In describing the errors in the Fourth National Climate Assessment, ‘NCA4’, I’ll use the words from the Executive Summary which purport to link climate changes in the USA to global climate change.
The first claim, “The last few years have also seen record-breaking, climate-related weather extremes,“ is shown to be false, simply by examining climate records, some from the National Climate Data Center.
Tornadoes have been decreasing over the past six decades as temperatures moderate from the significant cooling of the 1940s to 1970s. As a basic knowledge of meteorology teaches, it is the pole to equator temperature difference that drives the intensity of cold season storms and especially the spring-season storms which bring the extremely strong tornado outbreaks.
Figure 1. Annual count of strong to violent tornadoes from 1954-2014, showing a significant decrease of tornado activity the past 60 years, based on data from NOAAs Storm Prediction Center. [Note: This graphic replaces the original graphic that showed all tornadoes EF1 and stronger. Correction made 1/25/2019].
With the devastating Dayton, Ohio tornadoes fresh on our minds, it is useful to examine exactly why (modest) global warming has produced fewer – not more – of such events.
The simple answer is that tornado formation requires unusually cool air.
Very few thunderstorms produce tornadoes. In the hot and humid tropics, they are virtually unheard of. The reason why is that (unlike hurricanes) tornadoes require strong wind shear, which means wind speed increasing and changing direction with height in the lower atmosphere.
These conditions exist only when a cool air mass collides with a warm air mass. And the perfect conditions for this have existed this year as winter has refused to lose its grip on the western United States. So far for the month of May 2019, the average temperature across the U.S. is close to 2 degrees Fahrenheit below normal.
In the whirlwind that is 2018, there has been a notable lack of high-end twisters.
We’re now days away from this becoming the first year in the modern record with no violent tornadoes touching down in the United States. Violent tornadoes are the strongest on a 0 to 5 scale, or those ranked EF4 or EF5.
It was a quiet year for tornadoes overall, with below normal numbers most months. Unless you’re a storm chaser, this is not bad news. The low tornado count is undoubtedly a big part of the reason the 10 tornado deaths in 2018 is also vying to be a record low.
While we still have several days to go in 2018, and some severe weather is likely across the South to close it out, odds favor the country making it the rest of the way without a violent tornado.
If and when that happens, it will be the first time since the modern record began in 1950.
One of the main difficulties with tornado records is that a tornado, or evidence of a tornado must have been observed. Unlike rainfall or temperature, which may be measured by a fixed instrument, tornadoes are short-lived and very unpredictable. If a tornado occurs in a place with few or no people, it is not likely to be documented. Many significant tornadoes may not make it into the historical record since Tornado Alley was very sparsely populated during the 20th century.
Much early work on tornado climatology in the United States was done by John Park Finley in his book Tornadoes, published in 1887. While some of Finley’s safety guidelines have since been refuted as dangerous practices, the book remains a seminal work in tornado research. The University of Oklahoma created a PDF copy of the book and made it accessible at John Finley’s Tornadoes(link is external)