An unprecedented heat wave with temperatures exceeding 40 degrees Celsius has engulfed the United States and Europe, causing tens of millions to suffer from severe heat. In some regions of Spain, temperatures briefly soared to 46 degrees, setting a record for June. Scientists attribute the heat wave affecting both the U.S. and Europe to a heat dome.
The Financial Times of the United Kingdom reported on the 29th (local time) that a study by researchers at the University of Pennsylvania indicated that, due to climate change, the frequency of stagnant atmospheric phenomena affecting heat waves, floods, and wildfires has nearly tripled since 1950.
Recently, some European countries, including France, Spain, and Greece, have been experiencing unusual heat waves due to the formation of heat domes, which trap heat. According to the Spanish National Meteorological Agency, on the 28th, the temperature in the inland village of El Granado reached 46 degrees, the highest recorded for June. A large wildfire that broke out south of Athens on the 26th closed the coastal road leading to the Temple of Poseidon in Sounion, where temperatures had reached 40 degrees.
The United Kingdom's Meteorological Office noted that higher temperatures are likely early this week. In the southeastern region of the UK and eastern Wales, the highest temperatures are forecasted to reach 35 degrees, with London potentially also reaching 35 degrees. The Meteorological Office warned that, "In some areas, tropical nights will continue, and temperatures will not drop below 20 degrees for some time."
According to the weather media AccuWeather, over the past week, more than 3,000 locations in the United States have recorded all-time high temperatures. In the eastern U.S., dozens have been hospitalized due to heat-related illnesses, and New York set a record high for June at 39 degrees. The Korean Peninsula is also experiencing sweltering heat and tropical nights as warm, humid air flows in from the south. Scientists are seeking the cause of these extreme weather events in changes in the jet stream, which is a fast, narrow air flow circling the Earth's troposphere.
A heat dome is a meteorological phenomenon that traps hot air, generating extreme heat, similar to being covered by a lid or hat. A strong high pressure system remains abnormally long, hindering convection and precipitation while confining hot air to specific areas. Heat domes primarily occur when summer high pressure systems combine with weakened jet streams. According to the Royal Meteorological Society of the United Kingdom, a jet stream flowing between strong high and low pressures creates a barrier that disrupts the atmospheric flow resembling the Greek letter omega (Ω). The atmosphere stagnates in the area corresponding to the high-pressure zone of the omega letter, leading to prolonged hot weather.
A research team led by Professor Michael Mann from the University of Pennsylvania revealed that a phenomenon known as quasi-resonant amplification (QRA) is behind prolonged meteorological events in a single region, with its occurrence frequency nearly tripling since 1950, as introduced in the Proceedings of the National Academy of Sciences (PNAS) on the 16th.
Michael Wehner, a senior scientist at the Lawrence Berkeley National Laboratory, a co-author of this study, said, "While the thermodynamic effects of climate change due to rising temperatures are well understood, interpretations of the dynamical processes that trigger extreme weather events are much more complex" and added, "This study has identified significant changes in these complex aspects of atmospheric motion."
Quasi-resonant amplification is the phenomenon where the narrow, fast air flow known as the jet stream at altitudes of 8 to 15 kilometers becomes stronger and amplifies. It stagnates the weather patterns of a region for extended periods, causing extreme heat waves like the current heat wave as well as floods that have struck China and Asia, and recent wildfires repeatedly occurring in North America. This phenomenon maintains severe weather conditions only in specific regions, such as the heat wave striking the U.S. and Europe or heavy rain in China. Postdoctoral researcher Dr. Shekar Lee explained, "The quasi-resonant amplification phenomenon increases the intensity and duration of waves, resulting in extreme weather persisting in one area for an extended period."
Over the past 20 years, record-breaking, extreme summer heat events have occurred multiple times in the mid-latitudes of the Northern Hemisphere. Notable examples include the heat waves in Europe in 2003, Russia in 2010, and Texas, USA, in 2011. The heat dome case in the Pacific Northwest in 2021 is considered the most severe of these.
At that time, temperatures exceeded 47 degrees in Portland, Oregon, and 42 degrees in Seattle, Washington, continuing from late June to early July. According to the researchers' analysis, the quasi-resonant amplification led to soil moisture deficiency and amplified warming in the lower atmosphere, resulting in an unprecedented heat wave.
The research team analyzed the quasi-resonant amplification phenomenon from 1950 to 2024 using the fifth generation global climate reanalysis data (ERA5) produced by the European Centre for Medium-Range Weather Forecasts (ECMWF). This data provides a comprehensive four-dimensional perspective on the Earth's atmosphere, surface, and oceans from 1940 to the present and is updated hourly.
Professor Mann stated, "Human-induced global warming is regulating the jet stream and other circulation systems, increasing the frequency, duration, and severity of extreme weather events" and noted, "Recent Arctic warming and land-sea thermal contrasts are increasingly aligning with these environmental conditions."
The research team found that the frequency of quasi-resonant amplification increases during the maturation phase of the El Niño phenomenon. El Niño is characterized by the sustained higher-than-average sea surface temperatures in the tropical eastern Pacific. Researcher Ri explained, "During the El Niño Southern Oscillation (ENSO) phenomenon, heat and convection change across the tropical Pacific, causing waves to spread outward and altering the structure and intensity of the jet stream" and added, "This process subtly influences the atmospheric conditions where planetary wave resonance occurs."
The research team pointed out that despite the increasing frequency and impact of unusual climate phenomena, current climate models do not fully capture these occurrences.
Researcher Ri stated, "Current climate models do not accurately capture extreme weather events" and emphasized, "It is highly likely that current models underestimate the potential for destructive and deadly extreme weather events in summer."
Professor Mann also mentioned in an interview with the Financial Times, "As global warming continues, there is a higher likelihood of dual heat domes occurring simultaneously in Europe and North America."
Amanda Maycock, a professor at the University of Leeds in the UK, said, "What was once considered typical summer weather is now a concept of the past, and in the future, the changing weather patterns and jet streams will regularly produce extreme phenomena in temperature and precipitation" and added, "Temperature records are expected to be regularly broken by wide margins, and it is certain that heat will become more intense due to the continuous increase in greenhouse gases."
References
Proceedings of the National Academy of Sciences (2025), DOI : https://doi.org/10.1073/pnas.2504482122
Proceedings of the National Academy of Sciences (2024), DOI : https://doi.org/10.1073/pnas.2315330121
Science Advances (2018), DOI : https:// doi.org/10.1126/sciadv.aat3272