The forces of human-caused climate change triggered temperature spikes in both Tucson and Phoenix on well over half the days since our prolonged, record-setting heat wave began in late June, a science groupβs analysis found.
At the Starβs request, Climate Central used what it calls a Climate Shift Index to indicate how much climate change boosted each dayβs high, low and average temperatures in both cities from June 1 through Thursday, July 20. It found that since June 25 here and June 26 in Phoenix, the impacts of human-caused climate change triggered by fossil fuel burning and other greenhouse gas emissions began in a mild fashion and intensified through last week.
The analysis comes as the heat wave appears to have hit a peak in the past week. Temperatures have stayed stubbornly high for long enough now that this July could surpass the heat of July and August 2020, which have been Tucsonβs hottest two months on record, said Aaron Hardin, a National Weather Service meteorologist in Tucson, and Michael Crimmins, a University of Arizona professor and climate scientist.
Moreover, Climate Centralβs analysis concluded that despite Phoenixβs notoriously scorching temperatures compared to ours, climate change has spiked Tucsonβs temperatures during this heat wave more than Phoenixβs.
In that period, Tucson experienced 10 days in which temperatures topped 110 degrees, while Phoenix had 21 straight days of 110-degree-plus weather β record-setting sprees for both cities.
The capital cityβs temperatures peaked at 119 degrees for three straight days ending Thursday. Tucsonβs high temperature topped out at 112 those same days.
But the Climate Central analysis found that Tucson actually had more days in which climate change boosted its temperatures than Phoenix did. The difference in climate change impacts between the cities was particularly pronounced at night, when the Climate Central analysis found the strongest climate change effects overall.
Thatβs even though Phoenix had 11-straight nights ending Thursday when the low temperature never fell below 91; it was 97 on Wednesday night, July 19. Tucsonβs lows ranged from 78 to 87 on the days when climate change impacts were found.
The key to why Tucson had more days with high rankings on the climate index βis that weβre talking about how unusual a temperature is for that location and how much we can link to climate change (based on climate models and long-term trends),β said Andrew Pershing, Climate Centralβs vice president for science.
βThe simplest explanation is that while Phoenix is hotter, Tucsonβs temperatures have been more unusual,β Pershing said.
From June 25 through July 20, Tucsonβs maximum temperature exceeded its normal high by an average of 7.2 degrees daily. The daily difference in Phoenix averaged 6.7 degrees, Pershing said.
One reason for that could be that in Tucson, average temperatures in July typically decrease more rapidly than they do in Phoenix, which would naturally increase the gaps in the daily differences between current temperatures and normal ones, weather service meteorologist Hardin said. That would happen because Tucson normally gets a lot more monsoon moisture that lowers temperatures than Phoenix typically does, he said.
Also, the most intense area of temperature anomalies in this region has been centered a bit to Tucsonβs south and east, in Southwest New Mexico, around the Mexican border, Climate Centralβs Pershing added. Overall, Tucson has tended to be closer to the center of the βheat domeβ thatβs been linked to this heat wave than Phoenix has, Pershing said.
βThis is a monsterβ
βHeat domeβ is a commonly used term for a high pressure ridge that traps summertime heat beneath it, said Erinanne Saffell, Arizonaβs state climatologist. One or more heat domes have been elevating temperatures all over the Southwest and Deep South over the past few weeks.
βIt causes heat to build up, and with that heat buildup in the desert, it tends to stay here unless it gets pushed off to the north and east and west,β said Dalton Van Stratten, another weather service meteorologist in Tucson. βWhen the high is directly overhead, especially, and itβs very strong, you get really hot temperatures, a really hot pattern.β
Heat domes are common in this area in the summertime but they typically get pushed to the north and east as monsoon storms move in, said several weather experts. In fact, typically, a heat dome paves the way for the monsoon season.
βItβs the typical progression of our monsoon ridge. It starts in Mexico in May and June and typically builds north over Tucson in June,β said Crimmins, a professor and extension specialist for the University of Arizonaβs Department of Environmental Science.
If all is going well weather-wise, the heat dome pushes north and east into the Four Corners area by now, allowing monsoons to roll into this region from Mexico, Crimmins said. But this year, itβs stuck farther south, centered right now over Central Arizona and wobbling at times towards Southern Arizona, he said.
βThey are a normal, natural thing. This one is a little bit more extreme, longer lasting, impacting a lot more people than it normally does,β said state climatologist Saffell.
From Climate Centralβs Pershingβs perspective, the current heat dome βis just a super remarkable event β the insistence, the persistence of this. This is a monster β really, really scary.β
Climate Central started noticing the heat dome impact in its weather data starting βbasically in the middle of June, when we started to see heat in Mexico start to peak,β Pershing said. βThen, there was a three-day break. Then, a new pulse came in, even more intense and bigger,β he said.
It has covered parts of Mexico, California, Utah and Arizona of late β βthe fact that these conditions are sticking around for a week is really remarkable,β Pershing said.
Climate Shift Index
Generally, the Climate Shift Index employed by Climate Central examines the difference between a given dayβs temperature and its 30-year average temperature for the same day, Pershing said.
βThe best way to think of it is that weβre trying to estimate how frequently our current temperature, which is 1.3 degrees Celsius (1.8 degree Fahrenheit) warmer than pre-industrial temperatures, would occur in the current climate. Then, we compare that with the temperature that would happen without human-caused global warming,β Pershing said.
βThink of it as a world without global warming. The last time that happened was around 1900. But weβre not literally saying what the frequency would have been in 1900. Weβre literally trying to say what would happen today without global warming.β
The index ranks each dayβs temperatures analyzed on a scale of 0 to 5.
A zero means that dayβs temperature had a weak or no link to climate change, Pershing said. A 1 score means that dayβs temperature is one and one-half times more likely to occur due to climate change. Scores of 2 through 5 mean the dayβs temperature was 2 to 5 times more likely to occur because of climate change.
Hereβs how those rankings have played out in Tucson and Phoenix from June 25 through Thursday:
β Tucson had 18 days where daytime high temperatures scored at least 1 on the index, and 23 days whose average and overnight low temperatures warranted at least a 1 ranking.
β Phoenix had 15 days whose high temperatures merited at least a 1 score on the Climate Shift Index. It had 19 days whose overnight low was warm enough to warrant at least a 1 score, and 16 days whose average temperature warranted the 1 score.
β In Tucson, no high daytime temperatures were hot enough to deserve a 5 rating, although July 19th and 20th got 4 rankings. But 13 overnight lows earned a 5 ranking from the climate index, including every night since July 9.
β No daytime high temperatures earned a 5 rating in Phoenix and two days also scored 4βs. Ten nighttime low temperatures there got 5 rankings β the last 10 days studied through Thursday.
Big nighttime difference
That nighttime temperatures detected in the two cities during this heat wave were higher compared to normal nighttime temperatures than high temperatures were, compared to normal high temperatures, would come as no surprise to most knowledgeable climate-watchers.
Thatβs because the urban heat island effect β in which the use of concrete and other forms of hard surfaces on buildings, pavement, sidewalks and the like jacks up nighttime temperatures β often has a bigger effect on the climate than global warming, many experts have said.
But in the current heat wave, the way the data has appeared worldwide βmakes me feel very confident there is a strong global warming patternβ affecting the greater nighttime temperature differential from the norm compared to those in the daytime, Pershing said.
βWe see this nighttime pattern everywhere around the world, especially within 30 degrees latitude of the equator. Youβre pretty close to that in Tucson,β he said. βYouβre maybe like 35 degrees. The closer you get to the equator, we get a much stronger climate change signal at nighttime temperatures.β
Still, another factor is that the weather signal is quieter at night than during the day, making it easier to pick up the signal of climate change impacts, he said.
During the day, it a cloud passes over, it cools things down. If itβs not cloudy, that warms things up. But at night, thereβs less temperature variability than during the day, he said β βthe temperature you get at night is a really good indicator of heat in the system.β
Asked if he saw this summerβs heat wave occurring more in the future, he first referred a reporter to federal government climate assessment reports, then said, βThe short answer is more.β
βEvery time we look at data for Southwest, especially in the summer, our mouths just hang open,β Pershing said. βThe trends there are so strong compared to other places we look at around the country. Every summer, we expect a week or more of intense heat (there). Every year, a week stretches into two, then three. That trend has gone on for two or three years. When you see a trend like that, youβve got to bet on it continuing.β
βHighly credible, science-basedβ
Kathy Jacobs is a University of Arizona professor of environmental science and director of UAβs Center for Climate Adaptation Science and Solutions. While not a climate scientist herself, during the Obama administration she was director of the National Climate Assessment, which analyzed climate change trends across the U.S. She also served on a National Academy of Sciences panel that wrote a 2016 report on the science of attributing extreme weather events to climate change.
On reviewing Climate Centralβs findings about Tucson and Phoenix temperatures at the Starβs request, she said: βI was not previously aware of the Climate Central Climate Shift Index, but I think it is a good communication tool. It is a great way to give people an idea of how much the temperature in a particular day is affected by climate change.
βNot having been involved in creating it, I can only assume that they have used standard statistical processes for attribution, which have been developed over the last two decades. They are a highly credible, science-based group.β
βIt makes total sense that the index is measuring the influence of climate change against a background of weather data βpre-climate change,ββ Jacobs added. Since Tucsonβs temperatures are normally lower than Phoenixβs, βwe can still have significant climate change influence if the actual temperatures are lower.β
The UAβs Crimmins and State Climatologist Saffell said they couldnβt comment on the Climate Shift Index because they lacked time last week to review the detailed methodology used to produce it.
They both said they thought there was a connection between the past monthsβ extreme heat and climate change, but Crimminsβ support for that link was far stronger than Saffellβs.
βThe temperatures observed during this heat wave absolutely have climate change fingerprints on it. If we had a heat wave of this magnitude and pattern 50 years ago, the temperatures wouldnβt reach that level of todayβs temperatures,β Crimmins said.
βThe (temperature) values are much higher than you normally see in this kind of weather pattern. That is the temperature trend expressing itself everywhere at once β itβs just the overall trend of increasing temperatures in the planet. It makes every heat wave a little bit hotter because of that trend,β he added.
Thereβs no doubt that this persistent and strong ridge of high pressure has some impact from a warmer atmosphere, Saffell said. But as for simple temperature anomalies, as documented by Climate Center, βI donβt think they can be used exclusively to identify attribution,β she said.
She cited a chart, showing maximum June temperature anomalies for Arizona, that showed the unusual temperatures rotated repeatedly between those that were well above normal and well below normal.
βIβd hesitate to identify attributions from this chart,β she said.
At the same time, however, Saffell noted that recently published research has attributed a stalled jet stream over Greenland to increased warming at high latitudes on the planet and to a continued reduction in long-term snow cover across North America. The research was published this summer in the journal Nature Communications.