Too hot to handle

Summertime rolls into Phoenix, Arizona with as much welcome as a pimple on prom night. While people in other parts of the country head outdoors for picnics and baseball games, Phoenicians hole up in air-conditioned buildings as temperatures soar above 100 degrees F for months at a time.

Some years are worse than others. In July 2005, Phoenicians sweltered through nine consecutive days that topped 110 degrees. At least 21 people died as a result of that heat wave, one of the worst in the area’s recent history.

Obviously, homeless people and undocumented immigrants crossing the desert borderlands are at especially high risk during these heat waves. But the results of a new study show that even low-income people with homes are more vulnerable to the heat than wealthier folks. The reason has to do with what scientists call the “heat island effect.”

The Sonoran Desert is known for its hot, dry climate. But Phoenix gets an extra helping of heat because it is an urban heat island. In the desert wilderness, hot summer days turn into cooler nights when the sun sets. But in cities, structures such as buildings, roads, and parking lots retain daytime heat. As a result, the temperature is ratcheted up even in the dark of night. The city transforms into an “island” of heat.

The Phoenix heat island isn’t uniform. Within its boundaries, temperatures vary widely from neighborhood to neighborhood. It turns out that people living in the warmest neighborhoods typically have lower income than people living in cooler neighborhoods. The warmer neighborhoods also tend to have higher minority and elderly populations.

Phoenix heat map
This map shows land surface temperature in the Phoenix area taken by satellite on July 24, 2000 at 11 a.m. The surface temperature is the temperature you would feel if you touched the ground.

“There are significant differences in temperature even within a metro area and that’s correlated with humans—the people that live in those areas,” says Darren Ruddell, a former Arizona State University doctoral student who led the study. “It’s an environmental justice issue. The people who are most vulnerable are also living in the worst conditions. It’s a double whammy.”

Sidewalks are for frying eggs

The reason why has to do with land cover—the things people place or plant on the ground. Ruddell studied temperature and land cover in 40 different neighborhoods throughout the Phoenix metro area from July 15 to July 19, 2005. He divided land use/land cover into four categories:

  • urban (commercial/industrial)
  • xeric (residential drought-resistant landscaping)
  • desert (undisturbed natural land)
  • mesic (residential with mostly grass landscaping)

Ruddell determined daytime and nighttime temperatures using data from the Weather Research and Forecasting model developed by the National Center for Atmospheric Research. Susanne Grossman-Clarke is a researcher in ASU’s Global Institute of Sustainability. She calibrated the model for metro Phoenix and ran the simulations. Ruddell compared temperature readings from local weather stations to the model to ensure its validity.

He also analyzed census data on population, income, ethnicity, and age for each of the neighborhoods he studied. Ruddell compared the temperature and census data to land use information taken from satellite images.

map of extreme heat exposure for 40 neighborhoods
This image represents the amount of exposure to extreme heat (113 F or more) among the 40 neighborhoods.

The results showed that urban and xeric landscapes were hotter than mesic and desert areas. This might come as disappointing news for people trying to help the environment by xeriscaping their yards. Ruddell says that xeriscaping might not be the best environmental decision in the long run.

“You change the landscape and then these neighborhoods become hotter. So people use the air conditioner more and for longer periods,” he says. “And water is used in generating the electricity that runs the air conditioner.”

In low-income neighborhoods, neither grass nor desert landscapes are common. Instead, these areas sport layers of concrete or simply bare soil. These are the hottest neighborhoods of all. And their residents are the least likely to have the resources to combat the scorching temperatures.

Is it getting hot in here?

In 2006, more than 800 Phoenix area residents shared their perceptions about heat in a survey developed by Sharon Harlan, a professor at ASU’s School of Human Evolution and Social Change. Harlan’s Phoenix Area Social Survey asked residents if they think Phoenix is getting a lot hotter, a little hotter, or not hotter at all. It also asked them to compare their neighborhoods’ temperatures with other neighborhoods in the metro area.

Ruddell compared these answers with the temperature data. He found that perceptions were very accurate on the neighborhood scale. People living in cooler neighborhoods perceived their local environments as cooler and people living in warmer neighborhoods perceived their environments as warmer. But perceptions became less accurate on a regional scale.

“People can perceive climate change when they experience it,” says Ruddell. “People’s perceptions of environmental conditions become increasingly distorted as the scale broadens, however. If you continue that out to the global scale, people don’t perceive global climate change very well at all.”

The survey also asked residents if anyone in their households had experienced heat-related symptoms—such as leg cramps, fatigue, or fainting—during the summer of 2005. Ruddell noticed a spike in the number of symptoms in the very hottest areas. The results suggest that there is a tolerance threshold for heat.

Ruddell hopes the information from his study can help Phoenix area residents to avoid heat-related health problems. One way to do this would be to redesign neighborhoods.

“If we know that green space or shade could relieve heat stress we could change the design of neighborhoods,” he suggests.

However, Ruddell adds that redesigning neighborhoods is not as simple as it sounds. Growing trees requires time and money, but time and money don’t grow on trees. Many low-income residents can’t afford to water plants and don’t want to mow lawns after working multiple jobs.

Another possibility is improving heat wave warning systems. “Right now the heat wave warning systems aren’t that effective,” says Ruddell. “They’re applied at really broad scales. With the information that we have we could be a lot more effective. We could identify areas that are vulnerable and set up aid stations.”

Ruddell received his doctorate in geographical sciences from ASU in Spring 2009. But he is still working with ASU researchers to study water use and temperature on a household level. The scientists hope to learn the best way to landscape to minimize water use while maximizing the cooling effects of plants.

“I think we can design cities better to help reduce the intensity of urban heat islands as well as reduce human vulnerability to heat,” he says.

Diane Boudreau