Rising global temperatures could soon push the sun-baked cities of the Gulf across a threshold unknown since the start of civilization: the first to experience temperatures that are literally too hot for human survival.

A scientific study released yesterday warns that at least five of the region's great metropolises could see summer days that surpass the "human habitability" limit, with heat and humidity so high that even the healthiest people could not withstand more than a few hours outdoors.

The report in the journal Nature Climate Change says booming cities such Dubai, Abu Dhabi and Doha could cross the threshold by the end of the century, if temperatures continue to rise at current rates. Not far behind is the Saudi holy city of Mecca, a destination for millions of Muslim pilgrims every year.

On the hottest summer days, inhabitants of those cities could experience a combination of heat and humidity so high that the human body can't the excess heat through perspiration, say the report's authors, a pair of scientists from Massachusetts Institute of Technology and Loyola Marymount University.

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"Our results expose a regional hotspot where climate change, in the absence of significant mitigation, is likely to severely impact human habitability in the future," the authors write in the study.

The report examines different scenarios for climate change over the coming decades, focusing on a key heat measurement known as the "wet-bulb temperature", which includes humidity and evaporation rates, averaged over several hours.

A wet-bulb temperature of 35C is regarded as the survivability limit for healthy people.

Wet-bulb temperature is measured using a thermometer that has its bulb wrapped in cloth that is kept wet.

For years, climate scientists have postulated that parts of the Earth could cross the 35C mark in future centuries if global warming continues. But that day could come much sooner for cities in the Gulf, where temperatures soar well beyond 43C in the hottest summer months, the researchers said. Gulf cities such as Dubai, Abu Dhabi and Doha already suffer from high heat-index values that contribute to high rates of heatstroke among outdoor workers. But the authors warn that city planners will have to make major adjustments as temperatures begin to cross the lethal 35C threshold.

"It is an upper limit to adaptability to climate change due to heat stress," MIT researcher Elfatih Eltahir told reporters at a news conference.

A policy statement released yesterday by the American Academy of Pediatrics warned of new threats to children if global temperatures continue to climb.

"Children are uniquely at risk to the direct impacts of climate change," said Samantha Ahdoot, lead author of the policy statement published in the journal Science.

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