Study predicts deadly humid heat waves due to climate change

Life-threatening periods of high heat and humidity will quickly spread across the world with only a slight rise in global temperatures, a new study has found, potentially leading to a surge in deaths from the climate crisis.

According to The Guardian, extreme weather events that can be fatal to healthy people within six hours can affect hundreds of millions of people unaccustomed to such conditions. As a result, heat-related deaths could rise rapidly unless serious efforts are made to prepare the population as a matter of urgency, the researcher said.

Normally, the human body cools itself by producing sweat, which evaporates and removes heat. But at high humidity, evaporation decreases. The study used a limit based on human experiments showing that when the combination of heat and humidity, measured by the so-called wet bulb temperature, exceeds 31.5°C, the body is no longer able to cool itself.

The researchers called this threshold “uncompensated heat stress,” because sweating cannot compensate for extreme conditions. Without means of cooling, such as cold water, fans or air conditioning, death is likely to occur within a few hours.

The study analyzed data from thousands of weather stations around the world to show that 4% have already experienced at least one six-hour period of such extreme heat stress since 1970, and the frequency of such events is expected to double by 2020. However, until now they have been limited to hot places, including the Persian Gulf in the Middle East, the Red Sea and the North Indian Plain, where people expect extreme heat.

The analysis, which also used climate models, shows that extreme heat stress will quickly spread to other regions with global warming of just 2°C. The climate crisis has already caused global temperatures to rise by about 1.2 degrees Celsius. At 2°C, more than 25% of weather stations will experience extreme heat stress on average once per decade.

The East Coast and midwestern regions of the United States and Central Europe, including Germany, are among the places that will face unprecedented heat stress. In places that are already hot, such as Arizona, Texas and parts of California, periods of extreme heat stress would become annual events at 2°C.

The lack of mass deaths in places already under extreme heat stress showed that cooling measures can be effective in preventing deaths, the researchers said. But they said the rapid onset of these conditions in places where people historically had no need for cool buildings or air conditioning and were therefore unprepared was concerning.

“Of all the different manifestations of climate change, heat is the one I personally worry about the most, and these results made me even more worried,” says Carter Powis of the University of Oxford, UK, who led the study with colleagues at the Climate Research Center Woodwell in the USA.

“Everything is fine until it happens, and there are limits to what the human body can tolerate when it comes to heat,” the scientist said. “Everything will go on as usual until we cross these boundaries, and then very suddenly everything will go wrong.” Suddenly you will see an increase in mortality. What I worry about is that our bodies and our society will be more vulnerable to small changes than we think. It’s just another study that says we need to start cutting emissions – but that’s what we need to do.”

Dr Colin Raymond from the University of California, Los Angeles, who was not part of the research team, said: “Many areas are now only slightly below uncompensated levels, so as the planet continues to warm, the overall increase in impact will be exponential.

“We know the broad outlines of what lies ahead, but we don’t know every last thing,” Dr. Raymond said. “But the severity of the danger of extreme heat means it is incredibly important not to be caught off guard and treat heat waves near or above the uncompensated limit like other life-threatening disasters. In the most serious events likely to occur in the coming decades, lives will depend on the availability of artificial refrigeration.”

Previous studies of extreme heat and humidity have used the maximum theoretical limit of the human body – a wet bulb temperature of 35°C, at which even cooling agents cannot remove heat from the body. These studies have shown the high vulnerability of people in the Gulf countries, India and China to extreme temperature and humidity conditions. Scientists around the world estimate that the climate crisis has already caused millions of premature deaths over the past three decades.

The study, published in the journal Science Advances, used detailed weather station data and broader data from climate modeling and found “a very good fit,” Powis explained, which gives confidence in the results.

The researchers concluded: “The geographic range and frequency of uncompensated temperature extremes will increase rapidly, given only a modest continued increase in average global temperatures. This means that in the near future a significant portion of the world’s population will be exposed to these uncompensated environmental conditions.”

Scientists said there was a “real risk” of widespread impact, with “hundreds of millions of people” affected before they were sufficiently adapted to the heat to avoid the accompanying increase in death and disease. As The Guardian notes, researchers were unable to assess Europe and Japan using climate models in this study due to known difficulties in representing high levels of heat in Europe and humidity in Japan.

Even if adaptation measures are put in place to protect health, extreme heat stress will still prevent many people from working or going outside, Powis said. “Even where it is not fatal, the impact on quality of life will be dramatic.”