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Extreme heat took a toll in Pakistan this summer, and is expected to increasingly undercut daily activity worldwide as the planet warms.Shakil Adil/The Associated Press

Parched fields, rising seas and furious storms are among the many reasons to fear climate change, and a future that threatens to be less comfortable and more costly.

But one of global warming's biggest costs may lie in the simple fact that hot weather tends to make people sluggish.

The link between heat and the workplace is no surprise to anyone who has sweated their way through a sweltering summer day. But new research conducted by Harvard University and the World Bank calculates the tremendous cost to human efficiency from a warming planet.

On average, it finds that for every one degree Celsius rise in temperature, productivity falls 1 per cent to 3 per cent. The drop is diminished, although still evident, in developed countries with high rates of air conditioning. It's higher for industries that work outdoors, such as construction and mining.

"If this is true in the long run – and a key question is whether people will adapt – that would vastly overwhelm the damage estimates we have currently" for other climate-change-related costs, said Jisung Park, a Harvard economics researcher.

For example, studies in Malaysia suggest the cost of falling rice yields there equate to about 0.02 per cent of GDP. In Russia, predicted annual losses across the agricultural sector by 2050 equal roughly 0.2 per cent of current GDP.

That pales in comparison to Mr. Park's forecast, which suggests heat-damaged labour productivity could cut several percentage points from GDP.

"The numbers are sobering," he said.

His research looked at wealth data from 690,745 households in 52 countries, and found that heat is strongly linked with income. Even inside national borders, the poorest people often live in the warmest areas. The World Bank has warned that climate change threatens to pull 100 million people back into poverty by 2030.

Scientists have long known that turning up the temperature makes humans worse at completing some tasks; U.S. Navy research dates back to the 1950s. More modern studies have shown that in the United States, productivity is 23.6-per-cent lower on a day above 29 C compared with productivity at 15 C. At factories in India, output tumbles 8 per cent during weeks when temperatures soar above 32 C.

But in global-warming science, it's a cost that remains little recognized.

"Very few of the models, even today, explicitly include labour productivity, or direct heat stress-related impacts," Mr. Park said. His research shows that workers begin to lose steam at temperatures above 25 C.

A warming world stands to exact a toll on the young, too. Mr. Park is partway through a new study examining how heat hurts schoolchildren. "There's preliminary evidence that suggests there are pretty big impacts on things like test scores," he said.

For countries and companies seeking to keep productive workplaces and classrooms, installing more air conditioning is one answer, although not a definitive one.

"We find also that heat reduces performance when people work with air conditioning," said Stephane Hallegatte, senior economist with the World Bank. "What matters is not only the temperature when you're working, but if it's very warm at night the body cannot rest the same way."

The idea that heat hurts labour has gone overlooked, he said, because it's a creeping influence that doesn't grab attention like big storms or even deaths from heat waves. But the fact it stands to affect so many people gives it particular importance.

"It's across the board, so it could have a huge impact on aggregate numbers," he said.