As lethal heat waves have become more common over the last two years, most parts of the world will be in the grip of "extreme heat" by 2100, and more regularly, 20 to 50 times more in wealthy areas, according to the latest research paper.

Estimates show the tropics and subtropics, including the Indian subcontinent, large parts of the Arabian Peninsula and sub-Saharan Africa, will experience dangerously hot temperatures most days of the year by the end of this century, said the paper published in the Communications Earth and Environment journal on Thursday.

This is likely to happen even if we are somehow successful in limiting global warming to 2 degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial levels, the study warned.

If emissions go unchecked, large numbers of people in these regions could face potentially "nightmarish" periods of extreme heat.

"There's a possibility that if we don't get our act together, billions of people are going to be really, really overexposed to these extremely dangerous temperatures in a way that we just fundamentally haven't seen," said lead author Lucas Vargas Zeppetello of Harvard University.

Outside the tropics, they said deadly heat waves will likely become annual occurrences.

Severe heat waves – made hotter and more frequent by climate change – are already being felt across the world, threatening human health, wildlife and crop yields.

The researchers used a heat index that puts "dangerous" levels at 39.4 degrees Celsius (103 degrees Fahrenheit), while temperatures above 51 degrees Celsius are considered "extremely dangerous" and totally unsafe for humans.

The extreme measure was originally developed for people working in scorching indoor environments, like a ship's boiler room, and have rarely been observed outdoors, Zeppetello said. But by the end of the century, the researcher said it was "virtually guaranteed" that people in some parts of the tropics would experience this level of heat every year unless emissions are severely curtailed, with swathes of sub-Saharan Africa and India, particularly at risk.

"That's pretty scary," he told Agence France-Presse (AFP), adding that even walking outside would be dangerous under those conditions.

The study found that midlatitude regions around the world will, at the very least, experience intense heat waves each year. In much of Earth's wealthy midlatitudes, spiking temperatures and humidity that feel like 39.4 degrees Celsius (103 degrees Fahrenheit) or higher – now only an occasional summer shock – statistically should happen 20 to 50 times a year by mid-century, according to the study.

By 2100, that brutal heat index may linger for most of the summer for places like the U.S. Southeast, the study's author said.

And it’s far worse for the sticky tropics. The study said a heat index considered "extremely dangerous" where the feels-like heat index exceeds 51 degrees Celsius, which rarely happens, will likely strike a tropical belt that includes India, one to four weeks a year by century's end.

"So that’s kind of the scary thing about this," said Zeppetello. "That’s something where potentially billions of people are going to be exposed to extremely dangerous levels of heat very regularly. So something that's gone from virtually never happening before will go to something that is happening every year," he told The Associated Press (AP).

Zeppetello and colleagues used more than 1,000 computer simulations to look at the probabilities of two different levels of high heat – heat indexes of 39.4 degrees Celsius and above 51 degrees Celsius, which are dangerous and extremely dangerous thresholds according to the U.S. National Weather Service. They calculated for the years 2050 and 2100 and compared that to how often that heat happened each year across the world from 1979 to 1998.

The study found a three- to tenfold increase in 39.4-degree heat in the midlatitudes even in the unlikely best-case scenario of global warming limited to only 2 degrees Celsius since pre-industrial times – the less stringent of two international goals.

There's only a 5% chance for warming to be that low and that infrequent, the study found. What's more likely, according to the study, is that the 39.4-degree heat will steam the tropics "during most days of each typical year" by 2100.

In the U.S. city of Chicago, for instance, researchers predict a sixteenfold increase in dangerous heat waves by the end of the century. Chicago hit that 39.4-degree heat index level only four times from 1979 to 1998. But the study’s most likely scenario shows Chicago hitting that hot-and-sticky threshold 11 times a year by the end of the century.

Heat waves are one of the new four horsemen of apocalyptic climate change, along with sea level rise, water scarcity and changes in the overall ecosystem, said Zeppetello, who did much of the research at University of Washington state during the warming-charged 2021 heat wave that shattered records and killed thousands.

"Sadly, the horrific predictions shown in this study are credible," climate scientist Jennifer Francis of the Woodwell Climate Research Center, who was not part of the study team, said in an email. "The past two summers have provided a window into our steamy future, with lethal heat waves in Europe, China, northwestern North America, India, the south-central U.S., the U.K., central Siberia and even New England. Already hot places will become uninhabitable as heat indices exceed dangerous thresholds, affecting humans and ecosystems alike. Areas where extreme heat is now rare will also suffer increasingly, as infrastructure and living things are ill-adapted to the crushing heat," she warned.

The study focuses on the heat index and that’s smart because it’s not just heat but the combination with humidity that hurts health, said Harvard School of Public Health professor Dr. Renee Salas, who is an emergency room physician.

"As the heat index rises, it becomes harder and harder to cool our bodies," Salas, who was not part of the research team, said in an email. "Heat stroke is a potentially deadly form of heat illness that occurs when body temperatures rise to dangerous levels," she cautioned.

"Extreme heat contributes to chronic illnesses and is associated with regular losses of outdoor labor time, and ... has the potential to threaten the habitability of large swaths of Earth's land surface if greenhouse gas emissions are not curtailed," read the paper.

Most vulnerable to the severe health problems caused by extreme heat are the elderly, poor and outdoor workers, it added.

"Without adaptation measures, this would greatly increase the incidence of heat-related illnesses and reduce outdoor working capacity in many regions where subsistence farming is important," the study noted.

The researchers said that under all scenarios there could be a large increase in heat-related illnesses, particularly among the elderly, vulnerable and those working outside.

"I think this is a very important point that is receiving far too little attention," said Kristin Aunan, a research professor at the Center for International Climate Research specializing in emissions and human health, who was not involved in the study.

"Reduced workability in outdoor environments could have large economic impacts in addition to the human suffering arising from having to work under extreme temperatures," she told AFP, adding crop production and livestock can also be affected by temperature extremes.

But Zeppetello said it depends on how swiftly humanity can cut emissions.

"We don't have to go to that world. There's nothing right now that says it is a certainty, but people need to be aware of just how dangerous that would be if it were to pass," he said.