If CO2 emissions continue at their current pace, temperatures in Persian Gulf cities like Dubai will at times become too hot for humans to go outside, suggests a study in the journal Nature Climate Change.

Using climate models to take a close up, regional look at future temperatures based on local topography and weather, the paper’s authors figured out how those conditions would affect people by using a wet-bulb thermometer — which takes into account the cooling effects of evaporation and ventilation — to determined at which point mugginess overwhelms our ability to sweat off the heat, making the outdoors deadly.

Its the potentially deadly mix of hot and humid that can amount to a serious health risk for people in the Persian Gulf, where warm sea water and dry desert air are major climate drivers. Without access to air-conditioning, workers like these Asian migrants (above) labouring in Dubai are especially vulnerable to heat stroke or worse.

How hot is too hot?

Beyond 35 C, temperature regulation by sweating alone isn’t enough to dissipate heat from the body, meaning prolonged exposure would see a human’s core temperatures rise to a deadly level. That means the 77 C temperatures the study predicted for the Persian Gulf would overwhelm even the healthiest person’s ability to sweat off the heat.

Climate change to this effect would also have a big impact on the world’s largest annual public gathering, the hajj pilgrimage, as seen above. While the holy city of Mecca is on the Red Sea side of the Arabian Peninsula, and wouldn’t be quite as hot as cities on the Persian Gulf side, rising heat and humidity across t he region will surely cause more deaths, suggests the study co-written by engineers Jeremy S. Pal and Elfatih A.B. Eltahir

Hottest places on Earth

Over the course of human evolution, Earth hasn’t seen that kind of prolonged, oppressive heat and humidity before. But, if heat-trapping gas emissions — chiefly CO2 — continue to rise at current rates, the Persian Gulf (with its unique desert-meets-the-sea geography and climate) will see record-shattering heat waves every decade or so.

In contrast to the humid Gulf States, the legendary heat of Timbuktu, in the West Africa country of Mali, is dry, allowing Tuareg nomads, like the man above, to survive and thrive in temperatures that can top 54.5 C.

The African country of Djibouti, across the Gulf of Aden from Arabia, is also one of the planet’s hottest, and saltiest, spots. Lake Assal, in the Danakil Desert, can hit a scorching, but habitable, 50 C.

The trend continues south of Djibouti into Ethiopia where towns near the sulphur-rich Danakil Depression (above), is already one of the hottest and harshest places on Earth with an average temperature of 34 C.

On Libya’s western border with Algeria, the town of Ghadames near these sand dunes is regularly cited as one of the hottest places on the planet. During the summer of 2013, this UNESCO World Heritage Site averaged a scorching, yet dry and thus tolerable, 41 C.

In North America, California’s ominously named Death Valley (above), a U.S. national park, mistakenly held the record for highest temperature (56.7 C) until 1922, when it was discovered that Al’Aziziyah, Libya, was in fact the hottest.

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