One of the consequences of a warming planet is more heatwaves, and more heatwaves that are hot enough to kill. Those deadly heatwaves are set to keep growing in number, according to a new study.

Researchers compared projected temperatures with data from past heatwaves and found that a massive 74 percent of the world's population could be exposed to potentially deadly heatwaves by 2100, if carbon gas emissions continue to rise at the current rates.

If the nations of the world successfully cut back on the amount of carbon getting into the atmosphere, we're still looking at 48 percent of the people on the planet dealing with heatwaves that can kill in the next 80 years or so, according to the team from the University of Hawaii.

They also put together a web app showing how the situation could get worse as temperatures rise across the globe.

"We are running out of choices for the future," says lead researcher, Camilo Mora. "For heatwaves, our options are now between bad or terrible."

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

For the study, the team combed through 30,000 relevant references in publications from 1980 to 2014 to find information about deadly heatwaves of the past, including the 2003 European heatwave that claimed 70,000 lives and the 2010 Moscow heatwave responsible for around 10,000 deaths.

From their data the researchers picked out 783 deadly heatwaves covering 164 cities and 36 countries, using them to identify a temperature and humidity threshold beyond which heatwaves start to become killers.

The threshold varies from place to place, and depends on the way temperature combines with humidity and other factors like wind speeds, but the experts say people have died in temperatures as low as 23°C (73.4°F).

At the moment, about 30 percent of the world's population gets exposed to heatwaves beyond this threshold for 20 days or more every year, the researchers say, but much worse could be to come.

For humans, our core body temperature needs to be around 37°C (98.6°F), otherwise problems start happening – problems that can be fatal if the body temperature goes much higher.

And it's the most vulnerable people on the planet who don't have the shelter or the technology to protect themselves from the heat.

Atmospheric scientist Daniel Mitchell, from the University of Oxford in the UK, wasn't involved in the research but thinks it leaves out some relevant factors that contribute to mortality rates, such as city design and available medical support.

"There are lots of things that can lead to mortality that have nothing to do with the climate," he told Kendra Pierre-Louis at Popular Science.

However, Mitchell does agree that the study brings up some necessary points and is "a good step in the right direction".

According to Camilo Mora we're going down a path "that will become increasingly dangerous and difficult to reverse" if we don't start making serious cuts in our production of greenhouse gases.

"Actions like the withdrawal from the Paris agreement is a step in the wrong direction that will inevitably delay fixing a problem for which there is simply no time to waste," he says.

The research has been published in Nature Climate Change.