Europe has experienced two deadly heat waves in recent weeks, with a third expected in the upcoming week. A rapid study published last week found that the most recent European heat wave was the worst recorded in Europe.

The study also indicated that the temperatures seen during this heat wave would have been practically impossible a few decades ago. Heat waves are caused by intense heat domes, which are persistent areas of high pressure that trap hot air.

"Record sea surface temperatures we are seeing right now are due in part to El Niño and in part to long-term human-caused global warming," said Michael Mann, a climate scientist at the University of Pennsylvania. "Warmer oceans mean more moisture in the atmosphere, which is available to intensify storm systems and produce heavy rains." Mann added, "Record global [ocean] temperatures also imply more extreme heat."

"Make no mistake, the main factor driving the increase in deadly heat waves around the world is the burning of fossil fuels, as moderate base warming causes an exponential increase in extreme heat," said Kim Cobb, a climate scientist at Brown University. Cobb stated, "This El Niño phenomenon is unusually intense for this time of year and occurs in a fundamentally different, warmer climate than in past decades." The current El Niño phenomenon was declared on June 11.

El Niño amplifies global average surface temperatures and is expected to result in an exceptionally warm year in 2027. El Niño impacts, such as marine heat waves and extreme land heat, are usually observed months after the phenomenon forms and strengthens.

However, Michael Tippett, a climate scientist at Columbia University, stated, "On average, there is no strong link between El Niño and changes in summer weather patterns in the United States and Europe." Tippett added, "El Niño tends to significantly influence autumn and winter climate in these regions."

The jet stream is a high-altitude air current that directs weather systems. Heat domes tend to form where the jet stream turns north, ascending and passing over the dome. This pattern generates strong flows of hot air from south to north near the Earth's surface. "The tendency of the jet stream, during the warm half of the year, to adopt very stable wavy configurations favors the persistence of extreme weather phenomena," Mann said. Mann and colleagues have demonstrated an increase in the frequency of stalled jet stream patterns, known as planetary resonance events, in recent decades. "We know it has been associated with many of the most extreme heat waves we have seen in recent decades," he said.