July was just the beginning.
Phoenix experienced its hottest month ever recorded in July 2020, with reliable records going back to 1895. Now, the warmest city in the U.S. broke another record: On Sunday, Phoenix hit a high temperature of at least 110 degrees Fahrenheit for the 34th time this year, with a torrid August still ahead. The previous record was 33 days, set in 2011.
This sustained heating trend, like many extreme environmental events today, is a potent weather event amplified by human-caused climate change.
“This is a dramatic event,” said Jeff Weber, a research meteorologist at the University Corporation for Atmospheric Research.
The big driver of 110 F days is a lack of moisture and rainfall that typically sweeps into the Southwest beginning in mid-July, an occurrence known as the North American monsoon. During the monsoon season, which lasts through September, storms from the south bring moisture, clouds, and rains to the Southwestern U.S., cooling the region.
But the monsoon has been mostly absent this year. This absence, combined with a relentless warming Southwest, spells heat records.
“You have an extreme lack of a monsoon this year superimposed on a warming trend,” said Daniel Swain, a climate scientist at the National Center for Atmospheric Research.
Arizona experienced its second-hottest May through July on record this year, and overall, average temperatures in the Southwest have increased by at least 1.6 F since the early 1900s. This added heat has exacerbated an extreme drought in the region, prompting Western states to slash their water use from the diminished Colorado River.
Temperatures increased in the Southwest between 1901 -2016.
Image: national climate assessment
Without a monsoon this year, kept away by a mass of high air pressure over the region (called a ridge), there are few clouds to block out sunlight and the ground dries out. As a result, little of the sun’s searing heat goes towards evaporating moisture from the ground. Instead, solar energy just directly heats the surface, relentlessly.
“It just cooks,” said Weber, noting that as a former Phoenix-dwelling meteorologist he once called this unpleasant weather pattern “the heat ridge of death.”
Phoenix specifically has the added problem of amplifying already hot air, something known as the “urban heat island effect.” Cities dominated by roads, buildings, and concrete absorb heat and then reemit it into the atmosphere. “Today Phoenix is a massive sprawling city with a huge amount of concrete,” noted Swain.
But, importantly, the heat is widespread. High temperatures might be amplified in developed Phoenix, but they’re occurring all over the desert.
“It just cooks.”
“This isn’t something that is super localized,” said Swain. “This is something that has happened across a large part of the Southwest.”
The missing rains this year will have outsized impacts. Arizona gets about 90 percent of its rain from the monsoons, explained Weber. So a drier summer will mean the next year is also drier.
“This will have long-lasting effects,” said Weber. “It’s all about water.”
The National Weather Service expects highs between 112 F to 116 F in the lower deserts later this week, which the agency notes is “abnormal” for this time of year.
This can be harmful, if not perilous, for the residents of overheating communities. Among weather extremes in the U.S., heat is one of the deadliest events.
Yes, the Southwest, particularly the low-lying valleys, naturally heat up during the spring and summer. But now, an unusual consistency of searing 110 F (or higher) temperatures is hitting the desert.
It’s hard on the notoriously tough desert life, too.
“Even the desert plants have a hard time tolerating this drought and heat,” said Weber, noting the withered and dead flora after such prolonged heat events. “It’s very devastating.”
