By the end of this century, climate change could drive temperatures up in the Persian Gulf region to levels that humans cannot tolerate, a study suggests.

Without successful efforts to limit greenhouse gas emissions, some Middle Eastern cities could experience a combination of temperatures and humidity in excess of what humans can survive, researchers say their simulations show.

Heat waves are, of course, not uncommon in the world, but they seem to be increasing in severity; earlier this year, sustained temperatures approaching 118 degrees Fahrenheit in India led to the deaths of more than 2,500 people.

"The new study thus shows that the threats to human health [from climate change] may be more severe than previously thought, and may occur in the current century," says Christoph Schär of the Institute for Atmospheric and Climate Science in Zurich in a commentary on the study published in Nature Climate Change.

In that study, researchers warn that at least five of the Persian Gulf's major cities could see summer weather so hot and humid, even healthy young adults could not withstand being out of doors for more than a few hours.

The problem, the researchers explain, would be the combination of high heat and elevated humidity. In high heat, the human body attempts to cool itself by sweating, with the perspiration carrying heat away as it evaporates.

However, humidity levels that are high enough to impede evaporation make it much harder for the body to cool itself, which can lead to the body completely losing the ability to regulate its internal temperature, a condition known as hyperthermia.

The threshold for that to occur is at least six hours of 95 degrees Fahrenheit on what scientists call the wet-bulb temperature scale, a combination of heat, humidity and air pressure creating severely muggy weather.

At sea level, a temperature of 116 degrees combined with 50 percent humidity — producing a heat index of 177 degrees — would reach that threshold, researchers say.

Computer models show that, by the year 2100, five cities — Abu Dhabi and Dubai in the United Arab Emirates, Doha in Qatar, Bandar Abbas in Iran and Dharhran in Saudi Arabia — could experience heat waves hitting that threshold and beyond.

The Persian Gulf region, with its shallow waters that increase heat absorption and the already intense heat from the sun, are "a specific regional hotspot where climate change, in absence of significant mitigation, is likely to severely impact human habitability in the future," the researchers say.