In 2006, a heat wave led to nearly 150 heat-related deaths in California, according to coroners’ reports. (There were nearly 600 excess deaths during that period, suggesting an even greater effect.)

What made that particular heat wave dangerous was its humidity, which traps heat at night, resulting in unusually high nighttime temperatures that caught Californians off guard, said Tarik Benmarhnia, an environmental epidemiologist at the University of California, San Diego.

When cities are affected by extreme heat, poorer communities tend to be most vulnerable, he said. Heat-related deaths and hospitalizations in the 2006 California heat wave were higher in ZIP codes with fewer air-conditioners. The highest-income homes were more than three times as likely to use central air-conditioning compared with the lowest-income ones.

Some cities are seeking to alleviate the effects of heat waves by opening cooling centers, checking in on vulnerable people and providing bottled water. Generally, these are done in the day, however.

“It’s really the nighttime that is the problem,” said Rupa Basu, the chief of air and climate epidemiology at the CalEPA’s Office of Environmental Health Hazard Assessment.

In some parts of the Pacific Northwest recently, temperatures soared nearly 30 degrees Fahrenheit above their average, an extreme that “would have been virtually impossible without climate change,” said Geert Jan van Oldenborgh of the Royal Netherlands Meteorological Institute.

As temperatures rise, the air can hold more moisture. Water vapor accounts for around 85 percent of the greenhouse effect, according to Alexander Gershunov, a research meteorologist at the Scripps Institution of Oceanography at the University of California, San Diego. The water vapor doesn’t cause the initial warming, but there’s a feedback loop: Higher temperatures increase moisture in the air, and more moisture traps more heat close to the ground’s surface, like a blanket, which leads to more warming.

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