Searing temperatures will cost emerging economies up to 10 percent in lost daytime working hours, if countries do not cut planet-warming emissions further than they have promised so far, U.N. agencies and international labor bodies said on Thursday.
Global temperatures are predicted to rise by at least 2.7 degrees Celsius if emissions-reduction pledges made by nearly 190 nations for the new global climate change deal are met.
The Paris agreement, however, sets a goal of keeping average temperature rise to "well below" 2 degrees Celsius above pre-industrial times.
If the world continues with its current level of emissions, the impact on working hours - and lost GDP - is likely to be even worse, according to a joint report by the U.N. Development Programme, International Labour Organization, Climate Vulnerable Forum and other agencies.
"Excessive heat puts exposed working populations at greater risk from heat-induced stresses and undermines growth by compromising productivity," Cecilia Rebong, ambassador and permanent representative of the Philippines to the United Nations, said in a statement.
"Vulnerable groups need significant support to tackle rising heat in the workplace," Rebong added.
Countries likely to be worst affected by rising temperatures include India, Indonesia, Nigeria, Cambodia, Pakistan, Burkina Faso and parts of West Africa, the report said.
India is in the grip of an early-summer heat wave that has killed more than 100 people, forced schools to close and halted outdoor work like construction, government officials said last week. Temperatures have risen above 40 degrees Celsius in some states.
In the 1990s, several developing countries were already losing up to 3 percent of daylight working hours to intense heat. Since then, global temperatures have risen, according to the report which studied a sample of countries from each region.
In West Africa, the number of very hot days per year has doubled since the 1960s, with an extra 10 hot days every decade, the report said.
"Imagine working in a shoe manufacturer in Vietnam or a clothing factory in Bangladesh when it is 35 degrees Celsius," said Philip Jennings, general secretary of UNI Global Union.
"Governments and employers have to take this issue of the cauldron of a warming planet seriously and develop some effective policy responses and practical measures to protect workers," he added.
Countries like Bangladesh stand to lose the most as the planet heats up, said Saleemul Huq, advisor to the Climate Vulnerable Forum and director of the International Centre for Climate Change and Development.
"If we are to take sustainable development seriously, we have to scale up climate action across the board and fund real ways of adapting communities to these new everyday extremes," he said.