Exposure to dangerous levels of heat and humidity in cities around the world has exploded over the past three decades, according to new research. In a study of more than 13,000 cities, the number of people exposed to extremely hot and humid days in a given year (measured in “person days”) tripled between 1983 and 2016.
It is a symptom that two trends are colliding: urban population growth and rising global average temperatures. Cities often reach higher temperatures than surrounding rural areas because they are typically designed in a way that captures heat. So when people flock to city centers, they also flock to places where they may be at greater risk of heat-related illness and death. To make matters worse, extreme heat is already a leading weather-related killer, and climate change is exacerbating the problem.
Cities will have to find ways to stay cool in a warming world in order to better protect their residents, the authors argue. “In some cities, long-term adaptation is frankly gloomy. But in large parts of the planet, we can use tools we already have, ”says Cascade Tuholske, a postdoctoral fellow at Columbia University and lead author of the study, published today in the journal Procedures from the National Academy of Sciences. “Population growth and urbanization are not the problem for me. It’s a lack of planning. ”
About a quarter of the world’s population lives in places where the exposure to extreme heat and humidity grows, Tuholske and his colleagues found. For the study, they defined “extreme” as the least 30 degrees Celsius on the temperature scale with wet bulb. The wet ball globe is a detailed assessment of heat, humidity, wind speed, cloud cover and the angle of the sun. Thirty degrees on that scale can be compared to a day that feels like 106 degrees Fahrenheit. (This is not a direct conversion from Celsius to Fahrenheit because factors outside the temperature are taken into account.)
They relied on both infrared satellite images and readings on Earth to determine weather conditions for much of the last three decades. Using population data collected by the European Commission and Columbia’s Center for the International Earth Science Information Network, researchers multiplied the number of extremely hot days by the number of people in each city to get a total of “person days” where city dwellers experienced these extreme conditions. The number of personal days grew from 40 billion a year in 1983 to 119 billion in 2016 — which shows how many more people are now suffering from extremely hot weather in the cities of the world.
Rising temperatures are responsible for about a third of the global exposure in exposure, the study authors sussed out. The majority of the growth in exposure has to do with more people moving to urban areas. But this relationship can vary from city to city. Boom in population growth was the main driver of heat exposure in Delhi, India. But in Kolkata, climate change was a slightly bigger factor. The differences show that solutions probably need to be tailored to suit each city. As cities continue to attract new residents and global average temperatures continue with rise, they must act quickly.
Cities can reach temperatures several degrees warmer than surrounding areas due to a process called the Urban Heat Island Effect. Asphalt and other dark surfaces absorb heat. Exhaust from factories and exhaust pipes emit more heat. And there are fewer trees to provide shade or plants to provide the cooling effect of evaporation (a process similar to how people cool down by sweating). These effects tend to be worse in neighborhoods that have received less investment and more industrial activity over time, a phenomenon that other research has shown to take disproportionate tolls on colorful communities in the United States.
Despite the increasing risk, heat-related deaths can be largely prevented. To cool down, cities can paint rooftops and other surfaces white to reflect heat. Bringing more trees and greenery to neighborhoods also helps. There is also more that can be done to warn people of impending heat waves so they can find public refrigeration centers or other places where they can run out of heat with air conditioning. New York City and Los Angeles are just a few of the major metropolitan areas around the world working to do just that.
“For billions of people on Earth, our study is retrospective. This is their lived experience every day, ”says Tuholske. “We really need to learn from them and work with people in really hot cities to adapt.”