NORTHERN HEMISPHERE — Europe has experienced two deadly heat waves in recent weeks, and a third heat wave is forecast for the coming week. The United States is also experiencing a heat wave, particularly in its eastern region, at the same time.

These heat waves are attributed to heat domes, which are persistent areas of high pressure that trap hot air. Global ocean temperatures have reached record levels. Michael Mann, a climate scientist at the University of Pennsylvania, stated, "There is no doubt that record sea surface temperatures we are seeing right now — due in part to El Niño and in part to long-term human-caused global warming — are fueling more extreme weather events, as 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." A study published last week stated that the recent European heat wave was the worst recorded in Europe. The same study stated that the temperatures observed during this heat wave would have been "practically impossible" a few decades ago.

Kim Cobb, a climate scientist at Brown University, stated, "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." Cobb also said, "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 is known to amplify global average surface temperatures, and this El Niño is expected to result in an exceptionally warm year in 2027.

Mann identified the jet stream as a factor in recent heat extremes. The jet stream is a high-altitude air current that directs weather systems. Mann said, "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 events." Heat domes tend to form where the jet stream turns north, ascending and passing over the dome, which generates strong flows of hot air from south to north near the Earth's surface.

Mann and his colleagues have demonstrated an increase in the frequency of stalled jet stream patterns, known as planetary resonance events, in recent decades. He said, "We know that it has been associated with many of the most extreme heat waves we have seen in recent decades." Michael Tippett, a climate scientist at Columbia University, stated that, on average, there is no strong link between El Niño and changes in summer weather patterns in the United States and Europe, but it tends to influence autumn and winter climate in those regions.

Why It Matters

The simultaneous heat waves in Europe and the United States are attributed to heat domes. A third heat wave is forecast for Europe following recent record temperatures. Scientists have identified both natural phenomena like El Niño and human-caused factors such as fossil fuel burning as contributors to these conditions.

Record global ocean temperatures and an unusually intense El Niño are contributing to a warmer climate. Studies link these conditions to a potential increase in the frequency and intensity of heat waves. Changes in jet stream patterns also contribute to the persistence of extreme weather across different regions.