Let’s learn about why summer 2023 was so hot

Earth’s oceans and atmosphere have conspired to make a sweltering season

A photo of a temperature board in Phoenix, Arizona showing 110 degrees and the time 1:22 p.m.

Temperatures in Phoenix, Ariz., set records this past summer. In July, the city experienced 31 days in a row of 43.3° C (110° Fahrenheit) heat.

Brandon Bell/Getty Images

This summer has smashed heat records worldwide.

July was Earth’s hottest month ever recorded, following the hottest June on record. On July 6, the average global temperature reached a new peak at 17.23° Celsius (63.01° Fahrenheit). (If that number doesn’t sound scorching, remember that it doesn’t just account for the Northern Hemisphere’s current summer temps. It also accounts for temperatures in the Southern Hemisphere, which is currently in winter.)

Scientists are still teasing out all the factors that have led to this summer’s historic heat. But one thing is certain: human-caused climate change takes a lot of the blame.

Toasty oceans

Much of this summer’s brutal heat can probably be traced back to Earth’s oceans, experts say. Oceans worldwide have been warming for decades, largely due to humans pumping climate-warming greenhouse gases into the atmosphere. In April, the average sea surface temperature around the world hit a new record of 21.1° C (70° F). Now that Earth’s oceans are so balmy, they can’t absorb as much heat from the atmosphere. This leaves more heat in the air, leading to warmer summer weather.

El Niño may be worsening that problem this year. El Niño is part of a natural climate cycle that temporarily warms the planet every several years. It happens when westward winds over the Pacific Ocean are especially weak. That allows warm water to pile up in the eastern Pacific, burying cold deep-sea water that would otherwise well up. As a result, a blanket of warm water covers much of the tropical Pacific and leaks heat into the atmosphere.

This may have helped turn up the heat on summer 2023. But Earth’s new phase of El Niño only began in June and won’t peak for months. So it’s too early to say how much El Niño has added to recent sweltering weather, researchers say.

Heat waves

On top of Earth being generally warmer than before, devastating heat waves have recently roasted many places around the globe. Those regions include northern Mexico, China, southern Europe and the southwestern United States. There, Phoenix, Ariz., had 31 days in a row of 43.3° C (110° Fahrenheit) heat in July. And just after midnight on July 17, Death Valley, Calif., sizzled at 48.9° C (120° F). That may be the highest temperature ever felt anywhere at that time of night.

Heat waves happen when powerful winds called jet streams flow more slowly through Earth’s atmosphere. This can cause hot pockets of air to linger over the same places for days or weeks at a time. It’s not entirely clear why such atmospheric traffic jams happen. But research has shown that climate change is making heat waves more common and more severe.

One recent study revealed climate change’s influence on heat waves across the Northern Hemisphere this summer. Researchers made computer models of the world with and without climate change. Then, they compared how often heat waves happened in each case. In a world without climate change, the recent extreme heat seen in China would happen every 250 years, rather than every five or so years. And the heat waves now expected about once a decade in Europe and North America would be almost impossible without climate change, the models showed.

New normal

Another recent study looked at how much climate change drove July’s extreme heat around the world. Those models showed that more than 6.5 billion people worldwide experienced levels of heat that climate change made at least three times more likely. That’s more than 80 percent of the world’s population.

It’s too early to pin down the cost of all that oppressive weather. But hundreds of deaths have been reported around the globe. And the skyrocketing demand for air conditioning has heightened concerns about power shortages. Those problems will only become more urgent as climate change creates more viciously hot summers like this one.

Want to know more? We’ve got some stories to get you started:

Let’s learn about heat waves Heat waves often occur when a high-pressure system lingers over a certain area. These deadly events are on the rise due to climate change. (8/9/2022) Readability: 6.0

Deadly heat: Expected by century’s end, it’s here already Instances of hot and humid conditions that threaten human lives are on the rise. (6/19/2020) Readability: 7.7

Heat waves appear more life-threatening than scientists once thought This is bad news as a warming planet leads to growing numbers of excessive heat waves — and millions more people facing potentially deadly temperatures. (8/24/2022) Readability: 8.0

Learn why extreme heat can be so dangerous.

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July 2023 nailed an unfortunate world record: hottest month ever recorded (from Science News)

Here’s how much climate change increases the odds of brutally hot summers (from Science News)

What’s causing this summer’s extreme heat waves? (from Science News)

Last week was the hottest ever recorded — here’s why we keep smashing records (from Science News)

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Nikk Ogasa is a staff writer who focuses on the physical sciences for Science News. He has a master's degree in geology from McGill University, and a master's degree in science communication from the University of California, Santa Cruz.

Maria Temming is the Assistant Managing Editor at Science News Explores. She has bachelor's degrees in physics and English, and a master's in science writing.

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