Oct 9, 2023 - Energy & Environment

Study: Climate change to drive temperatures too hot for humans

A person cools off amid searing heat that was forecast to reach 115°F in July in Phoenix, Arizona, which became the first major U.S. city to reach an average monthly temperature higher than 100°F over the summer. Photo: Brandon Bell/Getty Images

Billions of people are at risk of temperatures exceeding survivability limits if global temperatures increase by 1°C (1.8°F) or more above current levels, a new study warns. Even young, healthy people could find it unbearably hot during part of the year, the study finds.

Driving the news: Regions in the Middle East and South Asia would "experience the brunt of deadly or intolerable conditions," researchers noted. Toward the higher end of warming scenarios, "potentially lethal combinations of heat and humidity could spread" to areas including U.S. Midwestern states.

Why it matters: The study, published in the journal Proceedings of the National Academy of Sciences on Monday, finds that temperatures are increasing and heat waves are becoming "more frequent, intense, and longer-lasting due to climate change."

  • Some areas are already exceeding the limits of the human body's tolerance for the combined impacts of heat, humidity, sun exposure and other factors, known as the wet bulb temperature.
  • This year, the world has endured some of the hottest summer and early fall temperatures on record, with global average surface temperatures temporarily exceeding the Paris Agreement's temperature target of 1.5°C (2.7°F) compared to preindustrial levels multiple times in 2023.

Yes, but: The Paris Agreement's threshold isn't truly broken until that higher level is maintained for about 30 years.

  • This study by scientists from Penn State and elsewhere warns this could occur if significant emissions cuts aren't made.

What they did: Researchers modeled temperature scenarios ranging from the Paris Agreement's target of 1.5°C of warming through 4°C and identified regions most at risk from rising heat and humidity for the study.

  • Researchers adopted a wet bulb temperature threshold, beyond which humans have great difficulty surviving, and used a slightly lower wet bulb temperature than the oft-mentioned 35°C (95°F) — based on the notion that humans may be more sensitive to humid heat than previously thought.

What they found: The regions projected to be the worst affected in scenarios that reach upward to 2 °C are equatorial and Sahel regions of Africa and eastern China, per the study. Researchers note this is "a viable outcome by the end of the century, perhaps sooner, without drastic reductions in greenhouse gas emissions."

  • "Continued warming above 3 °C and 4 °C, respectively, causes North and South America, as well as northern Australia, to experience extended periods of dangerous heat," per the study.
Composite map showing eastern US and western Africa. Subsaharan Africa is yellow and orange. The eastern US is yellow. This composite map shows land areas that could face extreme heat if the planet continues to warm, indicated in yellow and orange. The darker the color, the longer the projected exposure to extreme heat.
A composite map showing regions that could face extreme heat if the planet continues to warm, indicated in yellow and orange. The darker the color, the longer the projected exposure to extreme heat. Image: Daniel Vecellio, Qinqin Kong, W. Larry Kenney and Matthew Huber; composite image by Dennis Maney via Pennsylvania State University

Zoom out: The study bolsters other recent research showing human survivability limits being challenged in some parts of the world at current or higher levels of warming. A separate study last year warned of the emergence of an "extreme heat belt" from Texas to Illinois.

  • That hyperlocal analysis of current and future extreme heat events by the nonprofit First Street Foundation found the heat index in these areas could reach 125°F at least one day a year by 2053.
  • Another study published in 2020 found that intolerable heat was already occurring in some parts of the world, including the Middle East, and has been increasing in frequency.

Of note: Preliminary data from the Copernicus Climate Change Service shows the global average surface temperature for June through August was the hottest on record, as studies show climate change boosted the deadly heat in the U.S. and Europe.

The bottom line: "In the future, moist heat extremes will lie outside the bounds of past human experience and beyond current heat mitigation strategies for billions of people," per the study.

  • "While some physiological adaptation from the thresholds described here is possible, additional behavioral, cultural, and technical adaptation will be required to maintain healthy lifestyles."

What they're saying: "This will be a critical benchmark for future studies," said atmospheric scientist Jane Baldwin of University of California Irvine who was not involved in the research, to Reuters.

  • "Unfortunately, it's a somewhat grimmer picture than you would have gotten with the 35C limit."

Go deeper: Climate change-fueled heat waves swept the world this summer

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