By Environment Correspondent Deborah Zabarenko
WASHINGTON, Feb 24 (Reuters) - Earth's increasingly hot, wet climate has cut the amount of work people can do in the worst heat by about 10 percent in the past six decades, and that loss in labor capacity could double by mid-century, U.S. government scientists reported on Sunday.
Because warmer air can hold more moisture than cooler air, there's more absolute humidity in the atmosphere now than there used to be. And as anyone who has sweltered through a hot, muggy summer knows, it's more stressful to work through hot months when the humidity is high.
To figure out the stress of working in hotter, wetter conditions, experts from the National Oceanic and Atmospheric Administration looked at military and industrial guidelines already in place for heat stress, and set those guidelines against climate projections for how hot and humid it's likely to get over the next century.
Their findings were stark: "We project that heat stress-related labor capacity losses will double globally by 2050 with a warming climate," said lead author John Dunne of NOAA's Geophysical Fluid Dynamics Laboratory in Princeton.
Work capability is already down to 90 percent during the most hot and humid periods, Dunne and his co-authors wrote in the journal Nature Climate Change. Using a middle-of-the-road projection of future temperature and humidity, they estimate that could drop to 80 percent by 2050.
A more extreme scenario of future global warming, which estimated a temperature rise of 10.8 degrees F (6 degrees C), would make it difficult to work in the hottest months in many parts of the world, Dunne said at a telephone briefing.
Labor capacity would be all but eliminated in the lower Mississippi Valley and most of the United States east of the Rocky Mountains would be exposed to heat stress "beyond anything experienced in the world today," he said.
Under this scenario, heat stress in New York City would exceed that of present-day Bahrain, while in Bahrain, the heat and humidity could cause hyperthermia - potentially dangerous overheating - even in sleeping people who were not working at all.
Humans are endothermic creatures, which means they give off heat. If they can't get rid of it faster than they create it, they go into hyperthermia. Typically, humans cool off by doing less heat-producing activity, but it may get so hot and humid that even a sleeping person wouldn't be able to dissipate heat fast enough.
"This planet will start experiencing heat stress that's unlike anything experienced today," said Ronald Stouffer, a co-author of the study.
The only way to retain labor capacity, Dunne said, is to limit global warming to less than 5 degrees F (3 degrees C).
Global average temperature has risen by about 1.2 degrees F (0.7 degree C) compared to pre-industrial times. It is likely to rise another 1.8 degrees F (1 degree C) by mid-century, Dunne said.
The way some workers already adapt to heat stress - taking a siesta during the hottest hours of the day, working outdoor jobs like construction at night when temperatures drop or ceasing work entirely during periods of peak heat and humidity - could migrate to places where heat stress is increasing.
The U.S. West Coast and Northern Europe are likely to be two of the regions that will be affected last by the trend toward more hot and humid climate, the scientists said.
Part of the issue is how well-adapted certain regions are to extreme heat stress, Dunne said.
As an example, he noted that some 70,000 people were killed during a disastrous 2003 heat wave in Europe, where heat stress was highly unusual. However, the same kind of stress was normal for a place like India, where a similar heat wave killed 3,000.
"It's very regionally dependent and highly determined by adaptation," Dunne said. (Reporting by Deborah Zabarenko; Editing by Eric Walsh)