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Very soon, extreme heat waves could cease to be world news and become merely worldwide. That is because the chance of two or more northern hemisphere land masses being hit by prolonged, extensive and very high spells of summer heat at the same time has increased six-fold in the last 40 years.
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And those most likely to suffer the greatest discomfort will be city-dwellers: U.S. scientists in a separate study found that urban populations—in an America of sealed roads, paved sidewalks, glass walls and tiled roofs—could experience between two and six extra hours of uncomfortable heat each summer day , simply because they live in cities.
Country folk in the tropics, too, will pay an extra heat penalty, says a third study, as the tropical forests dwindle. Forest clearance between 2003 and 2018 alone has condemned 4.9 million people worldwide — more than half of them outdoor workers—to increasingly intolerable temperatures.
There can no longer be much uncertainty about the hazards of a hotter world: the UK Met Office predicts that 2022 will be the eighth successive year in which global temperature has exceeded 1°C above what was normal in the 19th century. The same institution has already warned that, at a rise of 2°C in a few decades, around a billion people could expect to experience real heat stress. What’s new in the latest round of alarms is the possibility that the sun’s heat will, increasingly, scorch multiple regions at the same time.
Scientists at Washington State University report in the Journal of Climate that between 1979 and 2019, the number of days on which extremes of heat hit two separate regions at the same time increased to 143 days a year—and there are only 153 days in the northern hemisphere warm months between May and September. In their definition of a heat wave, unusually high temperatures must have occurred for at least three days and extended over an area of 1.6 million square kilometres, about the size of Mongolia.
And these concurrent heat episodes got worse with time: their maximum intensity went up by 17%, while their geographic extent increased by 46%. And that should be cause for worldwide alarm about the seemingly inexorable rise in global temperatures, fuelled by ever-increasing fossil fuel use. Heat waves have been accompanied by multiple and devastating forest fires and by harvest loss, with concurrent heat waves linked to a 4% drop in global crop yield.
Researchers have already found an increasing likelihood that any one place could be hit by recurring extremes of temperature in one season. Extremes of heat hamper agricultural productivity: another piece of research has found that global crop yield could fall by 10% by 2050 because farmers would struggle to adapt. And yet another study concluded that one of the worst global famines in the 19th century happened because of concurrent extremes of heat and drought on three continents.
“As a society we are not currently adapted to the types of climate events we are experiencing right now,” said co-author Deepti Singh of Washington State University. “It is important to understand how we can reduce our vulnerability and adapt our systems to be more resilient to these kind of heat events that have cascading societal impacts.”
Extreme heat is well understood as a health hazard. And the greatest concentration of potential sufferers is to be found in the world’s cities, each of them a “heat island” surrounded by cooler countryside. A new study in Geophysical Research Letters has matched the two to reveal that cities in the eastern and central United States can be between 1.9 and 4.9°C hotter than the surrounding farms and woodlands.
The same data found that the heat of the night could be significantly more difficult to endure than the noonday sun, as all the brickwork, cement, and tarmacadam release the energy stored during the glare of daylight.
“People have known about the urban heat island effect for more than 180 years, but the fact that the differences between nighttime and daytime and among different cities is so substantial is somewhat surprising,” said co-author Yun Qian of the Pacific Northwest National Laboratory.
“Our findings show that urban living makes nighttime hours in all U.S. cities studied more uncomfortable.”
Famously, trees can help curb urban heat and cool the rural landscape. Infamously, the world’s tropical forests are still being burned, cleared, or degraded. According to a new study in the journal One Earth, accelerated forest loss has also compounded the heat hazard faced by outdoor workers: the risk was disproportionately high in the states of Matto Grosso and Pará in Brazil. In large, deforested areas, annual average maximum temperatures could reach 10°C higher than in the surrounding forest.
“Our findings highlight the vital role tropical forests play in effectively providing natural air-conditioning services for populations vulnerable to climate change,” said study lead Luke Parsons of Duke University. He added that “these are typically regions where outdoor work tends to be the only option for many, and where workers don’t have the luxury of retiring to air-conditioned offices whenever the temperature rises to intolerable levels.”