The tornado outbreak across the southern United States in late April 2011 was deadly, devastating, and record breaking.

These days, when the weather breaks records, it’s natural to wonder if global warming is to blame. So it’s not surprising that in recent weeks, climate scientists have been fielding lots of questions about the possible connection between global warming and tornadoes.

Wondering and questioning are the foundation of science, but they are only the beginning. At NOAA, the Climate Attribution Rapid Response Team (aka the “CSI” team, for “Climate Scene Investigations”), led by Martin Hoerling of the Earth System Research Laboratory, tries to move the process forward from questions to answers. Last week, the team turned their focus to tornadoes.

Even before the April 2011 outbreak, scientists have been looking for long-term changes in U.S. tornado activity. The research that’s already been done paints an inconclusive picture. The number of smaller tornadoes seems to have increased; the number of large tornadoes has not.

Between better technology — radars, satellites, the internet — and greater public awareness, it’s likely that the increase is due to more reports, not more tornadoes.

If tornado reports are biased by better reporting and detection, how would we know if global warming has affected U.S tornado outbreaks?

If we can’t detect a change in tornadoes themselves, says Hoerling, we might be able to detect a long-term change in the weather conditions that contribute to tornadoes.

Key among those factors are the instability of the atmosphere, the amount of water vapor in the part of the atmosphere known as the planetary boundary layer, and vertical wind shear.

Instability

Tornados start with thunderstorms, and thunderstorms start with convection — the rising of hot air away from Earth’s surface. When a column of air is warmer at the surface than it is at higher altitudes, the air in that column is unstable. The air on the bottom is buoyant and will naturally rise, initiating convection.