Is global warming responsible for a slow tornado season?

May 27, 10:10 AMDenver Weather ExaminerTony Hake

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Is global warming responsible for fewer tornadoes in 2009?  Stu
Ostro of the Weather Channel thinks so.  Image courtesy NSSL.

Stormchasers across Tornado Alley have been frustrated this spring by what seems to be a lack of tornadoes and severe weather.  Indeed, VORTEX2, the largest tornado field study ever, has been running for more than two weeks now and has not seen one twister.  Last week, Weather Channel Senior Meteorologist Stu Ostro speculated that global warming was the cause.

• Slideshow:  Amazing tornadoes

In a segment with on-air meteorologist Stephanie Abrams, Ostro explained that the jet stream lingering over Canada and a continual ridge of high pressure were suppressing severe weather.  Without the collision of low and high pressure (among other factors), tornadoes and supercell thunderstorms have a hard time forming. 

Stu Ostro of the Weather Channel believes these two graphs
show a correlation between atmospheric pressure (top) and
global temperatures (bottom).

Ostro continued his explanation saying that atmospheric pressure in the Northern Hemisphere's mid-latitudes between 30 and 60 degrees has increased steadily since 1970.  He then overlaid a chart showing this increase with that of a chart that shows global average temperatures having increased over the same period.  As he says in his blog entry, “That parallels remarkably well the trend of globally-averaged temperatures, which has also shown a significant rise since the mid 1970s.”

Previously having considered himself a climate change skeptic, Ostro now believes anomalous weather patterns can be traced to anthropogenic [manmade] climate change.  Ostro says, “Up until a few years ago I was convinced that there was nothing out of the ordinary going on. What's convinced me otherwise is the number, nature, and context of the anomalies happening around the world, and changes in circulation patterns associated with them.”

He closed out his blog entry with some rather pointed commentary saying, “There have always been extremes; however, their nature is changing. The large-scale long-term trends are affecting the weather, which is intimately and inexorably linked to climate within the continuum of time scales. Deal with it. We're all gonna have to. If we don't want to get zapped, we must adapt.”

Without entering a greater debate about global warming, what about Ostro’s comments on air that global warming and climate change are responsible for the slow tornado season?  Beyond what appears to be a cursory correlation between 500 millibar atmospheric pressure increases and temperature, is there any other empirical evidence to support this claim? 

Ostro failed to entirely state his case and the argument he presented left a lot to be desired. 

As global temperatures have increased, the number of pirates
has decreased.  Is there a relationship similar to the decrease
in the number of tornadoes?  Image courtesy Vengaza.org.

Certainly one could take any number of charts, overlay it with the chart of global temperature increases, and seem to establish a correlation.  Global warming skeptics are quick to point out that solar activity when overlaid on a temperature chart is remarkably similar.  Similarly, one could produce a chart showing the global average temperature versus the number of pirates and find that increased global temperatures are responsible for a decrease in the number of pirates. 

Throwing a chart out that apparently correlates air pressures with global temperatures and saying that is the reason behind the decrease in tornadoes in 2009 is not a particularly compelling or convincing argument.  Further, the fact of the matter is that if you set aside this one year, the number of reported tornadoes has increased steadily since 1950.  That in and of itself would go counter to Ostro’s argument. 

Granted it is widely accepted that the increase in the number of tornado reports has as much to do with an increase in population density and technology as anything – there are more people to see and report twisters and radar technology allows us to ‘see’ tornadoes that otherwise would go unreported in rural areas.  However, it does speak to the point that oversimplifying the matter does little to make the point.

Lastly, it is very important to note that NOAA’s Storm Prediction Center statistics actually show that the actual number of tornadoes in 2009 is virtually spot-on the 2004 to 2009 average (image at right).  Any claim that there are fewer tornadoes than normal simply does not bear out with the facts. 

In the end Stu Ostro failed to make his case that global warming is responsible for a decrease in tornadoes this season.  His attempt to correlate atmospheric pressure to global temperatures simply showed what on the surface is a cursory correlation between the two.  Further, the very basis of his claim that there have been fewer tornadoes is simply untrue.