Still, researchers said long stretches of heat could be less lethal in the future, if people become more accustomed to them.

Lethal heat waves like the one now broiling the American West are bigger killers than previously thought and they are going to grow more frequent, according to a new comprehensive study of such heat conditions.

WASHINGTON — The world’s deadly heat waves are getting worse, a new study has found.

A team of researchers examined 1,949 deadly heat waves from around the world since 1980 to look for trends, define when heat is so severe it kills, and make predictions on the trend.


They found that nearly one in three people now experience 20 days a year when the heat reaches deadly levels. But the study predicts that up to three in four people worldwide will endure that kind of heat by the end of the century, if global warming continues unabated.

‘‘The United States is going to be an oven,’’ said Camilo Mora of the University of Hawaii, lead author of a study published Monday in the journal Nature Climate Change.

The study comes as much of the country swelters through extended triple-digit heat.

Temperatures hit records of 106, 105, and 103 in the California cities of Santa Rosa, Livermore, and San Jose on Sunday, as a heat wave was forecast to continue through midweek.

American Airlines canceled 38 flights in and out of Phoenix for Tuesday afternoon as temperatures reached 118, matching a record for the day. The cancellations are for operations by smaller regional jets that have lower maximum operating temperatures than full-size airliners. Those jets can’t operate when it’s 118 degrees or above.

The heat wave, which is expected to peak Tuesday, is forecast to bring the hottest weather in Phoenix in decades, with temperatures near 120 degrees. Heat warnings were also issued for parts of Nevada.


The National Park Service warned visitors to Arizona not to hike into the Grand Canyon because of excessive heat. Temperatures below the rim of the canyon are expected to reach as high as 117 degrees this week.

In late May, temperatures in Turbat, Pakistan, climbed to about 128 degrees. If confirmed, that could be among the five hottest temperatures reliably measured on Earth, said Jeff Masters, meteorology director of Weather Underground.

Last year 22 countries or territories set or tied records for their hottest temperatures on record, said Masters, who wasn’t part of the climate study. So far this year, seven have done so.

Mora and colleagues created an interactive global map with past heat waves and computer simulations to determine how much more frequent they will become under different carbon dioxide pollution outcomes.

The map shows that under the current pollution projections, the entire eastern United States will have a significant number of killer heat days. Even higher numbers are predicted for the Southeast United States, much of Central and South America, central Africa, India, Pakistan, much of Asia, and Australia.

If pollution continues as it has, Mora said, by the end of the century the southern United States will have entire summers of what he called lethal heat conditions.

A hotter world doesn’t necessarily mean more deaths in all locales, Mora said.

That’s because he found over time the same blistering conditions — heat and humidity — killed fewer people than in the past, mostly because of air conditioning and governments doing a better job keeping people from dying in the heat.