Deadly heat waves—already a risk for 30 percent of the world's population—will spread around the globe, posing a danger for 74 percent of people on Earth by the end of this century if nothing is done to address climate change, according to a new study.

Nearly as alarming, the researchers project that even if greenhouse gases are aggressively reduced, at least 48 percent of the population will still face deadly heat waves by 2100 because of the amount of long-lived heat-trapping gases that already have accumulated in the atmosphere.

"We're running out of good options for the future," said lead author Camilo Mora, a biologist at the University of Hawaii at Manoa. "For heat waves, our options are now between bad or terrible."

The new study comes as near-record heat is forecast for this week in California and the U.S. southwest, with the temperature expected to soar to 120 degrees in Phoenix, and as severe heat grips parts of Europe, contributing to forest fires that have killed at least 60 people in Portugal.

A handful of deadly heat episodes have made headlines in recent decades, including the 2003 European heat wave that was blamed fore more than 30,000 deaths; the 2010 heat wave in Russia that, along with wildfire smoke, contributed to more than 50,000 deaths; and the three-day 1995 Chicago heat wave that killed more than 700 people. But Mora and his team, in analyzing heat mortality episodes reported in peer-reviewed scientific literature between 1980 and 2014, found that deadly heat episodes are far more common and widespread than previously thought.

The researchers identified 911 papers with data on 1,949 case studies where human deaths were associated with high temperatures. They found that lethal heat waves had occurred in 164 cities across 36 countries. The team obtained climatic data for the times and locations of those episodes, including surface air temperature, relative humidity, solar radiation, wind speed and several other metrics.

The team was able to plot a threshold beyond which conditions are lethal, based both on temperature and humidity.

Sustained exposure to air temperatures at and above the human body temperature—98.6 degrees Fahrenheit (37 degrees Celsius)—can result in dangerous body heat accumulation. But humidity is also a key factor. When relative humidity is high, air temperatures around 90 degrees can become lethal, as sweating becomes less effective for dissipating the body's heat.

The area of the planet where the deadly heat threshold is crossed for 20 or more days per year has been increasing, and now encompasses one-third of the world's population—primarily at tropical latitudes, the team said in their study published today in the science journal Nature Climate Change. But with climate change, the risk will extend both south and north. An online tool released with the paper allows counting, for any place on Earth, the number of days per year when temperature and humidity would exceed such a deadly threshold—both today and in the future under different climate change scenarios.

For example, by the time children born today are in their 80s, New York will have 50 days per year with temperatures and humidity exceeding the threshold beyond which people have previously died due to hyperthermia, if no steps are taken to curb greenhouse gas emissions. Sydney would face 20 deadly heat days a year by 2100 and Los Angeles would face 30 under a "business as usual" scenario. The study notes that the consequences of exposure to deadly climatic conditions will be aggravated by an aging population, since elderly people are more vulnerable to heat mortality, and by increasing urbanization, because of the heat-trapping effect of asphalt surfaces, building materials, and reduced vegetation.

For Orlando and Houston, deadly heat would last the entire summer by 2100 without steps to reduce greenhouse gas emissions, the study projects. Indeed, even though the degree of future warming is projected to be greater in temperate zones and at the poles, the greatest risk to people from deadly heat events will be at zones closer to the equator, because of the additional impact of humidity.

"With high temperatures and humidity, it takes very little warming for conditions to turn deadly in the tropics," said Iain Caldwell, a post-doctoral researcher at the University of Hawaii at Manoa, another of the paper's authors.

The study bolsters previous research projecting increasing risk to humanity due to heat waves because of climate change.

Howard Frumkin, a professor of environmental and occupational health sciences at the University of Washington School of Public Health, who was not involved in the new study, notes it is difficult to project how many people will die in future heat waves—and the new research does not try to do so—because it is hard to predict how people will adapt to the changing climate, by increasing use of air conditioning, for example.

"That's a two-edged sword," notes Frumkin, one of the lead authors of the U.S. National Climate Assessment's chapter on human health effects, most recently updated in 2014. "Until we power our air-conditioners solely with renewable energy sources, the steps we take to cope with the threat actually aggravate the threat," he said. Frumkin also noted that the risk of severe, prolonged heat will be greatest for the poor, who often don't have access to air-conditioning.

"Overall, the study reinforces what we already knew: that large areas of the inhabited world will experience unprecedented levels of heat exposure in the next several decades, and that this hazard shift has significant potential implications for health," said Jeremy Hess, another environmental and health sciences expert at the University of Washington, who was a co-author of the National Climate Assessment. Hess said a logical conclusion from the new research is that the U.S. exit from the Paris climate agreement, and the lessened effort to reduce greenhouse gas emissions that it represents, means that the risks will arrive sooner than if nations take aggressive action on climate change.

The increasing risk of heat mortality was one of the risks cited by the U.S. Environmental Protection Agency in 2009 when it adopted its finding that greenhouse gas emissions are a danger to human health and ecosystems. That endangerment finding was the legal foundation for the actions the Obama administration took to curb fossil fuel emissions.

Although the Trump administration is now seeking to undo most of those steps and to pull the United States out of the Paris climate accord, it has not yet sought to undo the endangerment finding, despite urging from some opponents of climate action.

Mora said that even though the team's research projects the spread of deadly heat even under the most aggressive international policy to reduce greenhouse gas emissions, he hopes that the study helps spur such action.

His greatest fear, he said, is that people read the grim results as a reason to abandon effort as useless. On the contrary, Mora said, a delay in curbing greenhouse gas emissions will make the spread of heat mortality more difficult to manage and reverse.

"As bad as this is," he said, "we cannot afford to give up hope."