Few sights on Earth are as awe-inspiring (or terrifying) as that of a tornado spindling down from a storm-choked sky. So we feed our obsession. We flood the Internet with twister footage and headlines about peculiar vortices and rampaging storms.
It all leads to an inevitable question: Is tornadic activity increasing?
"There's no evidence that it is," says Joshua Wurman, president of the Center for Severe Weather Research. "If you look at the frequency of tornado reports in the U.S., they're going up every decade, but there's pretty good evidence that it's due to improved reporting efficiency."
Scientists call this a reporting effect, meaning that reports of a phenomenon increase but actual occurrences do not. Yes, it might seem like more trees are falling in the forest, but it's only because more people are there to hear them.
Population growth and technological advances are at the heart of the statistical rise in tornado activity, Wurman says.
Research meteorologist Harold Brooks of the National Severe Storms Laboratory, however, warns that reporting effects may not be the only factor at work.
"We just don't know," Brooks says. "It could all be reporting effects, but we obviously can't be sure. There could be real meteorological changes going on, but changes in the reporting system make it impossible to discover."
The issue comes down to the way we record data on tornadoes versus other meteorological phenomena, Brooks says.
"Tornadoes are target-of-opportunity observations," Brooks says. "There has to be someone there who sees the event, or if the impacts of the event are noticeable, then there has to be a system in place to collect that report and to preserve it for posterity."
By comparison, temperature data collection is pretty straightforward: You place a thermometer at given location, check the findings frequently and record the data for a century.
Plus, tornado reporting in the United Sates largely aims to save lives by providing advance tornado warning, not climate data. Even the Enhanced Fujita Scale for rating tornadoes depends on damage to human structures. Tree damage is sometimes helpful, but trees fall easily and are scarce in twister hotspots such as the Great Plains.
"I've seen strong tornadoes going through open fields, and they don't really do a whole lot of damage," Wurman says. "They just break some grass, even though the wind might have been very, very strong. So it's difficult to rate them."
Wurman's Doppler on Wheels (DOW) provides meteorologists with detailed, on-site tornado analysis, but only a few radar systems of this kind exist. There's simply too much storm activity to cover. Even the 2009 VORTEX2 program, the largest field study of tornadoes in history, could only study a small fraction of U.S. tornadic activity.
What Would It Take?
What would it take for widespread tornadic activity to increase? According to Brooks, it essentially comes down to three key ingredients: warm moisture at low levels, cold air at high levels and changes in wind direction that produce rotation.
Meteorologists can predict how these ingredients might change in the future with the help of computer climate models.
"The big signal in North America will be an increase in the low level moisture," Brooks says. "So that's a favorable thing for tornadoes. The temperatures aloft will probably warm a little bit, but not enough to counterbalance. So we'll have a more favorable thermodynamic profile for tornadic storms."
Wind, however, depends largely on temperature differences between areas - such as the difference between the poles and the equator. Climate models predict the Earth's poles will warm more than its equator, which means less of a gap between temperatures and less favorable wind shear. This situation, Brooks says, would be unfavorable for tornadoes.
So it really comes down to chance. Will North America experience tornado-strengthening temperatures and moisture levels before poor wind shear conditions kick in? Or will the changes occur congruently, calming tornado-producing storms?
"At this point, it's really tough to tell which will win," Brooks says.