SIBI: After hours toiling at construction sites in 50 degree-plus heat, Lakhmir Brahmani finds little relief from the sun other than a donkey-powered fan during the dog days of summer in one of Pakistan's hottest cities.

Scientists have warned that swathes of South Asia may be uninhabitable due to rising temperatures by 2100 -- and in the desert community of Sibi in southwest Balochistan province, where the mercury hit 52.4 degrees Celsius (126 Fahrenheit) this summer, it feels like they could be right.

At night donkeys slowly crank giant hand-made fans to cool sleeping families -- an indigenous remedy for the region's excruciating weather where electricity is in short supply.

"I have no house or personal land... we have no electricity," explained Brahmani, saying he hopes to relocate his family to cooler climates but lacks the money to do so.

"How could I go to (provincial capital) Quetta or other areas where the cost of a truck or tractor ride one way is Rs 10,000 ($95), which I hardly earn in a whole month?"

The subcontinent -- home to one-fifth of the global population -- could see humid heat rise to unlivable levels by the century's end if little is done to put the brakes on climate change, according to a study released earlier this month.

Researchers outlined their findings in the journal of Science Advances warning of "summer heat waves with levels of heat and humidity that exceed what humans can survive without protection".

About 30 percent of the population across the region would be exposed to the scalding temperatures, up from zero percent at present, the report added.

The densely populated, rural farming regions of the subcontinent could be hit the hardest, where workers are exposed to heat with little or no chance to retreat to air-conditioning.

"Deadly heat waves could begin within as little as a few decades to strike regions of India, Pakistan, and Bangladesh, including the fertile Indus and Ganges river basins that produce much of the region's food supply," said the report.

Pakistan continues to be one of the world's most vulnerable countries to the effects of climate change, with its northern glaciers melting and population surging along with fast diminishing water supplies.

Living 'hell'

"Every year we say the heat is unbearable, but the next year when we face more heat, we forget the previous year's heat," says Mir Mohammad Luni, a farmer who lives near Sibi.

To escape the sun, Luni says he tries to finish most of his farming duties in the early morning before retreating to a hut made of scrub bushes that he douses with water every half hour or so to keep cool.

At high noon the city's market is transformed into a virtual ghost town, with shops shuttering and people crowding into any available shade or mud-soaked stream to beat the midday temperatures.

Luckily for the residents of Sibi the relatively dry, desert climate keeps the area on the fringes of livability, according to Mohammad Tahir Khan, the director of the Balochistan Regional Meteorological Centre.

If the air was the slightest bit more humid, Khan admits the city would be an uninhabitable living "hell".

Further east in the swampy port of Karachi, the sprawling megacity of over 20 million also remains at risk of being decimated by rising temperatures.

In 2015, a heatwave killed 1,200 people in the city, nearly two-thirds of whom were homeless residents unable to find sanctuary indoors or access to reliable drinking water.

During the height of the heatwave temperatures spiked to 45 degrees and hospitals were deluged with nearly 80,000 people treated for the effects of heatstroke and dehydration, according to medical officials.

Two years later residents said the city's authorities are failing to do enough to combat another scourge.

"The masses must be educated," says Shahid Habib, adding simple tips about what clothes to wear and how much water to drink during the hot season were vital.

"These things must be done in view of the intense heat. Such preventive measures should be taken that could protect lives."

'Heat beats heat'

Others said the metropolis also lacked the critical green spaces needed to help absorb the blistering summer heat.

"We should plant as many trees as possible," resident Imran Hussaini told AFP.

Back in the abandoned streets of Sibi's bazaar, tea seller Zafar Ali waits for the sun to retreat, watching over the occasional customer slurping a steaming glass of tea.

Ali swears by the hot drink as an effective means to combat the searing temperature.

"Heat beats heat," he says.