Extreme temperature and humidity events now occur twice as often as 40 years ago. The thermometer begins to exceed the limits of human tolerance.
During the hundreds of thousands of years of our existence on the planet, modern humans have managed to adapt to a wide variety of climates, from the arid heat of the Sahara desert to the icy cold of the Arctic. But we have our limits. If temperatures and humidity rise high enough, even a robustly healthy person, sitting in the shade, with access to water, will succumb to the heat.
As heat waves heat up and become more frequent, research has suggested that some places will start to see events that hit that human tolerance limit in the coming decades. But now a new study shows that they already happen. The findings, published Friday in Science Advances, underscore the need to rapidly reduce greenhouse gas emissions that trap heat and develop policies that help vulnerable populations stay cool.
High temperatures cause the human body to produce sweat, which cools the skin as it evaporates. But when air humidity is also involved, evaporation slows down and eventually stops. That point comes when the so-called wet bulb temperature, a measure that combines air temperature and humidity, reaches 35 degrees Celsius (95 degrees Fahrenheit). This poses a serious danger to natural human tolerance to high temperatures.
Previous analyzes using climate models suggested that parts of the Persian Gulf region, the Indian subcontinent, and eastern China would regularly see heat waves breaking this boundary later in the century. But they observed wide areas for several hours, which can mask more localized peaks in the short term under extreme conditions. To see what other researchers might be missing, "we decided to get a little closer," says Colin Raymond, who conducted the new study when he was a Ph.D. student at Columbia University.
Raymond and his co-authors examined temperature data from more than 7,000 weather stations around the world since 1979. They found that extreme humid heat occurs twice as often now as four decades ago and that the severity of this heat is increasing. Many places have reached wet bulb temperatures of 31 degrees C and above. And several have recorded readings above the crucial 35 degree C mark. Identifying that trend is "important because it is based on data from the weather station, which is the most direct evidence we usually have," says the Institute's climate scientist. Massachusetts Tech Elfatih Eltahir, who was not involved in the new research, but has previously done work on the issue.
These extremes of humid heat have already arisen in the same places that previous modeling studies had identified as future hotspots. Most are coastal areas that are close to warm bodies of water, which can supply abundant moisture and are subject to elevated land temperatures. Others, particularly in the Indian subcontinent, are regions where monsoon winds bring in moisture-laden air.
Given the shortage of weather stations in some of the places involved, like parts of Pakistan, "there are probably even higher wet-bulb values," says Raymond, who now works at NASA's Jet Propulsion Laboratory. The highest extremes were generally only reached for an hour or two, so they still don't necessarily reach the limit of human tolerance. But such events will start to last longer and cover larger areas in a warmer future. Additionally, even much lower wet bulb temperatures can be deadly, particularly for the elderly or those with underlying health conditions. The historic heat waves that killed thousands of people in much of Europe in 2003 and Russia in 2010 never had a wet bulb temperature above 28 degrees C. "These are very, very unpleasant conditions," says Eltahir.
The new paper also found that parts of the world will regularly see wet-bulb temperatures higher than the 35 degree C limit if global average temperatures rise just 2.5 degrees C above pre-industrial climate. The world has already warmed about 1 degree C above that level. "These kinds of events can become a regular occurrence with not much more warming than we've experienced," says Kristina Dahl, a senior climate scientist at the Union of Concerned Scientists, who was also not involved in the study.
That projection underscores the need to rapidly reduce greenhouse gas emissions to limit global warming as much as possible, which would restrict the frequency with which such events could occur in the future. It also raises a number of questions, including policies governments will need to develop to safeguard vulnerable groups, such as establishing cooling centers for elderly residents or issuing warnings before heat waves. And industries whose workers work outdoors, such as agriculture and construction, may need to change their hours to cooler times of the day. Even in the air-conditioned US, heat currently kills more people than cold, floods, or hurricanes.