Extreme heat in cities a growing problem as climate warms, study finds

The risk of extreme heat is a rising 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. 

As more people move to cities and the climate warms, extreme heat could harm and kill more people, reduce worker productivity and hurt economies, the study says. The urban poor are most at risk. 

The study, which assessed more than 13,000 cities from 1983 to 2016, found that global extreme heat exposure increased nearly 200 percent over that time period, a result of population growth, climate change and the fact that city infrastructure absorbs more heat. Nearly a quarter of the world’s population is in areas where extreme heat exposure is rising, the study says.  

Researchers have lacked a complete picture of heat’s impacts because some fast-developing parts of the world don’t have reliable weather station data, and climate models used to estimate temperatures tend to gloss over urban hot spots. 

The study took a novel approach and used satellite data to measure heat worldwide, giving researchers a sharper global view of the problem. The analysis reveals that many people flocking to cities in rapidly urbanizing areas such as southern Asia and sub-Saharan Africa are coming to places that are increasingly vulnerable to soaring temperatures and high humidity. 

“Population growth isn’t inherently the problem, much less urbanization,” said Cascade Tuholske, a postdoctoral research scientist at Columbia University’s ​​Center for International Earth Science Information Network and the lead author of the PNAS paper. “It’s the lack of planning and lack of investment in these rapidly urbanizing areas — but that can change.” 

A separate report published Wednesday on climate change’s health effects suggests the problem is intensifying and inequities between rich and poor countries are growing. 

The Lancet Countdown, a yearly assessment of health risks from climate, found that children and people 65 and older have seen a steady increase in exposure to heat waves over the last decade. Over the past 30 years, countries with low and medium levels of development have seen the largest increases in vulnerability to heat, which was made worse because many of these communities lacked access to air conditioning, cooling and urban green spaces.

The report also says climate change is increasing conditions suitable for infectious disease pathogens, reversing global progress in providing food and water security and increasing exposure to wildfires. 

Heat can harm or kill in multiple ways. The body’s organs can overheat dangerously if it loses the ability to regulate temperature, risking death. Heat can also exacerbate symptoms from underlying ailments such as cardiac disease, diabetes or kidney problems.

In June, a record heat wave in the Pacific Northwest in the United States killed hundreds when temperatures climbed to 108 degrees Fahrenheit in Seattle and 116 in Portland, Oregon. Scientists have said temperatures so extreme would have been nearly impossible if not for climate change.

Temperatures, though, remain just one concern. Humidity, 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 of heat risk across the world’s cities, the authors used a measure called wet-bulb globe temperature to assess these factors. Wet-bulb globe temperatures are often used to determine how heat affects people during strenuous activities such as military exercises, sports or outdoor work. 

When wet-bulb globe temperature measures reach 86 degrees Fahrenheit, conditions are unhealthy for many people and deaths rise among those vulnerable to heat, the PNAS paper says. Those conditions could feel roughly equivalent to a heat index of about 107 degrees, Tuholske said. 

To understand trends in heat impacts, authors of the PNAS study estimated wet-bulb globe temperatures and heat index measures for thousands of cities using satellite thermal imaging data and combining them with readings on the ground. Then, they compared temperature data to population maps to understand how many people were affected by extreme heat. 

The authors estimate that the global population experienced a total of about 40 billion days when wet-bulb temperatures hit at least 86 degrees in 1983. In 2016, that number had nearly tripled to 119 billion, the paper says. Two-thirds of the change was due to population growth. The rest of the increase was due to climate change and additional heat due to urbanization. 

The researchers argue that some previous studies of global urban heat have underestimated its impact because some areas don’t offer reliable weather station observations. In India, for example, just 111 of more than 3,000 cities assessed offered good observation data, the paper says. 

“Four billion people live 20 or more kilometers (about 12.4 miles) away from a weather station,” Tuholske said. 

Climate models often used in this type of analysis tend to minimize extremes and aren’t designed to evaluate important small-scale differences in heat across 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 others nearby. 

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

The study has limitations, though, she said. 

Communities have different vulnerabilities and thresholds for when heat becomes dangerous, Ebi said, something the paper does not take into account. 

The world already has warmed by more than 1 degree Celsius (about 1.8 degrees Fahrenheit) since the 19th century, and it’s “unequivocal” that humans are heating the planet, according to the United Nations’ Intergovernmental Panel on Climate Change report released in August. 

Scientists expect 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 categorize heat waves similar to how meteorologists treat hurricanes. 

“People go to cities because there’s more opportunities,” Ebi said. “There are reasons that cities are growing. The question, then, is how do you grow cities in ways that take into account a warmer climate?”