Hot and humid has worsened over the past six decades to inflict a 10% drop in labour capacity on those scorcher days. That's expected to double by 2050.
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.
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.