Extreme heat is accentuated in fast-growing cities and can be deadly. These are the reasons

By Evan Bush -

The risk of extreme heat is an increasingly present threat to fast-growing cities around the world, according to a new study published this month in the Proceedings of the National Academy of Sciences. 

According to the study, as more people move to cities and the climate warms, extreme heat could lead to health problems and kill more people, reduce worker productivity and deplete economies.

Poor people in cities are most at risk. 

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The study, which evaluated more than 13,000 cities between 1983 and 2016, found that exposure to extreme heat in the world increased by almost 200% during that period, as a result of population growth, the climate emergency and the fact that the City infrastructure absorbs more heat.

According to the document, almost a quarter of the world's population is in areas where exposure to extreme heat is increasing.

The researchers lacked a complete picture of the impacts of heat because some parts of the rapidly developing world do not have reliable data from weather stations and the climate models used to estimate temperatures tend to overlook urban hot spots. 

The study took a novel approach and used satellite data to measure heat around the world, providing researchers with a sharper global view of the problem.

The analysis reveals that many people who flock to cities in rapidly urbanizing areas, such as South Asia and sub-Saharan Africa, reach places that are increasingly vulnerable to high temperatures and high humidity.

"Population growth is not inherently the problem, much less urbanization," said Cascade Tuholske, a postdoctoral research scientist at Columbia University's International Earth Sciences Information Network Center and author. Main article of PNAS.

"It is the lack of planning and investment in these rapidly urbanizing areas, but that may change," he added.

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Another report released Wednesday on the health effects of the climate emergency suggests that the problem is intensifying and that inequalities between rich and poor countries are increasing. 

The Lancet Countdown, an annual assessment of climate-related health risks, found that children and people 65 and older have experienced a steady increase in exposure to heat waves in the last decade.

Ally Rogers, 5, plays in water from a fire hydrant with her father Allen, Tuesday, July 28, 2020, in New York. Mark Lennihan / AP

In the last 30 years, countries with low and medium levels of development have registered the greatest increases in vulnerability to heat, which was exacerbated because many of these communities lacked access to air conditioning, refrigeration and urban green spaces.

The report also notes that the climate emergency is increasing the right conditions for infectious disease pathogens, reversing global advances in providing food and water security, and increasing exposure to wildfires. 

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Heat can cause health problems or kill in multiple ways.

The body's organs can become dangerously overheated if the ability to regulate temperature is lost, leading to death.

Heat can also exacerbate symptoms of underlying ailments, such as heart disease, diabetes, or kidney problems.

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In June, an unprecedented heat wave in the Pacific Northwest of the United States killed hundreds of people when temperatures reached 108 degrees Fahrenheit in Seattle and 116 in Portland, Oregon.

Scientists have said that such extreme temperatures would have been nearly impossible if it weren't for the climate emergency.

However, temperatures are only one of the concerns.

Moisture, sun exposure, and wind also affect the body.

High humidity, for example, can reduce the body's ability to cool itself with sweat.

In the study on the risk of heat in the world's cities, the authors used a measure called the wet globe temperature to assess these factors.

This temperature is often used to determine how heat affects people during strenuous activities, such as military exercises, sports, or outdoor work. 

When wet globe temperature measurements reach 86 degrees Fahrenheit, conditions are unsanitary for many people and deaths increase among those most vulnerable to heat, according to the PNAS article.

These conditions could roughly equal a heat index of about 107 degrees, according to Tuholske. 

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To understand trends in heat impact, the PNAS study authors calculated wet globe temperatures and heat index measurements for thousands of cities using satellite thermal imagery data and combining them with readings from the ground.

They then compared the temperature data with the population maps to find out the number of people affected by extreme heat. 

The authors estimate that the world's population experienced some 40 billion days in which wet bulb temperatures reached at least 86 degrees in 1983. By 2016, that number had nearly tripled to 119 billion, according to the paper.

Two-thirds of the change was due to population growth.

The remainder of the increase was due to the weather emergency and additional heat due to urbanization. 

5-year-old Ellie Campbell refreshes herself in Missoula, Montana, on June 30, 2021.Tommy Martino / AP

The researchers argue that some previous studies of global urban heat have underestimated its impact because some areas do not provide reliable observations from weather stations.

In India, for example, only 111 of the more than 3,000 cities evaluated offered good observational data, according to the paper. 

"4 billion people live 12 miles (20 kilometers) or more away from a weather station," Tuholske said. 

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Climate models commonly used in this type of analysis tend to minimize extremes and are not designed to assess large small-scale heat differences between cities.

For example, areas with fewer trees and more pavement tend to absorb more heat, making some parts of cities 10 or even 20 degrees hotter than nearby ones. 

Kristie Ebi, a professor at the University of Washington Center for Global Health and Environment who studies health and heat waves, said the use of satellite data provided valuable new analysis and clarifies the extent to which population trends are contributing to increased vulnerability to heat. 

However, the study has its limitations. 

Communities have different vulnerabilities and thresholds when heat becomes dangerous, according to Ebi, something the document does not take into account. 

The world has already warmed more than 1 degree Celsius (about 1.8 degrees Fahrenheit) since the 19th century, and it is "unequivocal" that humans are warming the planet, according to the report of the Intergovernmental Panel on Climate Change of the United States. United Nations published in August. 

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Scientists predict more frequent and intense heat waves as humans continue to burn fossil fuels and warm the planet.

Some cities are preparing: The mayor of Seville, Spain, announced this week that his city will name and classify heat waves in a similar way to how meteorologists treat hurricanes. 

"People go to cities because there are more opportunities," Ebi recalled.

“There are reasons why cities are growing.

The question is how to get cities to grow taking into account a warmer climate ”, he added.