Millions of people across Europe are experiencing high temperatures during an early summer heat wave affecting France, Italy, Spain, and the United Kingdom. Temperatures are expected to reach as high as 104 degrees Fahrenheit (40 degrees Celsius), and the heat conditions are expected to last for several days.

Experts attribute the high temperatures to a phenomenon known as a heat dome. Heat domes are high-pressure systems that remain stationary for a few days, trapping heat and humidity. These systems result from a northward bulge in the jet stream, which is a river of fast-moving wind at high elevations.

Mireia Ginesta, a research associate at the Climate Litigation Lab at the Smith School of Enterprise and the Environment, explained that a high-pressure system compresses air as it descends to lower altitudes. "High pressure system means that the air is sinking, and as the air goes down to lower altitudes, it becomes compressed," Ginesta said. "So the pressure increases and the temperature also increases."

Jennifer Francis, a climate scientist at the Woodwell Climate Research Center, clarified the relationship between the jet stream and the heat wave. "The heat dome is really what the jet stream is doing," Francis said. "The heat wave is what we feel at the surface." She added that Europe is not accustomed to such heat. "In Europe, they're just not used to this," she said.

About half of France has been placed under a red heat wave alert by the national weather service. France has reported around 40 fatalities due to drowning as people sought cooling relief. Authorities in France have canceled trains, concerts, and sporting events and are restricting public alcohol drinking. Francis noted that France does not have widespread air conditioning. "It's really just in the last decade or two where these sorts of really brutal heat waves have been happening and killing a lot of people because they don't have the means to stay cool," she said.

Liz Bentley, chief executive at the Royal Meteorological Society, predicted temperature records will be broken. "We are going to see the June temperature records not just broken, but completely annihilated," Bentley said. Experts state that climate change is making the conditions for heat domes occur more often, making extreme temperatures more likely.

Bentley also stated that climate change is having an impact on the frequency, intensity, and persistence of these events. "Effectively, the world has turned up the thermostat temperature," Bentley said. "Climate change is definitely having an impact on the fact that they're more frequent, they're more intense, and they're more persistent as well."