- Exposure to dangerously high temperatures in cities nearly tripled between 1983 and 2016, according to a study that considered both warming and population growth.
- Cities are hotter than their surrounding areas because they are densely populated and tend to generate and trap more heat.
- The decade starting 2011 was the warmest in recorded history, and the proportion of the global population exposed to extreme heat is expected to multiply in the coming decades.
- Excessive warming of urbanized areas and the relentless influx of people points to an urgent need for policies that protect city residents, especially in developing countries.
Of all the extreme weather phenomena experienced by humans today, heat is the deadliest. A heat wave that scorched Europe in 2003 claimed 70,000 lives. At least 15,000 people died in France alone, with the Paris region reporting the most excess deaths.
The French capital is not alone; cities the world over are bearing the brunt of a warming world.
Urban residents’ exposure to dangerously high temperatures nearly tripled in the 34 years between 1983 and 2016, according to a paper published Oct. 4 in the Proceedings of the National Academy of Sciences. Both burgeoning populations and exceptional warming have fueled this health crisis in metropolitan areas.
While calls for tackling climate change on a war footing grow, communities are grappling with its impacts today. The Earth has already breached the safe limits for climatic change, one of the planetary boundaries defined by environmental scientists Johan Rockström and Will Steffen.
By calculating the number of days “felt” temperatures crossed dangerous levels and tracking the changing demographics of urban areas, the researchers estimated that heat exposure grew from 40 billion person-days in 1983 to 119 billion person-days in 2016.
“To my knowledge, there are no efforts to quantify how exposure has changed over time,” said Cascade Tuholske a climate researcher at Columbia University’s Earth Institute and first author of the paper.
The researchers also calculated how much population growth and urban heat individually contributed to heat exposure in 13,115 urban centers across the globe.
They conducted their analysis at multiple levels, from the global right down to individual cities. A higher-resolution map of temperature extremes is useful because local conditions vary greatly, even over short distances.
The decade starting 2011 was the hottest in recorded history, with average global temperatures peaking in 2016, 2019 and 2020. The planet has already experienced a 1° Celsius (1.8° Fahrenheit) spike in average temperature from the 1950s. In the next few decades, it could rise by another 4°C (7.2°F).
By 2090, the number of people exposed to dangerously high average monthly temperatures could swell tenfold, research published earlier this year found. Asia and Africa will see big increases in at-risk people, largely owing to rapid urbanization on the continents.
Paschal Arsein Mugabe, a climate researcher at University of Dar es Salaam, Tanzania, who was not involved in the study, said the findings were important because they emphasized the outsize role that anthropogenic factors are playing in triggering these extreme heat episodes.
Globally, about 66% of the increased exposure is due to swelling urban populations and the remainder because of warming, the paper found.
But extreme heat is more than just rising temperatures. This study uses the WetBulb Globe Temperature, which combines multiple atmospheric elements, including temperature and humidity, to capture how people are feeling the heat.
As warm-blooded mammals, we have evolved ways to regulate our body temperatures. Sweating is key. As sweat evaporates, it cools off the body. However, if humidity is also high on a hot day, this in-built thermoregulation falters because sweat cannot evaporate as effectively into moisture-laden air.
Cityscapes bereft of vegetative cover also stifle the terrain’s ability to moderate temperatures. Urban spaces plastered with human-made materials like concrete, asphalt, steel and bricks tend to absorb more heat and are slow to release it. Cities themselves generate a lot of heat from vehicles, buildings and industrial activities. This makes urban settlements warmer than surrounding areas — the “heat island” effect.
More than half of Earth’s 7.7 billion humans are packed into cities today, more than the world’s total population in 1975. This proportion is expected to grow to 68% by 2050, with the most significant gains in India, China and Nigeria.
In urban settlements, extreme heat exposure is highly unequal in how it impacts people; the urban poor suffer the most. Harsher working conditions and lack of shelter and access to health care turn this exposure into a health risk. The elderly, especially those over 64, are also especially vulnerable to these impacts.
“An increasing threat to human life from excess heat now seems almost inevitable, but will be greatly aggravated if greenhouse gases are not considerably reduced,” said a 2017 paper by Camilo Mora and colleagues at the University of Hawaiʻi, Mānoa.
Even if countries progress in the arduous task of curtailing greenhouse gas emissions, warming and its effects are not going away any time soon. The new study adds to knowledge about how to manage risks attached to this exposure.
Tuholske, who was at the University of California, Santa Barbara, when he started work on the paper, said he hoped the findings would “enable those most vulnerable to dangerous extremes to take action and reduce harm.”
One way to reduce harm is to be prepared. Mugabe at the University of Dar es Salaam said the data from the study could help design early warning systems. Philadelphia in the U.S. launched the first heat health warning system in 1995. Since the fateful summer of 2003, European countries have also been quick to adopt measures to reduce the toll from heat waves.
France installed a National Heat Wave Plan in 2004 and has developed an early-warning system to spur officials into action and raise awareness among the public. Despite facing an event hotter summer in 2019 than in 2003, the country reported fewer than 1500 heat-linked fatalities.
However, countries where the exposure is set to intensify the most are struggling. In some cases, this even extends to recognizing the magnitude of the problem.
The densely populated Indian city of Kolkata has seen a massive rise in heat exposure in recent years. In 2015, the city’s exposure stood at around 3 billion people-days. After the death of two taxi drivers that year, a taxi union in Kolkata decided to stop work during the hottest hours of the day. Severe spells of hot weather often trigger school closures. However, the city itself does not have an operational heat action plan.
These challenges cannot be readily overcome by technology or even infrastructure development, Tuholske said. Instead, they require policies that are sensitive to the conditions of city residents.
Tuholske C., Caylor K., Funk C., Verdin, A., Sweeney, S., Grace, K., … Evans, T. (2021). Global urban population exposure to extreme heat. Proceedings of the National Academy of Sciences, 118(41), e2024792118; doi:10.1073/pnas.2024792118
Mora, C., Dousset, B., Caldwell, I. R., Powell, F. E., Geronimo, R. C., Bielecki, C. R., … Trauernicht, C. (2017). Global risk of deadly heat. Nature Climat Change, 7(7), 501-506. doi:10.1038/nclimate3322
Klein, T., & Anderegg, W. R. (2021). A vast increase in heat exposure in the 21st century is driven by global warming and urban population growth. Sustainable Cities and Society, 73, 103098. doi:10.1016/j.scs.2021.103098
(Banner Image: A stock image of a cityscape courtesy of Photostockeditor)
Malavika Vyawahare is a staff writer for Mongabay. Find her on Twitter: @MalavikaVy
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