Does it seem to you that the recent spate of extreme weather has been more extreme than usual?
Record heat and more record heat. Flooding, flooding, and more flooding. Severe thunderstorms. All of the above in the same place at the same time.
People want to know why. People have always wanted to know why.
A couple of decades ago, I could provide a satisfactory explanation simply by stringing three random letters together: “It was caused by the PNA pattern.” “The AMO was strongly positive.” “The MJB had extra caffeine.”
Folks would nod their heads acceptingly and go on with their business. Somehow, knowing there was a scientific explanation was more satisfying than hearing that it was a random, freak event.
Nowadays, the most common explanations are “climate change” and “just ordinary weather.” Tell the truth: does one of those two explanations make you angry every time you hear it?
If so, I bet the following statements are true: you have a strongly-held position on climate change, and you think the other side is being either deceptive or stupid, if not both.
As a meteorologist and climatologist, it bothers me whenever either explanation is used wrongly. But sometimes, both explanations are valid, because “cause” can mean different things to different people.
Take the European heat wave, recently centered on France. You can find locations for which the heat was unprecedented since weather records have been kept.
From a weather perspective, it was caused by an enormous excursion of the jet stream, with one portion of the jet stream blowing northward from the western Sahara Desert all the way to Norway. By one measure, this excursion was the strongest ever. If you get that weather pattern, you’ll have remarkably hot and dry weather in Western Europe, no question.
Then you might wonder what caused that enormous excursion. From a weather perspective, I think I can trace it back to a storm over Japan a few weeks ago, which triggered a succession of jet stream waves and storms progressing eastward across the Pacific and Atlantic, finally reaching its apex in just the right (or wrong?) place.
Did climate change cause the weather pattern? Now it gets tricky. It seems obvious that climate change didn’t cause last week in France to be warmer than the week before or after. But the whole atmosphere is different because of the extra greenhouse gases and their effects, so none of the weather we presently experience would have happened the way it did without climate change.
Let’s try a slightly different question: Would the heat wave have been as intense without climate change? That doesn’t work either, because as I just said, this particular heat wave wouldn’t have happened without climate change. We would have had heat waves, sure, but they would have been different ones, on different days.
Okay, how about: Are intense heat waves (like this one) becoming less unusual? Or: are unusual heat waves (like this one) becoming more intense? These turn out to be the same question. If it’s yes for one, it’s yes for the other.
For the France heat, there’s already been a scientific study completed. It’s too soon to have been peer-reviewed, but it used established methods, and it’s by a team that focuses on this sort of thing, so I trust it. And, as it turns out, it matches what experts had been thinking in real time.
They found that, according to climate models, the extreme heat in France was about five times as likely to happen with climate change than without, corresponding to an increase in temperature of over 3 degrees Fahrenheit.
Don’t believe climate models? Try the observations, which say that the extreme heat was more like a hundred times as likely to happen with climate change, with heat wave temperatures warmed by about 7 degrees Fahrenheit.
Models can be unreliable because they’re, well, not real, and observations can be unreliable because it’s hard to tease out natural variability and local influences. But the combination — plus the basic fact that since the Earth is warming, heat waves ought to become more intense on average anyway — makes a darned strong case for global warming.
Not that the event could not possibly have happened without global warming, but we probably would never have lived to see it. Now we’re lucky enough to be able to expect (and plan for) another one within a couple of decades. Indeed, France’s experience with the 2003 heat wave, and the expectation that worse events were likely to come, aided its preparations for this one.
Meanwhile, if you’re a “just ordinary weather” fan, the big hailstorm that made a splash in Guadalajara was probably not affected one way or the other by climate change. So, there’s that.
John Nielsen-Gammon is Regents professor of Atmospheric Sciences at Texas A&M University and serves as the Texas state climatologist. He is a fellow of the American Meteorological Society. Follow him on Twitter at @ClimaTexas.