The big tornado outbreak, including a monster Oklahoma twister, have people asking again about a possible link to climate change. I’ll review the science in this post.
“The news helicopter from kfor.com caught this image of the shocking near-total destruction of a huge area of Moore, Oklahoma, on May 20, 2013.” Via Masters.
Tom Karl, the director of the National Climatic Data Center, explained in a 2011 email:
What we can say with confidence is that heavy and extreme precipitation events often associated with thunderstorms and convection are increasing and have been linked to human-induced changes in atmospheric composition.
Insured losses due to thunderstorms and tornadoes in the U.S. in 2012 dollars. Data and image from Property Claims Service, Munich Re.
Tornadoes “come from certain thunderstorms, usually super-cell thunderstorms,” explained climatologist Dr. Kevin Trenberth in an email today, but you need “a wind shear environment that promotes rotation.” Global warming may decrease the wind shear and that may counterbalance the impact on tornado generation from the increase in thunderstorm intensity.
Trenberth, the former head of the Climate Analysis Section of the National Center for Atmospheric Research, notes:
The main climate change connection is via the basic instability of the low level air that creates the convection and thunderstorms in the first place. Warmer and moister conditions are the key for unstable air.
The climate change effect is probably only a 5 to 10% effect in terms of the instability and subsequent rainfall, but it translates into up to a 32% effect in terms of damage. (It is highly nonlinear). So there is a chain of events and climate change mainly affects the first link: the basic buoyancy of the air is increased. Whether that translates into a super-cell storm and one with a tornado is largely chance weather.
After April 2011 saw records set for most tornadoes in a month and in 24 hours — “The Katrina of tornado outbreaks“ — I examined the climate/tornado link in great detail here, looking at the data, the literature, and expert analysis. That piece concluded:
- When discussing extreme weather and climate, tornadoes should not be conflated with the other extreme weather events for which the connection is considerably more straightforward and better documented, including deluges, droughts, and heat waves.
- Just because the tornado-warming link is more tenuous doesn’t mean that the subject of global warming should be avoided entirely when talking about tornadoes.
Early March 2012 saw what was likely “the most prolific five-day period of tornado activity on record for so early in the year,” as meteorologist Dr. Jeff Masters put it.
Then we had an unusually long “tornado drought” from May 2012 to April 2013, which has now come to a stunning end, punctuated by the devastating Moore, Oklahoma tornado yesterday:
A massive, mile-wide supercell tornado ripped through the suburbs of Oklahoma City, destroying homes, schools and other buildings. The tornado was on the ground for some 40 minutes, according to the National Weather Service (NWS), and police reported that an occupied elementary school was in the path of the cyclone. Early estimates had winds on the ground near 200 mph, which would have made the cyclone an F4 or higher. Witnesses said the damage was like something out of an atomic bomb strike, and there are at least 24 people dead, including many young children, with a toll that could eventually be far higher.
Masters says “the Moore tornado likely to be one of the five most damaging tornadoes in history,” which is particularly tragic because Moore had “previously experienced the 4th costliest tornado in world history, the notorious May 3, 1999 Bridgecreek-Moore EF-5 tornado.”
You can donate to the American Red Cross disaster relief here.
Below is an extended review of the scientific literature along with some analyses from this year and last year by leading experts.