Record rainfall in California. Record flooding in the mid-West. Record tornado activity in the central and southeastern U.S. And, while federal forecasters are calling for a near-normal level of hurricane activity this summer, the first named storm formed almost two weeks before the official start of hurricane season. In fact, extreme and record-setting weather seems to be the norm this year.
“This year has been, I think, what might turn out to be a classic example [of climate change],” said Jennifer Francis, an atmospheric scientist with the climate change think tank Woods Hole Research Center. “We've had the same jet stream pattern in place basically ever since the middle of the winter.”
That persistence, and what Francis calls the waviness of the jet stream, are fingerprints of human-caused climate change, particularly the disproportionate warming of the Arctic. And Francis says that persistent, wavy jet stream pattern is linked to much of this spring’s unusual weather, from late spring snow in the Sierra Nevada to a heat wave in the southeast.
Connecting the dots between climate change and tornadoes has been particularly tricky for scientists, though. Francis says that’s, in part, because we don’t even know if, or how, tornado activity has changed.
“Going back just a couple of decades, we really didn't have good ways to measure whether they occurred or not, and where they occurred,” Francis explained. “So, our records going back into the previous, say, 30 or 40 years are really quite sketchy. That makes it really tough to know whether there's been any changes lately.”
But Francis says that scientists are noticing some changes. For one, tornadoes seem to be forming in clusters. The hotspots for tornado activity also appear to be shifting, with more tornadoes hitting a swath of the southeast known as Dixie Alley.
“There's a lot to be learned still, but there's most likely going to be some change just because we are changing the atmosphere so much,” Francis said. “There's a lot more heat in the atmosphere, and there's a lot more moisture in the atmosphere. And that moisture is fuel for all sorts of storms, even tornadoes.”
The picture is much clearer when it comes to hurricanes. Rising ocean temperatures in the tropical Atlantic provide energy to feed stronger storms. Francis says that doesn’t mean an increase in the total number of hurricanes (that hasn’t been observed and isn’t expected), but an increase in the strongest storms.
Another change in hurricanes that’s clearly linked to climate change is a tendency to intensify more rapidly. Francis points to Michael, which developed from a disturbance to a Category 3 hurricane overnight, as well as Florence and multiple typhoons in the Pacific Ocean.
“It makes all kinds of sense because we have all this extra energy now in the atmosphere and in the ocean,” Francis explained. “So, once a storm does form it has a lot of fuel to work with.”
And then, there’s rainfall. With seven percent more water vapor in the atmosphere as a result of human-caused warming, Francis says it’s only logical that hurricanes and other storms would produce more rainfall.
There’s also evidence that hurricanes are showing more of a tendency to move slowly and even stall in place. That tendency can exacerbate the potential for higher rainfall.
“So, it could be that we see more of these Harvey- and Florence-like storms that reach the coast, sit in one spot, and dump up to four or five feet of rain,” said Francis.
Rising ocean temperatures also allow hurricane season to extend later into the fall, and Francis says that increases the chances of hurricanes interacting with a wavier jet stream. Those interactions can result in “very bizarre” storm tracks, like Superstorm Sandy’s sharp left turn into the New Jersey coast, or Hurricane Florence’s persistent westward path into the Carolinas.
Francis notes that when hurricane forecasters call for near-normal activity, they’re only talking about the number of storms. And that doesn’t reflect the many ways in which climate change is making hurricane season anything but normal.