Those most threatened will be outdoor workers in already-hot countries where temperatures and humidity are rising fast, possibly threatening the economic lifeline of South Asian migrants seeking jobs in Gulf nations.
A study published in Nature Communications found that the global economy already loses up to $311 billion per year as workers struggle in hot, humid weather.
It warned that sum would grow more than five-fold if the planet gets 2C (3.6 degrees Fahrenheit) hotter than now, on top of the 1.1C of warming already seen since preindustrial times.
Governments committed in 2015 to hold the increase in global average temperatures to "well below" 2C since preindustrial times, but are off track to meet that goal as humans continue to burn climate-heating fossil fuels.
If global warming hits 3C, today's adaptation tactic of moving outdoor work earlier or later in the day would be far less effective, the study said, as all hours would become too hot.
"More global labour will be lost in the coolest half of the day than is currently lost in the hottest half of the day," said study co-author Luke Parsons of North Carolina's Duke University.
That level of warming would also expose workers to significantly higher risk of injuries, kidney problems - and even premature death.
"To protect some of the people most vulnerable to climate change - outdoor workers in many low-latitude countries - we need to limit future warming," Parsons added.
The riskiest regions include the Middle East, where humidity is rising especially fast and which hosts 35 million migrant workers, according to the International Labour Organisation.
The new study found labourers in Qatar and Bahrain would suffer most with 3C of warming, losing more than 300 work hours per person annually, with even the day's coolest hour bringing significant heat exposure.
Nick McGeehan, a founding director at labour rights consultancy FairSquare who was not involved in the study, said migrant workers would bear the brunt of both economic and health impacts.
"The concern for me... is not that workers will lose money - it's that (employers) will maintain the status quo in the face of this very obvious risk, and severely damage more workers' health, and inevitably more workers will die," he said.
He called for legally mandated and regulated work-to-rest ratios in the Gulf.
Qatar already has some heat stress protections, with outdoor work banned between 10 am and 3.30 pm during summer and at any time if temperatures top 32.1C. Bahrain also bans outdoor work on summer afternoons.
But Barrak Alahmad, a medical doctor from Kuwait who did not work on the study, said such measures may not be enough, as morning shifts in some Gulf states have been associated with the highest intensity of heat exposure.
Instead, the doctoral candidate at the Harvard TH Chan School of Public Health urged more robust prevention and protection programmes, including acclimatisation for foreign workers and training to recognise symptoms of heat exposure.
"The evidence is quite overwhelming that systematically disadvantaged groups like migrant workers in the Gulf are at a high risk of adverse health outcomes from extreme heat," Alahmad told the Thomson Reuters Foundation. "How many more studies do we need before we take action?"
Gulf migrants' countries of origin are also warming and will lose work hours, the study noted.
On a hotter planet, India, China, Pakistan and Indonesia will face the largest labour losses among their working-age populations, it found.
That could mean workers from those countries will struggle to find safe outdoor employment, both in their homelands and traditional Gulf destinations.
"Ultimately this system will end at some point, and climate could be the catalyst for that," said McGeehan of FairSquare.