I find that the whole way this tornado discussion is going is disturbingly missing the point.
There is no compelling evidence that tornados are becoming more severe or more common, or for that matter, less severe or less common. There is no compelling modeling evidence that amounts to a meaningful prediction either way. So what should you say when you get hit by a tornado? You say "Dang! Shoulda moved to California when I had the chance." At least until the earthquake comes.
Does that mean that climate change had nothing to do with your tornado? Of course not.
This is a twenty-first century weather event. Everything is in a substantially changed and substantially changing context. Does it mean your bad fortune is "attributable" to climate change? Not in any reasonable sense of attribution. Unless, that is...
Unless your tornado happened to be part of an extraordinary subcontinental-scale tornado outbreak.
Now, this sort of outbreak event is not entirely unprecedented either, though it seems to be emerging as the single most severe instance in the satellite era.
But it's a, forgive me, black swan of a sort. One thing about really really severe events is that you can't do statistics on them. They are too rare to form a large enough collection to draw conclusions.
But you may have a flock of shouldn't-be-black stuff. Black doves. Black, um, pelicans. Black seagulls. Black other stuff. I'm not much on this bird business, but the thing is, although you know these things exist, and you know they are too rare to extract a trend, you shouldn't be seeing any of them very much.
Hot summers in Moscow. Year after year of flooding on the Mississippi. Huge tornado outbreaks. At what point do we get to look at the collection of weird events and say something is going on?
Let me admit, first of all, that there are all sorts of statistical warning signs associated with this question. Selection bias, observer bias, post hoc definitions. Our intuitions may well be misleading us.
On the other hand, there is the question of rolling a thirteen. The more we disturb the climate, the more excursions it will make into unfamiliar territory. As we perturb the climate, as it wobbles around more and more, it will more and more often hit these weird peaks.
Perhaps local events like tornados will show no trend, but tornado outbreaks, when they happen, will be more severe. The ocean, after all, is further from equilibrated with the atmosphere and with space. Air masses will encounter each other in unfamiliar ways. Perhaps strange things will happen. Perhaps (and I am not being Eli-style coy here, perhaps not) they already are happening.
The fact that the trend in tornados or strong tornados is too noisy to say very much doesn't change the fact that we just had an epic event, and that just about everywhere seems to be in a run of anomalous seasons. Are we enjoined from thinking about this for some reason? Maybe it's a misimpression, but maybe not.
Coupled GCM runs are expensive and not really mature. Atmosphere-only GCMs give us quasi-equilibrium responses and the atmosphere alone is strongly damped. But the bulk of the energy is in the ocean, which is a dynamic system with all sorts of bounce in it. What happens when you kick the ocean? You get wobbles on a decadal time scale. And what does this do to the atmosphere? Well, you're smarter than I am. You tell me.
The thing is, I see lots of people, some who ought to know better, buying into this sort of thinking, this instance from Revkin quoted by Kloor at the link above, emphasis added:
But that’s a meaningless assertion without asking whether there is evidence of a meaningful influence — meaning enough of a nudge to the atmosphere that the contribution from greenhouse gases is relevant to policy and personal choices, in this case in tornado zones.No, no, no! The idea is not: radiative imbalance causes warming everywhere which nudges the climate a bit. So many people think like this but it is wrong. Radiative imbalance pushes the climate around so much that eventually it changes enough to restore the balance. Almost surely surface warming is part of that. But a lot else is part of it.
In my opinion, people have it backwards, though, as to which is the nudger and which the nudgee. Forced climate change causes global warming as a response, not the other way around. And a lot else changes too. Talking about "nudges" is so massively wrong. The global mean surface temperature is very stable. It takes a hell of a kick to move global temperature as much as we are moving it, and the climate system is responding directly to the kick, not to the temperature change.
So what I think we should expect is changing climate on the time scale of the memory of the upper ocean, roughly a decade. And this process will accelerate over the lifetimes of anyone now living. The whole concept of "climate" weakens under the circumstances. And the more we disrupt the system on a large scale, particularly the more carbon we spew, the more and the bigger the black birds we will see.
This is why I'm baffled by arguments that start from the slightly more energetic and moist air columns. That would make sense if all else were equal, in other words, on nudge-world. But there is no reason whatsoever that I can see to expect all else to be equal.