Hot Time in the City

Concentrations of heat in urban areas can make life difficult for city dwellers.

By Violet Snow

July/August 2010

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As climate change concerns intensify, scientists are paying more attention to urban heat islands (UHIs). Cities, with their dense concentrations of man-made structures, tend to have significantly higher temperatures than surrounding rural areas, often with harmful effects on human health.

Researchers from Harvard Medical School have estimated that 1,500 people are killed by heat during an average summer in the United States (Climate Change Futures 2005). What’s more, a research team led by investigators from Texas Tech University has found that “the frequency of extreme heat events may double, or even triple, in many cities over the remainder of this century” (Distributional Impacts of Climate Change and Disasters 2009).

UHIs and climate change are related. “A UHI comes from local sources. Climate change is global and comes from the trapping of greenhouse gases in the atmosphere,” says Sharon Harlan, PhD, associate professor of sociology at Arizona State University. “They are related in that cities are also sources of gases, and because of climate change, the incidence of extreme heat events is predicted to increase.”

A city’s relative lack of plant life is responsible for UHIs. “When sunshine hits the earth and converts to heat, it evaporates water from plants, and heats the ground and air,” says Larry Kalkstein, PhD, director of the Univer­sity of Miami’s Synoptic Climatology Laboratory. “In the country there’s a lot of grass and trees, so evaporation can take place. In an urban area, [vegetation] is more limited, so all the energy is going into heating the ground and air.” Cars, air conditioning and other heat sources contribute to the UHI.

The temperature difference between a UHI and its rural surroundings is even greater at night, when the city’s buildings block the radiation of surface heat back into the cooling atmosphere. According to the Environmental Protection Agency, the temperature differential between city and country can be as high as 5.4°F during the day—and 22°F at night.

Kalkstein says that the elderly and infants are particularly vulnerable to heat’s effects. Obesity is another risk factor, as is diabetes because elevated glucose levels increase urination, leading to more rapid dehydration. Ironically, in cities like Phoenix and Miami, where air conditioning is more widely available, heat-related mortality is lower than in places such as New York and Chicago, where highly variable temperatures do not prepare residents for heat waves.

There are ways to counteract the effects of high heat. “The major problem is that people don’t drink enough during hot weather,” says Kalkstein. “We need moisture to allow evaporation of sweat, our main mechanism for cooling. Sitting in front of a fan can actually accelerate drying and prevent cooling. Sitting in a cool tub of water is the best way to cool down.”

Kalkstein has led the development of warning systems in major cities so that the National Weather Service knows when to issue heat advisories. “A study shows that in a three-year period in Philadelphia, we saved 117 lives,” he says.

Harlan’s research suggests that increasing urban vegetation, including green roofs that feature plantings on top of buildings, may help cool things down. Other solutions include using light colors to reduce heat absorption.

Avoiding UHIs requires both individual and societal measures. As Harlan warns, “We’re going to have a warmer world.”

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