornado Reports across the Central Plains”.
I have no particular comment to make about the model itself. WUWT has covered this issue well already.
I was intrigued, though, by the comment in the press release that “The model shows that it is likely that tornadoes are not occurring with greater frequency, but there is some evidence to suggest that tornadoes are, in fact, getting stronger”
Elsner reinforces this message with this quote:-
“The risk of violent tornadoes appears to be increasing. The tornadoes in Oklahoma City on May 31 and the 2011 tornadoes in Joplin, Mo., and Tuscaloosa, Ala., suggest that tornadoes may be getting stronger.”
We’ll leave aside the obvious fallacy that a few isolated events constitute some sort of trend, and look at what the facts tell us.
But first, a bit of detective work.
A Statistical Model for Tornado Intensity
A close study of the Elsner paper does not appear to reveal any reference at all to “tornadoes getting stronger”. (The paper is here – if anyone finds such a reference, let me know!) So where does he get this evidence from?
It appears to be based on another paper he has written this year, A Statistical Model for Tornado Intensity. There is already, of course, a well established system of measuring the severity of tornadoes, the Enhanced Fujita Scale, which uses damage surveys to estimate wind speeds.
Elsner, however, has taken a different approach, and uses a combination of tornado path length and width to estimate wind speeds. (The logic being that the strongest tornadoes tend to have the longest/widest paths).
This approach has a very fundamental drawback – the reliability of historical data relating to length and width. In Elsner’s provisional work, he seems to have used 1985 as his start point, but his paper now uses 1994 instead. Elsner told to me in May that “the data had problems before the mid 1990’s”, a fact which is confirmed in his paper.
And we also know already that, in 1994, NWS changed their recordings of width from “mean” to “maximum”. (Even then, discrepancies in the data where path width has been underestimated have been found more recently, such as this one in 1997 – see here).
But, even assuming that the data since 1994 is accurate, we are still only left with a trend of just 18 years. Is Elsner seriously suggesting that reliable trends can be calculated over such a short period of time?
Nevertheless, his paper concludes
More work needs to be done to understand the upward trends in path length and width. The increases lead to an apparent increase in tornado intensity across all EF categories.
So, how does analysis using the traditional EF Scale compare with Elsner’s study? Does it confirm his results?
Let’s start by looking at NOAA’s graph of stronger EF3+ tornadoes.
Certainly, 2011 sticks out , but it is absolutely clear that the last decade is unexceptional. It is also clear that there were many more strong tornadoes in the 1970’s, and to a lesser extent in the 1950’s.
This analysis surely casts huge doubt on Elsner’s conclusions, based as they are on such a short period.
But what about the strongest tornadoes, the EF-4’s and EF- 5’s? If Elsner’s theory is correct, surely we should be seeing an increasing frequency of these?
I have plotted below the numbers of these tornadoes since 1970’ using the data from NOAA’s Storm Prediction Centre.
The situation is quite clear on EF-4’s – the trend has significantly declined, and even 2011 had far fewer events than 1974 – 17 v 29.
The position with EF-5’s is less clear, with a slightly increasing trend. However, this trend has been heavily influenced by the single year of 2011.
With an average of just one EF-5 tornado a year, the numbers are simply too sparse to reach any proper conclusion. It still remains the case that 1974 had more EF-5’s than 2011, and that the decadal total in the 1970’s was significantly higher than the latest 10 year period.
- The Elsner paper relies on far too short a period to draw any meaningful conclusions.
- It also totally ignores the evidence of the 1950’s and 1970’s, which clearly points to a decrease in stronger tornadoes over the longer term , not an increase.
- Elsner’s conclusions seem to be heavily influenced by the EF-5 tornadoes in 2011.
- The analysis relies totally on the accuracy of path width and length data. While this may now be reasonably accurate, past data is highly suspect.
- There are attempts in the paper to link the supposed increase in tornado intensity to “climate change”. Elsner goes further in an earlier presentation, saying
“Evidence points to growing frequency and intensity of extreme weather worldwide due to global warming, but an effort to detect changes in the intensity of tornadoes has yet to be made.
Here we show compelling evidence for a growing trend in the ferocity of strong tornadoes across the United States.
But as there has been no warming since 1997, how can any increase in intensity since then be due to global warming?
- The paper’s conclusions state that “the EF Scale is not adequate for analyzing tornado intensity”. I am not sure if the tornado experts at NOAA would agree.
There is no doubt that the decreasing frequency and severity of tornadoes is a problem for those who claim that global warming has brought an increase in extreme weather.
There is already a well established system for measuring the strength of tornadoes, and a well established database to back it up. Yet Elsner has attempted to replace this with a statistical model, based on a small amount of potentially unreliable data.
Is this a case of moulding the evidence to suit the theory?
To avoid any ambiguity, in the Introduction to Elsner’s paper, he equates “tornado intensity” with “wind speed”.
When I ran the above graph for EF-5 tornadoes, I had left the zero fields blank. This apparently gives a false trend line, so I have now rerun it with zeros. The result, as below, now shows the true position, which is a declining trend.
Thanks to Donald Mitchell and Scott Scarborough for pointing this out.
1) FSU Press Release
2) “The Decreasing Population Bias in Tornado Reports across the Central Plains” by James B. Elsner and Laura E. Michaels
3) “A Statistical Model for Tornado Intensity” by James Elsner, Thomas Jagger & Ian Elsner.
4) NOAA Tornado Climatology Page
5) Storm Prediction Centre Database