Cruel summer: Punishing heat waves hit Europe, US, with a preview of what’s to come

Summer has barely started in the Northern Hemisphere, but already it's offering an oppressive preview of life on a hotter planet.

Europe has endured two deadly, record-shattering heat waves in a matter of weeks, with a third on the way next week. Now, the United States is experiencing its own blast of dangerous heat, particularly in the East, which is facing skyrocketing temperatures and high humidity.

These episodes have been triggered by intense heat domes — stubborn areas of high pressure that lock hot air in place — and are clearly supercharged by global warming, experts say.

The strengthening of El Niño and record-high worldwide ocean temperatures could be amplifying the heat, too.

"There's no question that record sea surface temperatures like we're seeing right now — which are due partly to El Niño, and partly to longer-term human-caused warming — fuel more extreme weather, since warmer oceans mean more moisture in the atmosphere which is available to intensify storms systems and produce heavy rainfall," said Michael Mann, a climate scientist at the University of Pennsylvania.

"The record global [ocean] temperatures also mean more extreme heat," he said.

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The link between climate change and heat waves like we've seen this spring and summer is especially strong, straightforward and well understood. As the planet warms, extreme heat events are becoming more common, intense and long-lasting.

A rapid study on Europe's most recent heat wave, released late last week, found that not only was it Europe's worst heat wave on record, but also those extraordinary temperatures would have been "virtually impossible" just a few decades ago, when human-caused global warming was less severe.

"Make no mistake, the primary driver for the uptick in deadly heat waves across the world is the burning of fossil fuels, given that a modest baseline warming drives an exponential increase in extreme heat," said Kim Cobb, a climate scientist at Brown University.

El Niño is well-known to amplify global average surface temperatures and is expected to lead to a record warm year in 2027, while boosting temperatures this year to some extent as well.

It can also lead to both marine heat waves and extreme heat over land, but these impacts are typically seen a few months after an El Niño has formed and strengthened, rather than right away. The ongoing El Niño was declared on June 11, making it a recent weather feature.

Michael Tippett, a climate scientist at Columbia University who studies the ties between El Niño and weather patterns, said that on average, there is no strong link between El Niño and shifts in summer weather patterns over the US and Europe.

Instead, he said El Niño tends to significantly influence the weather in the fall and winter in these regions.

However, Cobb said while there isn't a clear tie between El Niño and large-scale summertime heat waves, in general, this particular El Niño is unusual in multiple respects. This makes its impact potentially distinct from past events.

"This El Niño is unusually large for this early in the year, and it is occurring in a warmer climate that is fundamentally different than past decades. We learn something new from every event, driving new areas of research to improve our understanding of El Niño impacts in a warming world," Cobb said.

Even if El Niño is only playing a minor role in the extreme heat of late, it is likely to become a star player by next summer, which could yield even more extreme heat episodes around the world.

Mann pointed to another factor involved in recent heat dome-dominated extremes — the jet stream — which may have been part of the European heat wave that is finally beginning to subside, as well as the US heat wave that is just ramping up. The jet stream is a highway of air at high altitudes which steers weather systems.

When the jet stream is very wavy and slow to shift into a different pattern, it can lead to extreme outcomes, such as heat domes that don't budge for days to even weeks.

"The tendency of the jet stream, during the warm half of the year, to get locked into very stable, wavy configurations … favors persistent weather extremes," he said. The heat domes tend to form where the jet stream turns northward, moving up and over the dome itself and leading to strong flows of hot air from the south to the north near the Earth's surface.

Mann and his colleagues have shown an uptick in the frequency of such stuck jet stream patterns, known as planetary resonance events, in recent decades. That trend coincides with increased global warming.

"We do know that it has been associated with many of the most extreme heat waves we've seen in recent decades," Mann said.

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