When Tucson and the rest of the Southwest baked in record heat this month, the most obvious cause was an overpowering, stagnant ridge of high-pressure air known to weather experts as a heat dome.
But underlying that traditional cause of extreme heat waves is likely the increasing impact of human-caused climate change, four climate scientists told the Arizona Daily Star.
The heat dome hung over the entire Southwest from around June 11 or June 12 through June 19. It brought eight record-setting consecutive days of at least 110 degrees in Tucson and six record-setting consecutive days of at least 115 degrees in Phoenix.
Temperatures in other Southwest cities were just about as bad. The heat wave has since dissipated, with early signs now appearing of Southern Arizona’s traditional monsoon season, including lower temperatures and occasional drizzles.
But a very similar but separate high pressure ridge is blanketing the Pacific Northwest, threatening Portland and Seattle with temperatures this weekend of 30 or more degrees higher than their seasonal norms in the 70s and 80s.
The Southwest’s heat wave was triggered by “a broad, upper-level ridge of high pressure,” said Michael Crimmins, a veteran University of Arizona environmental scientist. “It’s pretty common, as part of the transition from the spring weather pattern that the West normally sees into the summer. It was just exceptionally strong, exceptionally expansive and it persisted for several days.”
While it’s impossible to pinpoint specific impacts of the long-term global warming trends on this event without detailed study, more and more studies around the globe are attributing heat waves like this one to long-term climate change triggered by fossil fuel burning, scientists said.
“There have been so many formal studies on extreme heat waves over the last five years or so and so few of them don’t find a human fingerprint on these events,” said Daniel Swain, a climate scientist at UCLA’s Institute of the Environment and Sustainability. “I’d be very surprised if we do studies on the recent one, or the one about to unfold, and we didn’t find an increased likelihood or severity of that heat wave due to climate change.”
Climate change impacts are likely to be a factor in this heat wave because temperatures in this region have risen at among the fastest rates in the lower 48 U.S. states since the 1980s and 1990s, scientists said.
Moreover, the kind of extreme heat seen the week of June 12 through 19 is quite consistent with what many climate-related computer models have forecast for a long time — more frequent and more extreme heat waves, Crimmins said.
“I think it’s really hard to explain this heat wave without having climate change at least in part of the equation,” said Crimmins, an extension specialist in climate science in UA’s Department of Environmental Science.
“The trends in temperatures we see in the Southwest just raise the floor. As the floor gets higher, it’s easier to reach those extreme temperatures. The trend basically gives us that extra little boost to be able to reach these absurd temperatures,” he said.
The kinds of high pressure systems that cause heat domes occur with some regularity whether you are in a cooler or warmer climate, said Jane Baldwin, a postdoctoral research fellow at Columbia University.
“What is happening with global warming is that because we are starting from a warmer baseline, when that ... high comes in, it can just reach even more extreme temperatures than if we were in a cooler climate,” she said.
Plus, she said, the Intergovermental Panel on Climate Change, a global body of climate scientists organized through the United Nations, has produced a report saying the extreme weather event most clearly tied to global warming is a heat wave.
“The mechanism is very direct,” Baldwin said. “When the temperature warms, all the day-to-day wiggles in temperatures we experience in cooler climates start to cross over into thresholds where rather than the temperature being just a little bit warm, it might be dangerous for human health.”
“It does feel absurd”
Tucson’s streak of 110-degree-plus temperatures from June 12 through 19 broke a previous record of six straight days set in 1994. Phoenix’s streak of 115-degree plus temperatures in that June stretch broke a record of four straight such days, set on six separate occasions, including twice in 2020. Tucson had six days in that period of individual record-high daily temperatures, while Phoenix had four.
Elsewhere, Las Vegas hit 116 degrees one day during the heat wave. Palm Springs and Salt Lake City tied all-time record high temperatures of 123 and 107 degrees, respectively. Record daily highs were also seen in Southern California’s Inland Empire and parts of New Mexico, Montana and Wyoming.
“These temperatures were outside of our normal experience. 110 degrees here is extreme and to get that many days in a row and to flirt between 110 and 115, it’s not within your normal experience, and it does feel absurd for Tucson. And the Phoenix string was bad, too,” Crimmins said.
High pressure ridges like the one we saw aren’t by themselves extraordinary, he added. The weather map for the region looked like what you’d normally see as you progress from May to June to July.
“It’s just that everything about it had values that were higher than we normally see. The pattern was very typical but the values were extreme,” Crimmins said.
Advances in research methods
That scientists today say a heat wave like this one is likely connected even to some degree to climate change represents a major shift in thinking.
In June 2020, UCLA’s Swain and three colleagues published a paper in a scientific journal noting that the news media and public often ask if climate change causes a specific extreme weather event.
“In a very literal sense, the answer to such a rigidly posed question will always be ‘no.’ All events in the dynamically coupled Earth system are ultimately the product of numerous complex, interrelated processes acting across a wide range of spatiotemporal scales,” the study said. “There will thus rarely (if ever) be a traceable singular cause for any specific event.”
Indeed, a decade ago, a common response from scientists to that question was, “No single weather event can be attributed to climate change,” the study noted.
But in the past 10 to 15 years, advances in research methods and in computer modeling, combined with a lengthening record of continued warming temperatures, have, in the study’s words, “transformed what was once an unanswerable hypothesis into (an amenable) scientific question” that can be answered.
Instead of asking simply if climate change caused a heat wave or a hurricane, these advances “have opened the door to systematically addressing the question of whether climate change has influenced the likelihood and/or severity of individual extreme events,” the study said.
Scientists can’t say with a broad brush that extreme events in general get worse due to climate change because the answer to that question depends on what events they’re talking about, Swain said. But the extreme event that can be attributed to climate change with the highest level of confidence is “an extreme, unprecedented heat wave — what we’re experiencing now across the entire West,” Swain said.
One reason Swain said he’d be surprised if studies didn’t find this heat wave had some influence from climate change is that “there’s been so much research, so many formal studies on extreme heat waves over the last five years or so, and so few of them don’t find a human fingerprint on these events.”
Recent experiences of Tucson and Phoenix’s heat waves show it really does matter how hot it is, he added.
“A long streak of 100-degree days in Phoenix is a not so big a deal. A long streak of 110 to 115 degrees, it does matter, and duration matters,” he said.
The broad scope of this heat wave and the large number of temperature records broken make it a good candidate for such an “attribution study,” said UA climate scientist Gregg Garfin, depending on what other big weather events occur this year.
“It’s not so much an issue of did anthropogenic climate change affect the heat wave. It’s what percent? How much? It undoubtedly affected it somehow,” he said.
He cited a combination of decades of temperature increases, repeated computer model projections of such events, and the plethora of records that were just broken to back up his view.
“What an attribution study would tell you: Did it increase the odds of this kind of heat wave occurring by 10% or 50%?” Garfin said.
Above-normal rainfall foreseen
While the Pacific Northwest is now sizzling with the same kind of heat wave Tucson and Phoenix just had, our region is settling — for now — into a more typical summer monsoon season of periodic rains and more moderately hot temperatures.
For the next week, the National Weather Service predicts Tucson temperatures will stray above 100 several days but may only reach 110 degrees once. From Monday through Thursday, it’s possible the high temperature won’t hit 100 degrees at all.
Plus, after more than a year of extreme dry weather, the weather service predicts a 30% to 60% chance of showers and/or thunderstorms every day from Monday through Friday.
If that trend were to continue, it would become a dramatic change from the “nonsoon” of summer 2020, which saw only 1.62 inches of rain during the entire June 15 through Sept. 30 season — the second lowest on record.
And that’s precisely the forecast from the UA’s monthly Southwest Climate Assessment report.
It says the outlook for July through September is for increased chances for above-normal precipitation across much of Arizona. It also calls for three months of above- or below-normal temperatures across much of the Southwest U.S. and portions of northern Mexico.