Temperature records toppled, and some were smashed, over a vast region of the U.S. this past fall week, including record October highs in Washington, D.C., Atlanta, Newark, Raleigh, Nashville, New Orleans, Pensacola, Indianapolis, and beyond.
And in the past week, over 300 daily record highs were broken.
But, it's not the number of records that's so exceptional, noted Daniel Swain, a climate scientist at the National Center for Atmospheric Research.
"It's the geographic scope that's striking," Swain said.
The expansive fall heat wave, which brought triple-digit temperatures to Alabama, demonstrates how rising global temperatures amplify weather events. In this case, big swings in the jet stream — a relatively narrow band of high altitude, powerful winds that separates cooler northern air from warmer southern air — let chilly air swoop down into the western U.S. while allowing warmer tropical air to move north and settle over a large zone of the central and eastern U.S.
But, with relentlessly rising background temperatures on Earth, this warm weather event was given a kick. That mean's boosted odds of breaking or shattering records. And that's precisely what happened.
"When we experience an unprecedented heat event, there is almost unequivocally a human fingerprint on that extreme heat event," noted Swain.
"Clearly, in a warming world, we expect to see more records occurring on the warm side of the distribution, so this [fall heat wave] would be in alignment with that expectation," said Karin Gleason, a meteorologist at NOAA's National Centers for Environmental Information.
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These October temperatures were more like those we'd experience in August, Gleason added.
Record low temperatures were also set in parts of the West, as a dip in the jet stream allowed chilly air to travel into the lower 48, and produce some big snowstorms. "It brought a handful of record lows," noted Swain.
When it comes to climate, it's long term trends that matter. And the long term trends are clear. Over the last decade, record highs outpaced record lows in the U.S. by two to one.
"The trend is in exactly the direction we would expect as a result of a warming planet," Michael Mann, a climate scientist at Penn State University, told Mashable earlier this year.
"In fact, we are seeing an increase in daily heat records, and we are NOT seeing an increase in daily cold records," Mann said.
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Though, extreme heat events are not just driven by hotter air. There's another hugely influential factor at play: the changing jet stream. Though it's very much an active, evolving area of atmospheric research, there's mounting evidence that the jet stream, which is naturally liable to bend (and allow cold or warm are to move with it), is getting locked into more persistent and amplified wavy patterns, explained Jennifer Francis, an atmospheric scientist at the Woods Hole Research Center.
These wavy patterns can allow warmer, or colder, weather to stay locked in and settled over a region. Francis noted the persistently warm pattern over Alaska this year, helping July become the warmest month ever recorded in the state's history.
What's more, a warming Arctic — which has reduced Arctic sea ice to dismal levels over the last decade — may have an outsized influence in driving the jet stream to get stalled in such persistent patterns. As the Arctic warms, there's a weaker temperature difference between the tropics and the Arctic, potentially allowing the jet stream to experience these bigger swings and wobbles.
"Overall, the preponderance of papers support the idea that the Arctic warming so fast is tending to make these wavy patterns happen more often," said Francis.
And overall, Earth is feeling the heat. The last five years have been the five warmest years on record since quality record-keeping began in the 1880s. This June was the warmest June on record. July was the hottest month ever recorded. The last 13 years have had the lowest 13 Arctic sea ice extents on record. September just tied for the warmest September on record.
On a related note, Earth's atmospheric carbon dioxide emissions are now skyrocketing. CO2 levels haven't been this high in at least 800,000 years — though more likely millions of years. What's more, carbon dioxide levels are now rising at rates that are unprecedented in both the geologic and historic record.
