The fall season is in full swing, but folks in the Southwest can be forgiven for not noticing.
Parts of Arizona, Nevada and New Mexico are in the grip of an extraordinary heat wave, with unseasonably warm temperatures 10 to 20 degrees Fahrenheit above average extending into mid-October. The heat is expected to finally break over the weekend as a cold front brings cooler conditions.
Phoenix in particular seemed locked into what feels like a never-ending summer. The city finally broke a streak of 21 consecutive days of record-breaking daily high temperatures on Tuesday. From September 24 through Monday, 19 new all-time highs were recorded, with two days tying previous records. All those temperatures were well into the triple digits.
"It's hard to shock me with things like this, but the weather in Phoenix was just incredible," said Jennifer Brady, senior data analyst and research manager at Climate Central, a nonprofit research group headquartered in New Jersey. "When you see those numbers, you think it's a typo. Even for those of us who follow climate change and know the trends, this is truly extraordinary and disturbing."
On Monday, Phoenix hit a record high of 103 F, according to the National Weather Service. A week before that, on Oct. 7, the city hit 110 degrees, and the day before the high was 113 degrees.
The extreme temperatures in the city are part of a trend of longer and more intense heat waves due to climate change. Such punishing conditions are expected to begin earlier in the spring, continue later into the fall, and generally become much more common in many parts of the country as the world continues to warm.
This summer was already the hottest on record in Phoenix, and 2024 will likely end up as the hottest year in the city's history. So far, Phoenix has had a total of 70 days with high temperatures of 110 degrees or more, a record that broke the city's previous record of 55 days set last year.
From May 27 to September 16, Phoenix was warm at or above 100 degrees for 113 consecutive days. Then temperatures rose into the triple digits again late last month.
"It's just amazing how long it went on without any kind of break," Brady said. "There is no end to it. And then you wonder: how can people live with so much persistent heat?"
Cooling centers, hydration stations and other heat mitigation efforts in Phoenix were scheduled to end on September 30, but remained open through October 7 due to the fall heat wave, the Arizona Republic reported. The Maricopa Association of Governments also announced that its Heat Relief Network map, which tracks cooling, water and donation locations, will remain in place until October 31.
Outside of Phoenix, record high temperatures were also recorded in Nevada and New Mexico in October. Temperatures in Las Vegas have reached 100 degrees six times so far in October, a figure that the National Weather Service says is unprecedented in the 87 years since record-keeping in the city began.
In New Mexico, the Albuquerque branch of the National Weather Service said on Oct. 5 that temperatures in the state at the time - which ranged from the mid-80s to 95 degrees - were "more typical for mid-August."
According to the National Weather Service, extreme heat causes more deaths nationwide each year than any other weather event.
Maricopa County, where Phoenix is located, has had 389 confirmed heat-related deaths so far this year, with 292 still under investigation, according to county data. Last year, the province reported a record 645 heat-related deaths. It's too early to know how this year will compare, officials said.
Part of the problem for Phoenix, like many large metropolitan areas, is that it tends to experience higher temperatures than the more rural parts of the state, due to a phenomenon called the "urban heat island effect."
Densely populated urban areas have buildings, roads, and other man-made structures that absorb and retain more heat than natural landscapes, putting cities like Phoenix at particular risk from extreme heat.
In a study published in July, Brady and her colleagues at Climate Central analyzed the urban heat island effect in 65 major US cities, where about 15% of the country's population lives. They found that temperatures in Phoenix were about 7.4 degrees warmer on average, due to the city's built environment.
The researchers also found that underserved and disadvantaged communities bear a disproportionate burden of that urban heat, putting them at even greater risk when temperatures soar.
The phenomenon is not limited to the downtown area, Brady added.
"If you have a suburb with a lot of pavement, a lot of buildings and no trees, it's going to be a lot warmer there too," she said.
In addition to the threat to human health, intense and prolonged heat has a broader impact on the planet. More months of summer-like conditions mean longer wildfire seasons, said John Mejia, an associate professor at the Desert Research Institute in Nevada. And warm temperatures earlier in the spring and later in the fall also disrupt the natural flow of Earth's water cycle, changing the availability of water circulating between the oceans, soil and atmosphere.
"Ecosystems are highly dependent on a lull, with cycles of hot and cold, hot and cold," Mejia said.
Last month, the National Oceanic and Atmospheric Administration said the planet's global surface temperatures in August were more than 2 degrees Fahrenheit above the 20th century average, making it the warmest August on record and extending a 15-month streak.
"If you average all temperatures around the world, temperatures have been above record levels for 15 months in a row," Mejia said. "We pretty much expect that because we're increasing base temperatures due to global warming."
Still, the spate of heat records broken in recent weeks has left many experts baffled, as such high temperatures are rare in October.
"We are quite perplexed," Mejia said, "because this is really abnormal."
This article was originally published on NBCNews.com
