When tornadoes form and reach down to the Earth in sinister columns of 100 mph winds, they do so for so many highly specific reasons that it's almost impossible to predict where they might hit or how strong they will be.

The recipe for a tornado has to be Goldilocks-specific. Air temperatures on the ground have to be just right, as do air temperatures in the upper atmosphere. The air has to mix in the right place and at the right time of year, and there has to be a storm strong enough to stir it all together.

It's for reasons like this that tornado occurrences are so unevenly distributed. Tornadoes have hit every continent on the planet except Antarctica, but the vast majority of tornadoes hit in the United States—specifically in the south and southeast United States, or "tornado alley," where cold air blowing east from the Rocky Mountains can mix to deadly effect with warm air blowing north from the Gulf of Mexico.

Roughly 1,000 tornadoes touch down in the United States every year, the most for any country according to the National Climatic Data Center. Canada is a distant second with about 100 every year.

Given the high sensitivity inherent in tornado formation and the lack of any clear pattern in tornado events, scientists have been cautious in linking the storms to climate change in any way. But they are starting to notice certain trends, namely when tornadoes are occurring.

Fewer but fiercer tornado days
New analysis of tornado counts in the United States dating back to the 1950s has found that, while America has consistently seen around 1,000 tornadoes each year, the number of days in which those tornadoes have occurred has been steadily decreasing. This trend equates to an increase in "tornado density"—in other words, the United States has been seeing the same number of tornadoes, but on fewer days.

When those days do come, however, they come with even greater ferocity, according to James Elsner, a geography professor at Florida State University and lead author on the research, published earlier this month in the journal Climate Dynamics.

The study itself doesn't go much further than that. Elsner and his team point out the statistical trend—that "the risk of big tornado days featuring densely concentrated tornado outbreaks is on the rise"—but only a suggest a hypothesis for what environmental changes might be behind this trend.

The hypothesis relates to an important component in tornado formation: the mixing of warm air on the surface and cold air in the upper atmosphere. When these different air flows are mixed together in a stormy environment with specific convective pressures, they can spiral together to create a tornado. Elsner thinks that climate change warming the atmosphere could be stifling tornado formation throughout the year, until a pocket of cold air comes along.

"We speculate that because there's warming both at the surface and above, that's kind of stalemate that leads to fewer days with tornadoes," Elsner said. "But when they come, they come in bunches."

There have been notable events in recent years when large numbers of tornadoes hit a single area on the same day. On April 27, 2011, for example, 62 tornadoes touched down in Alabama, leaving 250 people dead and 23,000 homes damaged or destroyed (ClimateWire, April 30).

Kevin Trenberth, a senior scientist at the National Center for Atmospheric Research, said that the study provides further evidence for what many climate scientists already suspected. In an interview with Scientific American last year, Trenberth discussed the faint connection between climate change and more extreme tornado activity.

Still, he said the study of tornadoes is "fraught with difficulty" and suggested that scientists may only be able to ultimately gain a better understanding of the probability that tornadoes form in certain weather conditions.

"That's probably about the best we can ever do," Trenberth said.

'Different playing field' for researchers
Harold Brooks, a senior research scientist at the National Oceanic and Atmospheric Administration's National Severe Storms Laboratory, said that the trend in increasing tornado density is something that he has been talking about at scientific conferences for almost two years.

"I think that [trend] is real," Brooks said. "Now why [it's real] is a completely different question."

Brooks is more skeptical of the physical mechanism Elsner's team proposes. Elsner himself described his hypothesis as "tantalizing, but you can't conclude with certainty" that the trend his team has highlighted is a result of climate change. He added, however, that the findings could push the scientific debate over tornadoes and climate change "onto a different playing field."

"I think prior to this work scientists had looked at only the number of tornadoes and concluded, based on that metric, that climate change was not apparently affecting tornadoes. But what this study shows is there are other metrics," Elsner said.

Elsner added that in the past at scientific conferences on tornadoes, there was very little discussion of climate change.

"I think that's going to change now," he said.

"Our hypothesis of course is just one of several, so we need to try and understand which one might be the case, and I think we're going to see a lot of research on this topic," Elsner said.

Brooks said tackling the physical mechanism is the next big challenge for scientists, and he said several scientists are already working on it. Elsner and his team "identified an interesting result," Brooks said, "we just aren't sure what to do with it yet.

"A bunch of us have ideas as to how it might work, but getting enough quality data to verify it, that's hard," he added.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500