Three-quarters of the world’s inhabitants, including those living in Hawaii, will be exposed to deadly heat waves by the end of the century unless greenhouse gases are not substantially reduced, according to a study published today in Nature Climate Change.

And even if bold action is taken to curtail emissions, nearly half of the world’s population still faces living with the dangerous hot spells, with tropical regions feeling the worst of it, the study said.

“We’re left with a choice between bad and terrible,” said lead author Camilo Mora, a University of Hawaii professor.

In Hawaii, according to Mora, the extreme heat might last two or three months at a time, driving residents and tourists indoors in search of air-conditioned relief — all the while straining island power grids.

The episodes will be reminiscent of the oppressive heat and humidity felt here a few days each year now, he said, and for about a week during the strong El Nino summer two years ago.

Mora, an associate professor of geography in the UH Manoa College of Social Sciences, said that when the project began a year and a half ago, little was known about how common such killer heat waves are, and the researchers were expecting to find relatively few.

That’s because the scientific literature usually describes only a few, including the 2003 heat wave that killed 70,000 people in Europe, the 2010 event in Russia that killed 55,000 and the 1995 Chicago hot spell that killed more than 700.

Digging deeper, Mora’s team found more than 1,900 deadly heat waves going back to 1980.

“That was a big shock, I’m telling you,” he said.

Most of the lethal heat waves were recorded in developed countries at the cooler climes of the mid-latitudes. Some of the cities that have endured killer heat waves include New York, Washington, Los Angeles, Chicago, Toronto, London, Beijing, Tokyo and Sydney.

But the researchers uncovered many more, especially in developing countries, including a hot spell in Pakistan last year that killed more than 1,000 and another in India two years ago that killed more than 2,000.

After analyzing the weather conditions during the lethal heat episodes, the researchers identified a threshold of heat and humidity beyond which conditions defeat the body’s natural cooling system. It’s a threshold that’s variable because lower temperatures can become lethal as relative humidity goes up.

According to their data, at 80 percent humidity, killer heat waves have occurred at temperatures above 68 degrees Fahrenheit. At 40 percent humidity, they have occurred when the mercury tops 75 degrees.

But to be conservative, the study used a higher threshold — a curve that includes 81 degrees at 80 percent humidity and 91 degrees at 40 percent humidity.

In Honolulu Sunday, the humidity was 70 percent and the high temperature reached 87 at the Daniel K. Inouye International Airport. That qualifies as killer heat under the study, but the isles also were cooled by an 11 mph breeze.

The study found that the areas around the globe where the threshold is exceeded for 20 or more days per year have been increasing and are expected to grow even with significant cuts in greenhouse gas emissions.

Currently, about 30 percent of the world’s human population is exposed to such deadly conditions each year.

The study also found that tropical areas are the most vulnerable to the deadly conditions. The reason: The tropics are hot and humid year-round, whereas the risk of deadly heat in higher latitudes is restricted to summer.

“Warming at the poles has been one of the iconic climatic changes associated with the ongoing emissions of greenhouse gases,” co-author Iain Caldwell, a UH-Manoa postdoctoral researcher, said in a press release. “Our study shows, however, that it is warming in the tropics that will pose the greatest risk to people from deadly heat events. With high temperatures and humidities, it takes very little warming for conditions to turn deadly in the tropics.”

Ironically, while deadly heat events have been on the rise, there have been fewer deaths over time. Mora said that’s because governments have been doing a better job warning people and taking precautions.

But poorer people in developing countries generally have fewer opportunities to take refuge from the heat, as there is less air conditioning, he said.

The death toll from future heat waves in Hawaii probably won’t be great, Mora said, because the islands have no shortage of air conditioning. But power grids are known to fail when they become taxed, he said, and tragedies can happen.

Mora said that to help cope with a hotter future, Honolulu needs to start becoming a greener city. The city needs to plant many more trees to help counter the effects of its urban heat.

The state, he said, can also work to restore its natural ecosystems, which help to store carbon and counter the effects of global warming.

While Mora said President Donald Trump’s move to pull out of the Paris climate agreement was a step backward for the planet, he remains hopeful the world can still rally to minimize the damage.

“This is something we cannot afford not to fix,” he said. “It’s like climbing a building and choosing between jumping from the second floor or the eighth floor. I prefer to jump from the second floor rather than the eighth floor.”