Coping with heat conjures a range of images. Mom pulls down the shades. Dad turns off the lights. Kids jump into the pool or run through the sprinkler. Grandpa sips a cool drink on the breezy porch.

But now researchers say those tactics might not be enough to cool the effects of the hot days, hot nights, and intense heat waves that are likely to be a consequence of unabated global climate change.

Matthew Huber, a professor of earth and atmospheric sciences at Purdue University, and Steven Sherwood, of the University of New South Wales in Sydney, Australia, analyzed the highest tolerable heat-wave conditions in a study published in the Proceedings of the National Academy of Sciences. Their research found that Earth's future temperatures could exceed livable limits in some regions.

"We used the reasonable worst-case scenario if the world continued on a carbon-intensive trajectory. You could call it a business-as-usual trajectory," Huber said, adding that the study looked well beyond the year 2100. The research found that a rise of 12 degrees Fahrenheit in the global average surface temperature would cause some areas to be so hot and humid that conditions would make it impossible for those without air conditioning to cool off through their skin. A 21-degree warming scenario would put half of the world's population in an uninhabitable environment.

In the study, Huber and Sherwood for the first time tested the limits of habitability by measuring the highest tolerable "wet-bulb" temperature, the equivalent of what is felt when sweating skin is exposed to a strong breeze in the shade. Their analysis determined that the highest tolerable wet-bulb temperature could be exceeded for the first time in human history in future climate scenarios.

Huber explained that humans and many mammals have an internal body temperature of approximately 98.6 degrees Fahrenheit and cannot tolerate a wet-bulb temperature above 95 degrees for longer than six hours. Humans cool themselves through the skin. Internal heat can dissipate when the external temperature is cooler than internal body temperature. But when the external wet-bulb temperature is 95 degrees or above, the body can't cool itself and begins to experience hyperthermia. Extended hyperthermia is associated with ill health and eventually death.

For the analysis, the researchers looked at the wet-bulb temperature impacts on a fit, acclimated person who is naked, drinking unlimited amounts of water, in the shade, standing in front of giant fan, and not doing any labor. "We asked the question, what kind of conditions would be lethal to that person?" Huber said. The answer is that 100 percent relative humidity and a wet-bulb temperature of 95 degrees Fahrenheit are likely to be lethal.

"Most people are more familiar with the heat index, or the feels-like temperature they see on the weather report. The wet-bulb temperatures we are talking about would have a feels-like, or heat-index, temperature of between 170 to 196 degrees Fahrenheit," Huber said.

With this information on the limit of human adaptability, the researchers then ran climate models based on the global-warming scenarios that are possible if our recent emissions trends continue. They found that wet-bulb temperatures of 95 degrees is likely to occur within three centuries "in many places people care about, such as the eastern United States, Spain, Italy, India, Australia, Brazil, and coastal China, among other locations," Huber said.

Right now, no place on Earth regularly experiences 95-degree wet-bulb temperatures. The highest such temperatures in the world's hottest places, such as Arizona and the Sahara, are 59 to 68 degrees Fahrenheit. In those regions, even though air temperatures regularly exceed 110 degrees, the humidity levels are low. In India, China, and Brazil, wet-bulb temperatures of 77 to 81 degrees Fahrenheit occur, and sometimes they reach 86 degrees. The world's wet-bulb temperatures don't rise above 86 degrees because heat and humidity lead to thunderstorms, which provide cooling. However, as the planet gets warmer, the atmosphere is expected to get hotter, and wet-bulb temperatures are likely to be much higher when thunderstorms occur.

Future weather fluctuations across the world will be similar to those we experience today. There will be cold spells and rainy months. Intense heat waves may happen every few years and catch people by surprise. But the difference between today's heat waves and those expected in the future lies in the extremely high temperatures and high humidity levels to come. "You'll have a heat index of 170 degrees Fahrenheit. Heat waves won't affect just the elderly or young children, they'll affect everyone," Huber said.

Reducing carbon emissions is one way to stave off or delay some of these effects. However, heat-trapping gases have already been released, and some level of warming is assured for our future. Huber said that the extreme heat effects presented in his paper will probably happen even if emissions are reduced by 50 percent if Earth's climate is as sensitive to heat-trapping-gas radiative forcing as scientists expect. "The way I think about it, if you spend 30 years of your life smoking five packs of cigarettes a day, and then you stop, you are not going to suddenly be healthy," Huber said. "Carbon sticks around in the system for a long time, and warming sticks around in Earth's system for a long time. It is hard to stop burning carbon. Like smoking, carbon is addictive. If we burn carbon for 100 years and then we say, 'Okay, now we are going to stop,' we would have already built in quite a lot of future warming damage."

The current emissions trajectory—if allowed to continue (and if Earth's response is at the higher end of values of expected sensitivity to heat-trapping-gas forcing)—indicates that in the next couple of centuries some parts of the world are likely to be too hot for humans. "In our study, we just asked the basic questions. If we burn a lot of carbon and the climate system is sensitive to greenhouse-gas emissions, how bad could it get for humans?" Huber said. "We were disturbed to find that it gets really bad."

Huber said the study has prompted many economists and policy makers to think through the emissions scenarios that will push the limits of habitability in parts of the world. The health effects of heat stress point to the need for adaptation and climate preparedness.

For more information:

Sherwood, S. C., and M. Huber. 2010. An adaptability limit to climate change due to heat stress. Proceedings of the National Academy of Sciences 107: 9552–9555. Available online at www.pnas.org/cgi/doi/10.1073/pnas.0913352107