Despite 2011, study shows tornadoes not getting worse

• Little evidence of any link to climate change
• Damage may have actually decreased over the decades
• 2011 was an outlier when it comes to tornadoes

As 2012 shapes up to be the least active year on record for tornadoes, new research suggests climate change is not necessarily driving up the number and force of killer twisters.

The research, accepted for publication in the journal Environmental Hazards, counters assertions that climate change is fueling this form of severe weather. The findings are limited to tornadoes, and don't delve into whether climate change could be affecting other weather extremes such as heat waves, floods or droughts.

"There is absolutely no evidence of an increase in damaging tornadoes," the authors write in the study, and they add that the number of U.S. tornadoes might actually have decreased since accurate records began in the 1950s.

On average, just over 1,300 tornadoes form each year in the USA, the vast majority of which cause very little or no damage.

The study was led by Roger Pielke, Jr., a professor of environmental studies at the University of Colorado. His co-authors are Kevin M. Simmons of Austin College in Sherman, Texas, and Daniel Sutter of Troy University in Troy, Ala. .

As awful as 2011 was, when 550 Americans died in tornadoes, last year appears to have been unusual and not part of a trend. Tornadoes and tornado damage can fluctuate wildly from year to year: 2012 is on track to be the quietest year on record for tornadoes in the USA, according to data from the Storm Prediction Center in Norman, Okla.

Additionally, the study finds that the damage from tornadoes has actually decreased since the 1950s, once the figures are "normalized," or adjusted, for inflation, wealth, and population.

This is the first paper to comprehensively "normalize" historical economic losses from U.S. tornadoes, Pielke says.

Another way of looking at it is that this study attempts to "adjust for the amount of stuff that's in the way," says Harold Brooks of the National Severe Storms Laboratory in Norman, who finds no fault with the study: "It's pretty reasonable," he says, adding that there is no sign that tornado incidence has worsened over the decades. "There is no long-term trend in tornadoes," he says.

Another expert, meteorologist Mike Smith of AccuWeather Enterprise Solutions in Wichita, Kan., agrees: "His findings do not surprise me. I do not see an upward trend in disasters in general and do not see one for tornadoes in particular, " Smith says. "I believe there is some evidence (but not enough to form a robust conclusion) that tornadoes were worse in the first half of the 20th century."

Smith, though, does acknowledge that "2011 sticks out like a sore thumb." In the dataset used in the study, 2011 was one of the top three years for tornado damage since 1950.

When it comes to climate change, Pielke adds that his study "provides strong counter-evidence to claims found in the scientific literature that the atmospheric environment that spawns tornadoes has intensified," leading to more destructive storms.

Indeed, as he notes, the Intergovernmental Panel on Climate Change's report on weather extremes earlier this year -- while claiming a link between global warming and other weather extremes -- found that "long-term trends in economic disaster losses adjusted for wealth and population increases" can't be laid at the feet of climate change.
The IPCC report adds that "there is low confidence" in a link between climate change and tornadoes. which can be affected by "competing physical processes."

Pielke's study provides a counterpoint to a report released earlier this month by Munich Re, the world's largest reinsurance firm, which noted that climate change is driving the increase in natural disasters such as the severe storms and tornadoes that hammered the U.S. last year.

Some of the other findings from the Environmental Hazards study include:

• Since 1950, the damage from tornadoes even adjusting for population and inflation has been half the amount for hurricanes, but twice that of earthquakes.

• The strongest two categories of tornadoes (called EF4 and EF5) represent about 1% of all reported events but have caused almost 45% of all normalized damage.

• The most damage overall from 1950-2011 has occurred in Texas and Alabama.

• The most damaging months are April (31% of total dollar damage), May (20%) and June (16%).