By now you’ve all heard of the EF-5 tornado that devastated Moore, Oklahoma earlier this week. (READ HERE about why we now use a different scale to determine tornado strength) A tragedy by all accounts and I hope FEMA acts swiftly and thoroughly.
As someone who studies climate for a living, I often get asked: What does climate change mean for extreme weather, like tornadoes and hurricanes?
First, tornadoes and hurricanes are entirely different beasts. They often get lumped together since they are both rotating systems. Their mechanics, however, are more dissimilar than they are alike. The relationship between hurricanes and climate change is a lot more clear-cut that that of tornadoes and climate change.
For this post, let’s talk just about tornadoes.
Tornadoes are typically associated with thunderstorms, when there are large values of convective available potential energy (CAPE) and deep-tropospheric wind shear. The intensity of tornadoes is dependent on the shear, and only slightly on thermodynamics.
That’s a lot of fancy technical talk for: tornado activity depends largely on how much energy is in the air when the air starts to lift, or move upward. All severe weather is a function of the amount of energy in the atmosphere.
Climate models have shown that CAPE will increase – a reflection of an overall increase in energy in the atmosphere due to the burning of fossil fuels. More CAPE means more days with tornado-favorable weather conditions (Brooks 2013). So, more tornadoes likely to happen.
What about strength?
Lee (2011) determined that EF-2 and stronger tornadoes will increase by as much as 12.7 percent over the next 70 years. The majority of the increase will occur early (April-May) during tornado season (normally April-June). An overall increase in tornado days (weather favorable for tornadic activity) will increase for most regions of the USA, especially for the Lower Great Plains, Lower Great Lakes, Mid-Atlantic States, and the Southeast. And more EF-2 and higher tornadoes will appear during these times.
Can we ever look at a single storm (Hurricanes Katrina and Sandy, the Moore tornado) and say “This storm was caused by climate change?” Not yet. Our atmosphere is a complex system of inputs, outputs and offsets, and extreme weather has been around for a while. But climate change is a threat-multiplier. What we have seen and will continue to see is that our “extreme weather” is becoming more and more extreme as global temperatures rise and we continue to pump fuel for these storms.