If you ever have driving from the country into the city and wondering how the temperature rose dramatically, you have felt the hot island effect in the cities. The streets and buildings of a metropolis absorb the energy of the sun during the day and release it gradually at night. The built environment essentially bakes itself, and temperatures can rise as much as 20 degrees Fahrenheit higher than the surrounding land, taking advantage of shards of trees that “sweat”, release water vapor and cool the air.
As global temperatures rise rapidly, scientists, governments and activists are fighting for ways to counter the heat island effect. According to the World Health Organization, the number of people exposed to heat waves jumped by 125 million between 2000 and 2016. Extreme heat kills more Americans than any other natural disaster and is especially dangerous for people with pre-existing conditions such as asthma.
By 2050, seven out of ten people will live in cities, says the World Bank. It will be a lot of bulging people. “I really see cities as a kind of canary in the situation of coal mines, where one has a bit of an idea of what the rest of the planet can experience,” says climate adaptation researcher from Portland State University, Vivek Shandas, who has studied the heat island effect over 50 American cities.
Shandas’ research has shown that even in cities, one neighborhood can be 15 degrees warmer than another, and that differences map income inequalities. A big prediction for a quarter of an hour of heat is how much green space it has. Richer parts of a city tend to have more green, and poorer parts have more concrete; they are highly developed and filled with large checkout counters, highways and industrial facilities that absorb the sun’s radiation. A concrete landscape is actually so good at keeping warm that it stays warm at night. When the sun comes up, a poor neighborhood is already warmer than a rich neighborhood.
Researchers have just begun to investigate whether they can lower the temperature in urban structures by using “cool” roofs, walls and sidewalks – those that are bright and escape the sunlight. Lighter surfaces reflect more of the sun’s radiation than dark surfaces. (Think about how you feel while wearing black instead of white on a sunny day. This albedo effect is also part of the reason why the Arctic heats up so fast.) But while the thermodynamics are straightforward, it turns out , that the spread of cool surfaces is be strangely complicated.
Take the problem of cooling roofs, says environmental engineer George Ban-Weiss, who studies cool infrastructure at the University of Southern California. In theory, it’s simple to paint the large, flat tops of commercial buildings white or light gray. Homeowners can choose lighter tiles – plain old clay actually reflects sunlight quite well. These changes would cool the air coming from a roof, as well as the structure itself, meaning residents do not have to run air conditioners as often. If a building can support the extra weight, the owners could even create a garden on the roof filled with plants, which would cool the entire area by releasing water vapor.
But while these changes would make life more bearable for the people inside each modified building, if enough owners followed suit, it could have an unintended regional side effect in some areas. In a coastal metropolis like Los Angeles, the hot city generally stands in contrast to the cold of the ocean, a differential that drives a reliable ocean breeze. When land and sea temperatures get closer to each other, there may be less wind. “So that means less clean air is coming into the city, which would tend to make polluting concentrations higher,” says Ban-Weiss plus the loss of the breeze that even keeps people cool.