(CNN) Deadly heat waves are going to be a much bigger problem in the coming decades, becoming more frequent and occurring over a much greater portion of the planet because of climate change, according to a study published last summer in Nature Climate Change.

Extreme heat waves, such as the several torching the Northern Hemisphere this week are frequently cited as one of the most direct effects of man-made climate change.

The study says, by the year 2100, three out of four people on Earth could be subject to at least 20 days per year of heat and humidity associated with deadly heat waves, if greenhouse emissions continue to rise at their current rates.

Currently, that number is about one in three people.

Even if humans aggressively cut back on fossil fuel emissions, such as outlined in the Paris climate agreement, rising temperatures and humidity levels will combine to increase the intensity and frequency of deadly heat waves, the study said. President Donald Trump recently announced he would be pulling the United States out of the Paris Agreement.

"Our attitude towards the environment has been so reckless that we are running out of good choices for the future," said Camilo Mora, associate professor at the University of Hawaii at Manoa and lead author of the study.

"For heat waves, our options are now between bad or terrible," Mora said.

"Many people around the world are already paying the ultimate price of heat waves, and while models suggest that this is likely to continue to be bad, it could be much worse if emissions are not considerably reduced."

Determining what makes a heat wave deadly

Heat waves are notoriously deadly, as the human body can only function in a narrow range of body temperatures around 98.6 degrees Fahrenheit (37 degrees Celsius).

An international team of researchers, led by Mora and the University of Hawaii at Manoa, analyzed hundreds of historic heat waves to quantify what weather conditions posed the greatest risk of death in humans. Using data from 783 lethal heat waves in 164 cities and across 36 countries, the researchers discovered a common threshold where the heat wave becomes deadly.

In summer 1995, more than 700 people died of heat-related causes in Chicago.

"This threshold now allows us to identify conditions that are harmful to people. And because it is based on documented cases of real people across the globe, it makes it that much more credible and relevant. The scary thing is how common those deadly conditions are already."

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Not surprisingly, the threshold is driven not only by the air temperature, but also the relative humidity. Sweating, the body's main process to remove heat by evaporative cooling (think the process that makes your body cold after you get out of the shower), becomes much less effective as relative humidity increases, because the air is already more saturated with moisture.

The team was able to use this threshold to distinguish between deadly and non-deadly heat waves in the historical record, and then use the same threshold to observe how these conditions will play out in the future using climate models run with different greenhouse gas emission scenarios.

Future of deadly heat

The study allows us to peer into the future for any location on Earth and see how many days per year, on average, that location will cross the potentially deadly threshold the group has identified.

web application was also developed so you can view any location under any of the potential emission scenarios.

For example, New York City, which now sees about two days per year that cross the threshold, rises to about 50 days per year by 2100.

The results paint a grim picture, where entire summers exceed the threshold in warmer and more humid cities like Orlando and Houston.

Currently, about 30% of the world's population (and about 13% of the land area) experiences at least 20 days per year on which the deadly threshold is reached. By 2100, this percentage jumps to 74% of the population (47% of the land area) if emissions continue unchecked.

Aggressive cutbacks now on emissions could help, though only slightly.

"If we do the best that we can, which is the Paris Agreement, you are still going to have nearly 50% of the human population impacted, which is pretty bad," Mora said.

"But if we don't do anything, we are going to have [more than] 70% of the population affected by these heat waves, which is terrible," said Mora.

"Unfortunately there isn't much we can do to make the situation better," Mora said while speaking of the Paris Agreement. But by doing nothing, "We can certainly make it a lot worse."

Greatest risk in the tropics

While higher-latitude locations will warm more than the tropics under global warming, this study indicates that a greater threat to life will be in the more humid, tropical locales.

According to the study, because areas in the tropics "have higher year-round warm temperatures and higher humidity," they require "less warming to cross the deadly threshold."

In the deep tropics, such as in Jakarta, Indonesia, the entire year is pegged to be above the potentially deadly level.

Unfortunately, many of these parts of the world are less able to adapt to worsening heat waves as well, with limited access to air conditioning and unreliable electrical grids

Heat stroke patients seek relief in Karachi, Pakistan, in 2015. That year's heat wave claimed over 4,000 lives in Pakistan and India.

Dr. Marshall Shepherd, director of the University of Georgia's Atmospheric Sciences Program, was not involved in the study but agreed with it's findings.

"Study after study finds discernible fingerprints of climate change in contemporary heat waves," Shepherd said.

"Most people know that summer is also the most uncomfortable in terms of heat and humidity. Basic physics relationships explain why a warmer climate will likely mean more oppressive heat indices and heat-related health issues."

Shepherd also said that we often overlook the impact that cities have on the intensity of heat waves, through the "urban heat island effect."

"We have overlooked that some of the most dangerous warming is associated with increasingly intense heat waves plus urban heat," Shepherd said. "Globally, more people live in cities and that trend will continue."