Urban Areas Getting Hotter Faster

On a sizzling summer day, the center of a city's downtown can make you feel like a turkey baking in an oven -- and it's only going to get worse.

Not only do cities retain more heat than rural areas do, found a new study, hot cities will grow even hotter as the climate warms and cities grow. By mid-century, nighttime temperatures in cities could rise by more than 10 degrees Fahrenheit.

At stake are the comfort and health of people who live in cities around the world, especially those who don't have access to air-conditioning.

"If you've been exposed to hot temperatures during the day and you expect relief over night, that becomes increasingly difficult as temperatures at night get warmer," said Richard Betts, a climate scientist at the United Kingdom's Met Office. "We have to prepare to live in a warmer world."

In a concrete jungle, roads and buildings absorb sunlight and trap heat, which also flows as waste out of cars, air-conditioning units and even just the breathing of millions of people crammed into a busy grid of streets. As a result, cities create their own, warmer microclimates -- a phenomenon called the urban heat island effect.

Scientists have known about urban heat islands for many decades. Still, most climate models are based only on conditions in rural areas, where soil, trees, lawns and fields don't absorb and reflect sunlight the same way that asphalt crosswalks and concrete skyscrapers do.

To see how climate change might interact with urbanization and population growth, Betts and colleagues created a new model that compared future climate scenarios in cities with predictions for rural areas.

The results, published in the journal Geophysical Research Letters, found that the urban heat island effect is most extreme in dry, subtropical regions, where the differences between nighttime and daytime temperatures are greatest. Consider Las Vegas, for example, where nights often feel warm in the glitzy city, even though temperatures plummet in the surrounding desert when the sun goes down.

With projected increases in heat-trapping carbon dioxide, the model predicted that between now and 2050 daytime temperatures will increase equally in cities and rural areas. But over the same time period, city nights will get much warmer than rural nights.

In the Middle East, where the effect will likely be most extreme, the study predicted that CO2 emissions will lead to a nighttime rise of about 5.4 degrees Fahrenheit both inside and outside of cities, with an extra 5.4 degrees Fahrenheit rise within city limits. Among other places, nights will also get significantly warmer in the urban areas of East Africa, Central Asia and the western United States.

"Most people have assumed to this point that urban areas are warming about the same amount as rural areas," said Keith Oleson, a climate researcher at the National Center for Atmospheric Research in Boulder, Colo. "I think that study shows that urban and rural areas can act differently."

As cities grow warmer, it will become even more important to invest in urban cooling strategies, the study suggests, such as white roofs, green spaces, calculated window placement and other architectural decisions that allow buildings to spit out fewer greenhouse gases and less heat.

"We need to think more creatively about building design if we're dealing with a hotter climate," Betts said. "More people are expected to live in cities, and more people will be affected by warmer temperatures."