A person who is physically active at a wet-bulb temperature of 80 degrees will have trouble maintaining a constant core temperature and risks overheating. A sedentary person who is naked and in the shade will run into the same problem at a wet-bulb temperature of 92 degrees. A wet-bulb temperature of 95 degrees is lethal after about six hours.

In the Indian state of Andhra Pradesh, the highest wet-bulb temperatures of the latest heat wave have peaked around 86 degrees — levels approaching the worst of the 1995 Midwest heat wave, which set records in the United States for humid heat.

Heat waves are the natural disasters easiest to tie to climate change. Statistical analyses and climate modeling indicate that the 2010 Russian heat wave was about five times more likely to have occurred in 2010 than it would have been in the cooler 1960s. An analysis conducted after the 2003 European heat wave concluded that it was twice as likely as it would have been before the Industrial Revolution. A recent study in the journal Nature Climate Change found that the 1.5 degrees of global warming since the start of the Industrial Revolution had quadrupled the probability of moderate heat extremes.

In work one of us (Robert Kopp) led for the Risky Business Project, we found that over the period from 1981 to 2010, the average American experienced about four dangerously humid days, with wet-bulb temperatures exceeding 80 degrees. By 2030, that level is expected to more than double, to about 10 days per summer. Manhattanites are expected to experience nearly seven uncomfortably muggy weeks in a typical summer, with wet-bulb temperatures exceeding 74 degrees, about as many as residents of Washington have experienced recently.

That increase over the next couple of decades is locked in by the greenhouse gases we’ve already emitted and by our current energy system. Since we can’t avoid it now, we must make our communities more resilient to heat and humidity extremes. One step is to expand access to air-conditioning for those who can’t afford it. We must also improve cooling in stiflingly hot factories and warehouses, strengthen public health systems, improve public warnings when heat and humidity are dangerously high, and be willing to shift outdoor work schedules.

Of course, air-conditioning poses its own problems. Air-conditioners use a lot of electricity, and generating it with our current power system along with the leakage of coolants from these machines will add to the heat-trapping gases in the atmosphere.

Still, as a society, we can influence the weather of the future by the decisions we make today. If we choose not to reduce emissions of heat-trapping gases and instead continue to rely upon fossil fuels, the average American could expect to see about 17 dangerously humid days in a typical summer in 2050 and about 35 in 2090.