Let’s learn about why summer 2023 was so hot

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.

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