At the same time, the authors used widely accepted occupational health and safety standards to gauge when temperature and humidity conditions become unsafe for unprotected outdoor work.
In terms of temperature alone, this metric put one goal post at 77 degrees Fahrenheit, a point at which work activity is not limited by weather. On the other end, 90 degrees was viewed as a "black flag" condition, in which no outdoor activity would be considered safe.
By establishing the 1861 to 1960 period as a base reference, the study authors determined that today's current temperatures reveal that the globe has already warmed by nearly 2 degrees.
That figure, they said, is expected to double by 2050, prompting unsafe heat-stress conditions that will cause global working capacity to fall to 80 percent, on average. This means that, all things equal, during the hottest months laborers in such heat-stress zones would have to work 20 percent less than they could have before 1960, Dunne said.
Going forward, many possible scenarios could come into play, Dunne said, depending on shifting greenhouse-gas emission levels, which the team characterized as an "uncertainty" driven by technology, policy and population growth.
But the team calculated that even a relatively good scenario would see a nearly 3.6-degree bump by 2200, translating into a labor-capacity slide down to 75 percent.
And the worst-case scenario? A global drop in worker capacity to less than 35 percent.
"It's also important to understand that that's just a global average," Dunne stressed. "In some places the situation will be much worse than others. For example, in New York the temperature will rise to [84 degrees], whereas in Bahrain, which is at [nearly 83 degrees] now, it will go up t
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