Heat and humidity beyond human tolerance is occurring in some parts of the world faster than expected under global climate change, scientists report in the May 8 issue of Science Advances.
Colin Raymond of Columbia University and colleagues are the first to present observational data showing that a well-known measure of humid heat called wet-bulb temperature has, in some places, already exceeded 35°C (95°F), the temperature at which humans can no longer regulate body heat.
They also found that dangerous bouts of wet-bulb temperatures of over 27°C (80.6°F) have more than doubled since 1979. These extreme temperatures were not found in previous studies because occurrences were brief and highly localized, concentrated near the Persian Gulf, South Asia and the Gulf of California.
"That these conditions are already occurring, and that people didn't realize they were already occurring gives extra incentive for figuring out how to adapt," said Raymond, who largely performed this research as a doctoral student at Columbia before beginning post-doctoral work at NASA's Jet Propulsion Laboratory.
To keep cool, humans shed excess heat through sweat, which evaporates into the air. High humidity prevents sweat from evaporating as readily, making humid heat more dangerous than dry heat. Without the opportunity to cool off, people can suffer from elevated heart rate, high blood pressure and even organ failure and death.
Wet-bulb temperature is a measure of heat and humidity that expresses how human bodies will experience the temperature. It is so named because it is calculated by wrapping the bulb of a thermometer in a wet cloth. In low humidity, water will evaporate from the cloth, carrying away heat and cooling the thermometer in the same way sweat cools the human body. In these conditions, the wet-bulb temperature will be lower than the air temperature. In high humidity — when the air is more saturated with water vapor — the water cannot evaporate as easily so the cloth stays hot. If the wet cloth cannot cool below the air temperature, neither can human skin.
"Physiologically," Raymond said, "there's a point when heat and humidity will become not just uncomfortable, but actually impossible to acclimate to."
In 2010, scientists Steven C. Sherwood and Matthew Huber suggested that a person in perfect health, at total rest, in full shade and with unlimited water will still overheat if heat and humidity combine to stop sweat from cooling their skin below 35°C (95°F). Since these ideal conditions are rarely realized, even lower wet-bulb temperatures are dangerous. The 2003 European heatwave and the 2010 Russian heatwave, for instance, each killed tens of thousands of people with wet-bulb temperatures no higher than 28°C (82.4°F).
Most climate scientists believed wet-bulb temperatures never rose above 35°C, and never would until the effects of global warming grew stronger later in the 21st century.
But these temperatures are already occurring, said Raymond and colleagues. Researchers had not yet noticed them because of a quirk in how global climate researchers study data from individual weather stations.
Computer climate models use and project weather data as if weather stations were organized in a neat grid around the planet and as if they collect data at synchronized time intervals. In reality, many factors affect where weather stations are built and when they can collect and transmit their observations. Scientists are restricted by geography, land access and funding when building stations. The stations also need maintenance and they can transmit faulty readings. In rural or undeveloped countries, weather stations are often few and far between.
To integrate this messy data with more ordered computer models, climate researchers average the data between weather stations to estimate what the weather might be at neat grid points. During the averaging, the most extreme values captured by individual stations are "smoothed" to look more like what adjacent stations experienced. No previous study verified if the most extreme wet-bulb temperature values at these stations were actually occurring or if they were just bad readings.
"It's intimidating to try to disentangle good data from bad data, wading into the weeds, figuring out what's a weed and what's a flower," Raymond said. "We spent the majority of our time actually working on quality control, so we're as confident as any scientist would be in the final result. But it took a lot of work to get there."
Not only did extreme heat and humidity occur at single weather stations, the researchers found, but they lasted only an hour or two each due to the unique meteorology required to bring about such extremes.
"The number one ingredient is a source of very large amounts of moisture," said Raymond. Stations that experience these extremes were usually near water bodies that get very hot, like the Persian Gulf. The surface water of such bodies evaporates rapidly and drives up the humidity over the water. Towns or cities on the shore are very hot but not as humid. But when a sea breeze blows the humidity onto the hot shore, the wet-bulb temperature skyrockets.
"Normally a sea breeze is refreshing because the water is cooler than the air, and that's also the case in the Persian Gulf," Raymond said. "Except it's not refreshing because the water is so hot. It's more like a breeze in a sauna."
As the global mean temperature rises, such events are going to become even more extreme, longer lasting and more widespread, said Raymond, occurring regularly in the American South, South America and the tropics. The cities and towns in these places will need to adapt mitigation measures to keep their citizens safe, allowing more activity at night, building underground, and ensuring their most vulnerable populations have access to air conditioning.
"It's an all-of-the-above kind of need, because these conditions are already happening and only getting worse," he said.
In the meantime, researchers will need to better understand how sea temperature, irrigation, clouds and other factors lead to extreme, localized bouts of heat and humidity, Raymond said. These insights could help better predict when these events will strike and how to mitigate their effects.