What happened to autumn? Scientists point to climate change
Regarded by millions of Americans as their favorite season, autumn for many regions of the United States has traditionally been marked by the gradual transition from hot summer weather to frigid winter temperatures. But in recent years, fall seems to have all but disappeared — especially in the Northeast — and experts say climate change is partly to blame.
Throughout October of this year, a time normally associated with crisp weather and changing leaves, many parts of the Northeast saw temperatures upwards of 70 degrees Fahrenheit. And then, seemingly overnight, the weather turned much colder.
In Spokane, Wash., the warmest October on record was quickly followed in early November by the season’s first freeze and snowfall. By the end of the third week in November, snow had fallen all over upstate New York, and Buffalo, N.Y., received a record-breaking 6 feet of snow.
“We’re seeing this weather whiplash here in the fall, where it can be so warm, it can have record warm temperatures, and then very quickly we can transition into a very cold period,” Judah Cohen, director of seasonal forecasting at Atmospheric and Environmental Research, told Yahoo News.
Experts such as Cohen say climate change is a factor. Weather is fluky, and it’s impossible to ascribe most individual events to climate change, but climate change is creating the conditions, starting with a longer, hotter summer, that make fall-less years more likely.
Last year, the Associated Press reported that the fall foliage season was delayed by warmer weather from Maine to Oregon, and in some places it was ruined altogether by an unusually hot, dry summer that caused leaves to die prematurely.
“Summers are growing longer,” Cohen said. “September, a lot of times, acts more like August than what you traditionally consider a fall month. Summer is definitely encroaching on the fall season.”
“It’s staying warm later, for sure,” Matthew Barlow, a professor of climate science at the University of Massachusetts Lowell, told Yahoo News. “We’ve actually looked in the Northeast for fall weather patterns, and you can see that you get summer patterns later into fall.”
One might expect, then, that typical fall weather would just be shifted until later in the year, as it would take longer for temperatures to drop to those historically associated with winter. But there’s another wrinkle attributed to climate change that explains why winter temperatures suddenly crash onto the country, plunging millions into winterlike conditions: warmer Arctic temperatures in places like Alaska that send polar vortices southward.
“I would summarize the whipsawing from weather extremes this fall as the summation of two competing factors: ambient warming due to increasing greenhouse gases and an increase in polar vortex stretching,” Cohen said.
This is the same reason that we are seeing more extreme winter weather, such as a massive snowstorm that hit the South last January. Recent research finds the polar vortex is more frequently getting stretched out, which also brings cold air south. The reason is uncertain but appears to be related to the dramatic loss of Arctic sea ice, according to a study led by Cohen that appeared in Science magazine last year.
But while extreme winter weather gets all the attention, the same phenomenon can happen in the fall, making a week in November feel more like January.
“Climate change — but specifically the changes in the Arctic — lead to more disruptions of the polar vortex, where the polar vortex kind of stretches, or elongates, like a rubber band,” Cohen said. “And we’re definitely seeing an increase in those types of events in October through December.”
“I think it’s applicable to what happened this fall, because we’ve had these unusual polar-vortex-stretching events,” Cohen added. “So in October, there was this early cold weather snap and early snow. And I know places in the Southeast were having the earliest freezes ever.”
For example, Baton Rouge, La., saw its earliest freeze in history in mid-October.
“The cold snap that we’re seeing in November and extreme lake effects that we’re seeing here in western New York, that’s also associated with the stretched polar vortex event,” Cohen said, referring to the Buffalo snowstorm.
In a complementary phenomenon, the jet stream, a band of warm air flowing west to east, is also being destabilized by climate change. The jet stream is powered by the temperature differential between the Arctic and other regions, and the Arctic is warming four times faster than the rest of the Earth. As the difference in temperature decreases, a weakened jet stream is more frequently diverted southward, pulling a band of colder air above further south.
Scientists caution that the research on whether winter weather now strikes more frequently in fall, and why, isn’t as robust as the research showing that global warming is causing hot weather to last longer into the year.
“It’s certainly gotten cold here in a way that’s felt sudden, but I’m not really aware of anybody who has run those numbers to get at that variability piece of it,” Barlow said. “There isn’t a consensus on that, I think, yet, on whether there’s an increased breakdown or stretching of the polar vortex, or whether the jet stream is getting wavier, at least in the North Atlantic. There definitely is some evidence of that, but I don’t think there’s a consensus.”
The good news for people who want to take advantage of autumn’s outdoor rituals is that because polar-vortex-stretching events are temporary, the warm weather can sometimes bounce back. In Rhode Island, for example, the first freeze of the year in late October was followed by temperatures climbing back up to the 60s, just in time for trick-or-treating on Halloween.
