El Niño heat waves are adding stress to an already overheated summer, with Europe, parts of Asia and the western United States facing another burst of dangerous temperatures. Climate scientists cited by WIRED point to a stack of causes: unusually warm seas, high-pressure systems that trap heat, a bent jet stream, the newly formed El Niño and the background warming caused by human greenhouse gas pollution.
Western Europe has already had its hottest June on record, according to WIRED, and July brought severe wildfires. Reuters reported that French President Emmanuel Macron said Tuesday that the combination of heat and fires had created “the toughest since the Second World War.” WIRED reported that heat records are also being broken across parts of Europe including Spain and Germany.
The western US is next in the blast zone. AZFamily reported that Phoenix could reach 115 degrees Fahrenheit, or 46 degrees Celsius. CNN reported that Salt Lake City could hit a record 106 degrees. Montana is under an extreme heat watch, with some areas forecast to approach 110 degrees, conditions that could worsen fires already burning there.
Daniel Swain, a climate scientist at the University of California Agriculture and Natural Resources, told WIRED that interior Western cities are expected to be near or at records for early August. He also warned that nighttime temperatures may offer little relief. In Phoenix, Swain said, temperatures may not fall much below 100 degrees, following a recent night when the city tied its record highest low temperature at 97 degrees.
Why is El Niño making heat waves worse?
El Niño is a climate pattern marked by warmer-than-usual surface water in the tropical Pacific. It can shift weather patterns far from the ocean where it forms, and it tends to raise the global average temperature after its effects spread through the atmosphere over several months.
Swain told WIRED that Pacific waters were already unusually warm before El Niño developed in June. As the event strengthens, he said, it is expected to push sea surface temperatures higher. This year’s El Niño is forecast to be exceptionally strong and is developing faster than other major El Niño events did at the same point in the year, according to Swain.
That timing matters because the planet is already more than 1.5 degrees Celsius warmer than it was before industrial-era warming, Swain told WIRED. He said the current event could become the strongest observed El Niño by a large margin, calling it a potentially unprecedented event in a climate that is already unusually hot.
How heat gets stuck over cities and farmland
The immediate machinery behind many of the current heat waves is less exotic than the headlines: high pressure. These heat domes suppress clouds, keep air sinking and allow sunlight to keep cooking the surface. If the pattern stalls, the heat piles up day after day.
Jennifer Francis, a senior scientist at the Woodwell Climate Research Center, told WIRED that the jet stream helps steer much of the Northern Hemisphere’s weather. When that fast-moving west-to-east band of air becomes wavier, warm tropical air can surge farther north and linger over places that are usually cooler.
Francis said seas around Europe are unusually hot, and that those warm waters are feeding heat into the atmosphere. She attributed the ocean warmth to decades of carbon pollution trapping extra heat, much of which is absorbed by the ocean. She also said unusually warm waters near Japan and China are helping fuel heat waves in Asia.
A recent analysis reported by the Associated Press found that Europe’s June heat wave, which killed more than 1,000 people, would not have occurred without the additional heat added by burning fossil fuels.
Regional weather is also adding its own shove. Swain told WIRED that the western US heat wave is partly connected to Hurricane Genevieve, a Category 5 storm that formed over warm waters off Mexico and is injecting heat and energy into the atmosphere downwind. The physics are messy. The result is not.
This story draws on original reporting from WIRED.
