The researchers resolve the old argument over whether the source of summer misery is the heat or the humidity by saying that it is both. They rely on a method of measuring atmospheric conditions known as wet-bulb temperature, which, while less well known and understood than the standard method of measuring temperatures, describes the extent to which evaporation and ventilation can lower an object`s temperature. A wet-bulb thermometer has, literally, a wet bulb: It is wrapped in a moistened cloth.
If the wet-bulb temperature is 95 degrees Fahrenheit (35 degrees Celsius), that means that even a person drenched in sweat cannot cool off. Wet-bulb readings are not the same as the heat-index measurements used by the National Weather Service, Eltahir said. (This is the figure used by weather forecasters to say what a hot day "feels like" when the humidity is added.)
A wet-bulb measure of 95 F, he estimated, would roughly translate to a heat-index reading of 165 F. Since even today`s heat waves cause premature deaths by the thousands, mainly affecting very young, elderly and infirm people, the more extreme conditions envisioned in the new paper "would probably be intolerable even for the fittest of humans, resulting in hyperthermia" after six hours of exposure.
Erich M. Fischer, a senior scientist at the Institute for Atmospheric and Climate Science at the science and technical university ETH Zurich who was not involved with the paper, explained the role of humidity.
"Anyone can experience the fact that humidity plays a crucial role in this in the sauna," he said. "You can heat up a Finnish sauna up to 100 degrees Celsius since it is bone dry and the body efficiently cools down by excessive sweating even at ambient temperatures far higher than the body temperature. In a Turkish bath, on the other hand, with almost 100 percent relative humidity, you want to keep the temperatures well below 40 degrees Celsius since the body cannot get rid of the heat by sweating and starts to accumulate heat."
As climate change causes temperatures to rise around the world, it should come as no surprise that the warm-water coasts in the Middle East could be the first to experience brutal combinations of heat and humidity. The conditions would not be constant, but spikes would become increasingly common.
A temperature that today would rank in the 95th percentile "becomes approximately a normal summer day" by the end of the century, the researchers said. Wet-bulb temperatures that even exceed the 95-degree threshold could be expected to occur once every 10 or 20 years, Eltahir said. "When they happen, they will be quite lethal," he said.
The research raises the prospect of "severe consequences" for the hajj, the annual pilgrimage that draws roughly 2 million people to Mecca to pray outdoors from dawn to dusk. Should the hajj, which can occur at various times of the year, fall during the height of the summer, "this necessary outdoor Muslim ritual is likely to become hazardous to human health," the authors predicted.
If the nations of the world reduce their greenhouse-gas emissions, the authors concluded, the predicted disasters can be prevented - "Such efforts applied at the global scale would significantly reduce the severity of the projected impacts."
An essay published with the new paper by Christoph Schaer, of the Institute for Atmospheric and Climate Science at ETH Zurich, said that the message of the new research is clear. "The threats to human health may be much more severe than previously thought, and may occur in the current century," he wrote.
A heat wave in July of this year got very close to the 95-degree wet-bulb threshold described by the authors, reaching about 94.3 degrees. "It is credible that it will sometimes rise above 35 C within this century," he wrote.
In an interview via email, Fischer said that he found the research "robust and noteworthy," though he said some uncertainties remain in the temperature measurements and the models. "Whether it exceeds or just gets close to the adaptability limit and for what period (which is probably quite relevant) may need further research," he wrote.
Steven Sherwood, a researcher whose work in 2010 suggested that parts of the world could become uninhabitable within 200 years if fossil-fuel burning continued unabated, said he saw no reason to doubt the results of the new study. However, he added that "we really need to learn how to improve these models" to build confidence in the results.
Still, he said he was startled by the prediction that many cities on the Persian Gulf coast could be essentially uninhabitable by the end of the century for those without air-conditioning. "That is truly shocking," he wrote in an email exchange, and added that he found it ironic, "given the region`s importance in providing fossil fuels."
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