To create a tornado, you need to blend four ingredients: warm, moist air, wind shear, atmospheric instability, and a cold front, or something else that lifts air.

But we’d need to look at the molecular level to see when the start button on the blender is being pushed.

“We basically have gotten to the point that we know the mechanisms that occur, but our ability to detect those mechanisms, in real time … doesn’t exist,” said Stephen Strader, a tornado researcher at Villanova University.

This past week, Strader has been involved in many conversations about another sector of tornado research that’s unclear: attribution science, which determines how much climate change contributed to an event compared to other factors, such as natural fluctuations in the atmosphere and oceans.

Research shows that climate change will increase the number of warm, moist days — Memphis and a dozen other cities hit record high temperatures last Friday. But there isn’t clear evidence on whether there will be an increase in other atmospheric conditions, like wind shear, which is wind going in different directions and speeds at different elevations.

“Observed and projected future increases in certain types of extreme weather, such as heavy rainfall and extreme heat, can be directly linked to a warmer world. Other types of extreme weather, such as tornadoes, hail, and thunderstorms, are also exhibiting changes that may be related to climate change, but scientific understanding is not yet detailed enough to confidently project the direction and magnitude of future change,” authors wrote in the Fourth National Climate Assessment.

Climate models struggle with the small size of tornadoes. The latest outbreak included a tornado that traveled more than 100 miles, but it was still just a mile wide at most. This inability to create high-resolution models makes it hard to predict future risks and draw the link between climate change and tornadoes.

But models are improving.

“What we do in our model systems now is look for those ingredients in a climate change world, and that helps us understand where tornado environments might be changing,” Strader said.

Tornadoes are not increasing. But they are clustering. 

On a broader scale, there has not been an observable increase in the quantity or intensity of tornadoes in the past 50 years in the U.S. But there is some evidence that tornadoes are shifting eastward due to warming temperatures.

From 1950 to 2020, Middle Tennessee averaged nine tornadoes per year. That jumped to 17 per year from 1995 to 2020, which partially reflects better tornado monitoring.

There is research that suggests tornado outbreaks are becoming more clustered, though scientists aren’t sure why. In the U.S., the total number of annual tornadoes has remained relatively stable, but there are fewer days with at least one tornado and more days with multiple tornadoes.

The latest event will rank as Middle Tennessee’s fifth-largest tornado outbreak ever. There were 16 tornadoes in Middle Tennessee on Saturday, and that’s on top of five other tornadoes earlier in the month.

The timing is also unusual. From 1811 to 2020, there had only been 16 tornadoes in December in Middle Tennessee, according to the National Weather Service.

So, there are plenty of puzzles. But Strader says the attribution science should not obscure the role of deficient planning and infrastructure in weather fatalities.

“The window of opportunity to act — and not, sort of, make climate change the scapegoat — is now,” Strader said.

Either way, he says making society more resilient won’t hurt. Climate change from burning fossil fuels is making many forms of extreme weather worse, and, with every new storm, it’s getting harder for communities to recover before the next disaster.

This story and headline was updated on March 13, 2022, to reflect revised numbers from the National Weather Service. Originally NWS reported 15 tornadoes on Dec. 10-11, tying it for fifth place. They later revised it to 16 tornadoes.