The summer of 2023 has featured the most intense heat in modern records averaged over the planet. June was Earth’s hottest on record, and the oceans are exhibiting unprecedented warmth. Far and wide, already-exceptional weather events are being pushed into record territory by the effects of human-caused climate change.
Inside the most extreme heat wave the Southern U.S. has faced
The exceptional heat will be remembered for its intensity and duration
and
July 21, 2023 at 6:00 a.m. EDT
In the Lower 48 states, global warming has manifested itself in a historically intense and prolonged heat wave, stretching from California’s interior to South Florida. The zone from Arizona to Texas has sat at the center, with record-shattering heat enduring for at least three weeks and showing little sign of relenting.
A concentrated sphere of heat, known colloquially as a “heat dome,” has powered the excessively high temperatures. Over the coming week, the heat dome will reach from coast to coast, inching northward and parking smack dab in the middle of the country. Over the next eight to 14 days, the National Weather Service is calling for above-normal temperatures nearly everywhere in the continental United States.
Max temperature, July 18-21
Less than
80° F
80° F
90° F
100° F
110° F
and
greater
A relentless and punishing heat dome has set temperature records over the southern United States since the beginning of July.
Heat dome
Concurrently, historically severe heat domes have sprawled over southern Europe and parts of Asia.
Heat domes
Source: NOAA
Max temperature, July 18-21
Less than
80° F
80° F
90° F
100° F
110° F
A relentless and punishing heat dome has set temperature records over the southern United States since the beginning of July.
Heat dome
Concurrently, historically severe heat domes have sprawled over southern Europe and parts of Asia.
Heat domes
Source: NOAA
Max temperature, July 18-21
Less than
80° F
80° F
90° F
100° F
110° F
A relentless and punishing heat dome has set temperature records over the southern United States since the beginning of July.
Heat
dome
Heat domes
Concurrently, historically severe heat domes have sprawled over southern Europe and parts of Asia.
Source: NOAA
This massive heat dome is one of several affecting the planet, bringing all-time records in parts of Europe and in China and alarmingly hot sea-surface temperatures to the Atlantic. While heat domes form every summer, recent years have featured a string of particularly anomalous heat waves.
“Human influence has likely increased the chance of compound extreme events since the 1950s,” reads the most recent assessment from the United Nations Intergovernmental Panel on Climate Change. “This includes increases in the frequency of concurrent heat waves and droughts on the global scale.”
In other words, what we’re witnessing now — multiple simultaneous record-setting heat domes globally — is exactly what scientists expect as increasing greenhouse concentrations from human activities warm the planet.
A heat dome of exceptional strength and duration
Beneath the heat dome baking the southern U.S., numerous cities have set record highs. Reno, Nev., and Grand Junction, Colo., tied all-time records of 108 and 107 degrees on Sunday and Monday, respectively. Other cities came very close. Salt Lake City fell a degree shy of its all-time record at 106 degrees, as did Las Vegas, at 116.
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We’re tracking dangerous heat waves across the United States daily. Look up your city to see extreme heat risks near you.
The nation’s most extreme temperatures have occurred in Arizona and Southern California. Death Valley, Calif., reached 128 degrees Sunday, two degrees from the highest temperature globally over the past 90 years. Meanwhile, Phoenix has tied or broken calendar day record highs seven times in eight days; it hit 119 degrees Wednesday and Thursday, its highest temperature since 2017.
Phoenix also registered its all-time warmest low temperature on Wednesday, dropping to just 97 degrees, to produce an average daily temperature of 108.0 degrees, its highest on record.
The top 10 hottest nights on record in Phoenix have all occurred in the past 20 years, despite nearly 130 years of continuous bookkeeping. That’s probably a symptom of both warming from greenhouse gas emissions and the urban heat island effect, or the expansion of buildings and paved surfaces that elevates city temperatures.
As a testament to the heat wave’s longevity, Phoenix has reached a high of at least 110 degrees on a record 21 straight days, while its low has been at or above 90 degrees on a record 11 nights in a row. Weather models indicate there’s a chance that Phoenix will continue its string of 110-degree highs through at least the end of the month.
Phoenix is also on pace to be the first American city to have an average temperature of 100 degrees or greater for any calendar month.
It’s not just Phoenix that can’t shake the heat. El Paso has reached at least 100 degrees on a record 35 straight days and counting; the city’s previous record was 23 days during a streak back in midsummer of 1994.
New Orleans, which hasn’t received as much attention as locations to the west, is heading for its warmest summer on record. The brutal heat has routinely combined with oppressive humidity to push heat indexes into the 100-to-108-degree range.
And in Florida, Miami has had 40 days in a row with a heat index over 100, during which there was a 16-day stretch when heat index values eclipsed 105. The previous records for both were 32 days and eight days, respectively.
Simply stated, there are no analogues, or comparable heat waves, in the data that rival the unusual synergy between intensity and duration that the Southern U.S. is facing right now.
Heat around the world
The Southern U.S. isn’t alone. Three other heat domes have been shattering records globally:
- A European heat dome helped Rome spike to 109 degrees on Tuesday; the city’s previous record was 105. All-time heat records were also set in Spain.
- In Asia, Sanbao, China, hit 126 degrees Sunday, a national record. It represented the highest temperature ever observed north of 40 degrees north latitude.
- A heat dome over the Atlantic has contributed to record warm water temperatures. The margin by which the record has been achieved is staggering, too. The North Atlantic’s average temperature is a little more than 1 degree warmer than the previous record holder.
A climate connection
Hot weather, and even extremely hot weather, is expected during the summer. But the human influence on the climate system is supercharging extremes.
“If global warming increases, some compound extreme events with low likelihood in the past and current climate will become more frequent,” warned the Intergovernmental Panel on Climate Change. “There will be a higher likelihood that events with increased intensities, durations and/or spatial extents unprecedented in observational record will occur.”
In plain language, things that we haven’t seen before — including from a magnitude and duration standpoint — are now entering the realm of physical possibility, and will only become more severe as the climate warms further.
Heat dome animation by Artur Galocha
