Three Times More City Dwellers Face Dangerous Heat, Humidity

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The number of city dwellers exposed to dangerous levels of heat and humidity has tripled in little more than one human generation, from 1983 to 2016, concludes a new study published this week in the Proceedings of the National Academy of Sciences (PNAS).

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By 2016, at least 1.7 billion were exposed to multiple days during which temperatures and humidity rose to levels at which even very healthy people found it debilitating to work outdoors, the study of more than 13,000 cities states.

And in a separate study, researchers warn that the combined impact of climate change and forest loss in Brazil could expose more than 11 million people to extreme enough heat stress to produce dehydration, exhaustion, collapse ,and even death.

Researchers have been warning for decades, and with increasing alarm, about the damaging consequences for human health of ever-higher temperatures. The two new studies are concerned with what could happen as levels of atmospheric carbon continue to rise in response to ever-greater combustion of fossil fuels.

But the first is a based on a careful, detailed examination of what has already happened on a day-to-day basis in 13,115 cities worldwide over a sequence of 34 years. Cities already hold more than half the global population. In a few decades, this will have risen to two thirds of all the people on Earth.

And cities, notoriously can be dramatically hotter than the surrounding countryside: this is the “urban heat island effect.”

The U.S. researchers behind the PNAS study used satellite data and ground instrument readings to assemble their figures, and matched it with population statistics for those cities over the same period. They defined their heat wave danger zone as 30°C on a measure called wet bulb globe temperature to take account of the impact of humidity on human health: humans can stand higher temperatures at low humidity, because they can cool by perspiration, but as humidity levels rise, this becomes more difficult. At a high enough wet bulb temperature, humans overheat, so that exhaustion, collapse, and death from a range of sudden conditions become an increasing hazard.

The data tell a clear story measured in person-days: in 1983, 40 billion people were exposed to potentially dangerous wet-bulb temperatures for at least one day. By 2016, this number had risen to 119 billion, almost three times as many. During those 34 years, the planet’s human population increased by nearly three billion, and the proportion of city dwellers rose, too, so the growth in human numbers accounted overall for two-thirds of the extra hazard.

Planetary heating accounted for the rest. But the proportions depend acutely on location. Population growth accounted for most of the extreme heat person-days in rapidly growing cities such as Shanghai and Guangzhou in China, or Yangon in Myanmar. In Baghdad, Cairo, Kuwait City, Lagos, Kolkata, and Mumbai, more than half the hazard came from the warming climate. Some 17 cities added an entire month of extreme heat days over the 34-year period. Many were in warm climates dominated by big river systems.

“A lot of these cities show the pattern of how human civilization has evolved over the past 15,000 year,” said Cascade Tuholske, of Columbia University’s Earth Institute, who led the study. “The Nile, the Tigris-Euphrates, the Ganges. There is a pattern to the places where we wanted to be. Now those areas may become uninhabitable. Are people really going to want to live there?”

The question is not a new one. It has already been raised again and again. The next step is to identify which communities could be most at risk. These include, according to research in the journal Communications Earth and Environment, 12 million now living in Brazil’s northern states of Pará and Amazonas.

The threat comes from global heating driven not just by fossil fuel combustion, but also by deforestation: intact forests almost everywhere around the globe have an insulating effect, moderating summer heat and alleviating winter cold, and Brazil’s rainforest, notoriously, has been increasingly at risk. The latest projections of temperature increase find that of the nation’s 5,565 municipalities, 16%—with a total population of 30 million—might by 2100 meet thermal stress as a consequence of the conversion of forest to savannah. Of these, 42% are in Brazil’s northern region and among “the most socially vulnerable,” the scientists say.

There will be damage, too, to the national economy, as conditions for outdoor work become increasingly difficult to tolerate. By 2030, a 1.5°C increase in global temperatures could reduce the nation’s work capacity by the equivalent of 850,000 full-time jobs.

“Extreme heat conditions induced by deforestation may have significant and long-lasting effects on human health,” said Paulo Nobre of Brazil’s National Institute for Space Research, one of the authors. “If deforestation continues at its current rate, the effects for our civilization will be dramatic.”