As large swathes of the southwestern United States currently swelter in blistering temperatures, scientists are warning in a new study that thanks to climate change, heat waves like these will only get worse - putting more lives at risk.

With the mercury hitting 47 degrees Celsius (118 degrees Fahrenheit) in Phoenix, Arizona, on Monday, Camilo Mora of the University of Hawaii said that in the future, "the United States is going to be an oven."

Climate change will sharply increase the frequency of lethal heat waves across the world, according to the study published this week in the journal "Nature Climate Change."

Currently, nearly a third of the planet's population is already exposed to deadly temperatures. The risk of falling ill or even dying from extreme heat is set to surge such that by the end of this century, it's estimated almost half of people across the globe will suffer periods of dangerous heat - even if emissions are drastically reduced.

By 2100, the proportion of people at risk worldwide will grow to 48 percent in a scenario where greenhouse gases fall significantly. If emissions aren't curbed, the proportion of the global population under threat will soar to around three-quarters.

The overall risk of heat-related illness or death has climbed steadily since 1980, with nearly one in three people globally now experiencing 20 days a year when the heat reaches deadly levels.

"We found that killer heat waves around the world are becoming more common - and that this trend already seems unavoidable," said Mora, lead author of the study.

"The empirical data suggest it's getting much worse."

Keeping cool under a fountain in New Delhi, India - a country set to be among those hardest hit by rising heatwaves

Little warming needed for tropics to turn 'deadly'

The study analyzed 1,949 lethal heat waves from around the world since 1980, with heat and humidity during such deadly episodes enabling researchers to calculate the point at which conditions can become fatal.

The team created computer simulations to determine how much more frequent heat waves will become under different carbon dioxide pollution scenarios.

It showed that in the future, the tropics will be hit hardest - regions including Sri Lanka and southern India, most of West Africa, and northern Australia will face more than 300 potentially lethal heat wave days each year under the planet's current business-as-usual emissions trajectory.

According to Mora, "with high temperatures and humidity, it takes very little warming for conditions to turn deadly in the tropics."

Licking an ice slab at Pakistan's Karachi Zoo during a heatwave in April 2017 - animals are also set to suffer under extreme heat

Like sunburn 'but inside the body'

Mora said the threshold for deadly conditions varies from place to place, with some people dying in temperatures as low as 23 degrees Celsius. Humidity levels combining with the heat is the crucial factor, explained Mora, as high humidity reduces the human body's ability to cool via perspiration.

"When it is both very hot and humid outside, heat in the body cannot be expelled," he said. "This creates a condition called 'heat citotoxicity,' that is damaging to many organs.

"Think of it as a sunburn, but inside the body."

Since the start of the 21st century, heat waves have claimed tens of thousands of lives, even in countries best equipped to help their citizens cope. The blistering heatwave that struck Europe in the summer of 2003 killed as many as 70,000 people.

Higher temperatures and dry conditions have been made worse in urban areas by clearing trees, which provide shade and cooling moisture.

After applying mud on their bodies, people cool off on a hot day on the banks of the Kanchon Mala lake in India

Protection from the heat - for those who can afford it

But a hotter world doesn't necessarily mean more deaths everywhere across the globe, Mora pointed out. He found that over time, the same sweltering conditions of heat and humidity killed fewer people than in the past - mostly because of air conditioning and governments doing a better job keeping people from dying in the heat.

So the number of "lethal heat days" cannot predict how many people will die, said the study; if everyone is living in air-conditioned environments in the future, they will be shielded. However, that's only in countries that can afford it - in those that can't, heat will become even more unbearable in the future.

Western countries can better endure the heat thanks to infrastructure including air conditioning

Professor Dave Reay, chair in carbon management at the University of Edinburgh, told DW: "As we bask in the early summer sun here in Europe, it's easy to forget the sinister side of the hotter summers that result from climate change.

"Responses such as provision of air conditioning, altered working hours and regular checks on elderly neighbors can all be very effective in reducing the short-term risks," he said.

"For the longer term though, it is how fast we can cut our greenhouse gas emissions that will ultimately decide how hot things get."

Simon Bullock, a senior campaigner on climate change at Friends of the Earth, told DW that world leaders "must put far more effort - far faster - into keeping coal, oil and gas in the ground."

"Burning fossil fuels is the main driver of the heat waves, droughts and floods [that are] putting people's lives in jeopardy."