An Indian girl splashes her face with water to keep off the heat as her mother works at a nearby road construction project on a hot summer afternoon in New Delhi in 2014. (AP/Saurabh Das)

From construction workers in Dubai to farmers in India, workers around the world are suffering from excessive heat fueled by climate change. This heat is leading to huge productivity losses and mounting economic strain for dozens of countries, according to research published Monday ahead of a U.N. forum.

The study builds on research detailing how extreme heat in some places prevents employees from working during the hottest hours of the day. People simply tire faster and accomplish less the hotter it gets. That lost work time translates into significant hits on the gross domestic product in nations across the globe, and it is a problem that could deepen as the Earth continues to warm.

“For certain tropical countries that are not so well-economically developed, they might lose up to 10 percent of working hours during daylight,” said Tord Kjellstrom, one of the co-authors of the research and a visiting professor at Australian National University. “It’s a whole working month that would be lost because it’s so hot you can’t work.”

Kjellstrom and fellow researchers found that in dozens of countries, daylight work hours lost to excessive heat have increased since the 1990s. They also estimate that at the current rate of global warming, that trend will continue. For instance, countries such as India, Vietnam and Indonesia could see the number of lost work hours more than double by 2055 and more than triple by 2085.

The idea that heat and work productivity are intertwined is not a new concept, of course. Researchers have long studied whether the high heat in the southern U.S. hampered economic activity there, even as the North benefited from an industrial boom.

More recently, researchers have begun increasingly to look at the issue through the lens of climate change.

By the mid-1990s, persistently hot, poor countries such as Bangladesh were estimated to have lost 1 to 3 percent of all daylight work hours to extreme heat, which can cause exhaustion, stroke and sometimes death among exposed workers. In West Africa, research found that the number of very hot days per year had doubled since 1960. Serious heat waves have become more prevalent in various parts of the globe. Those figures could only be getting worse over time.

“In Southeast Asia, as much as 15% to 20% of annual work hours may already be lost in heat-exposed jobs, and this may double by 2050 as global climate change progresses,” Kjellstrom and a colleague wrote in a separate study published in the Asia-Pacific Journal of Public Health. They estimated that by 2030, reduced labor productivity could account for losses of “several percent of GDP, which means billions of U.S. dollars even for medium-sized countries.”

The effects of heat stress aren’t felt only at the country level, but also at the human level.

Not only does extreme heat put the health of individual workers in danger, but it also hurts workers and their families financially.

“The worker faces income loss when less is achieved within the same period of time, or a loss of leisure/family time if more work is required,” the authors of Monday’s paper wrote, noting that the health of children, women and elderly relatives face increased risks when family incomes are reduced.

“It’s extremely likely that the poorest countries and the poorest communities are the ones most affected,” Kjellstrom said. “It slows down and undermines efforts to reduce poverty in the world.”

This week’s research adds to data detailing productivity losses due to scorching heat.

A 2010 paper that studied countries in Central America and the Caribbean found that once the temperature surpasses 26 degrees Celsius (78 degrees Fahrenheit), economic output in labor-intensive sectors of the economy starts falling. Specifically, output drops about 2.4 percent for every increase in degree Celsius. This was true of both agricultural and non-agricultural sectors.

study released in late 2014 by the National Bureau for Economic Research suggested that warmer temperatures result in quantifiable economic costs.

That paper showed a fairly dramatic negative influence of heat on economic productivity. In particular, the authors found that  for a single very hot day — warmer than 86 degrees Farenheit — per capita income goes down by $20.56, or 28 percent.

Last year, a sweeping study published in the journal Nature found a strong relationship between a region’s average temperature and its economic productivity. Culling economic and temperature data for more than 100 wealthy and poorer countries over 50 years, researchers found that the optimum temperature for human productivity seems to be around 13 degrees Celsius or about 55 degrees Fahrenheit, as an annual average in a particular place. When it gets much hotter than that, the researchers found, economic productivity declines strongly.

“The relationship is globally generalizable, unchanged since 1960, and apparent for agricultural and non-agricultural activity in both rich and poor countries,” the authors wrote.

Kjellstrom thinks that there is at least some potential for optimism. If the world’s nations live up to their promises in an agreement made last year in Paris to sharply cut greenhouse gas emissions in coming decades, that could help slow the warming of the climate. In addition, he said, new technologies to cool workplaces — even in poor countries — could help some workers avoid dangerously hot working conditions, although that also could place more strain on infrastructure.

But the problem will not subside anytime soon. Monday’s report estimates that more than a billion workers “already grapple with dozens of additional extremely hot days in a year due to climate change alone.” That number probably will grow, with the biggest burden falling on some of the world’s poorest workers.

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