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Hotter Heat Waves Deadly, but Cities Can Use Tricks for Cooling Off

Cities can combat the urban heat island effect under global warming with trees, white roofs and other fixes
San Francisco skyline


View of downtown San Francisco, California from Dolores Park.
Credit: sagesolar via flickr

Dense metropolises of concrete, glass and asphalt are poised to warm faster than their surroundings as the planet heats up.

The higher temperatures mean more severe heat waves, which, according to the Centers for Disease Control and Prevention, are already the deadliest weather phenomenon in the United States. By midcentury, scientists expect between 3,500 and 27,000 additional heat-related deaths.

High temperatures make it harder for the body to cool off, and they exacerbate air pollution, leading to more hospital visits from people with vulnerable hearts and lungs, particularly the very young and the elderly.

However, there are tricks cities can use to cool off, and some recent research shows these tactics can save lives, even as the climate changes.

In a study published last month in the journal PLOS ONE, researchers reported that white roofs, reflective pavement and trees can counteract temperature increases in cities.

'Climate responsive design'
"No studies had looked at how cities could mitigate those impacts through what we would call 'climate responsive design,'" said Brian Stone Jr., an associate professor of city and regional planning at the Georgia Institute of Technology and lead author of the report.

The researchers analyzed Atlanta, Philadelphia and Phoenix using global and regional climate models under a business-as-usual scenario and under an aggressive heat management program. "We had to balance a need for diversity of climate types and diversity of growth patterns," Stone explained.

Philadelphia is an older, slower-growing city with more of an aging population than Atlanta or Phoenix. It's also at a higher latitude, which makes its population more vulnerable to heat waves, since they are less acclimated. Meanwhile, Phoenix resides in the desert Southwest and Atlanta is in the more temperate Southeast.

In the intervention scenario in these cities, the researchers modeled how the three cities would respond to a minimum green space ratio on land parcels, setting a floor for areas covered with grass, gardens or trees. Vegetation tends to have a cooling effect by circulating moisture in the air that draws away heat during evaporation. Tree canopies also provide cooling shade.

The team also modeled how Phoenix, Philadelphia and Atlanta would behave with more reflective streets, sidewalks, parking lots and rooftops. Higher reflectivity, or albedo, means the area absorbs less sunlight, thereby lowering the temperature.

Stone and his collaborators then overlaid a health impact model. They found that combinations of increased vegetation and albedo could cut into projected increases in heat deaths, reducing them between 40 and 99 percent. "On average, we reduced the rate of increase by about 60 percent," Stone said.

The magnitude of the risks each of the three cities faces is different, as well as the responses they have available. Phoenix, for example, is in "a climate that's not conducive to vegetation enhancement strategies," he said, noting that drier regions will have to lean harder on strategies that increase albedo. However, cities with less sunshine may not see benefits as drastic from cool roofs and the like.

But in order to have the greatest health benefits, cities will have to implement these tactics on wide scales, applying them to the entire metropolitan region around cities. This also means greening and whitening more than just government-held properties.

Private sector is key
"You really have to move beyond publicly owned parcels to modify the climate," Stone said. "You can't just rely on the public response."

Laurence Kalkstein, a professor in the Department of Public Health Sciences at the University of Miami Miller School of Medicine who was not involved in the study, observed that there are other variables that influence how much a city can benefit from warming mitigation.

Population density, for example, influences how many people are at risk in a given hot spot and alters a city's willingness to invest in local cooling. "The denser the urban area, the better the benefit," Kalkstein said, pointing out that a city like New York with roughly 27,000 people per square mile could save many more lives with vegetation and albedo enhancement than Los Angeles with its 7,500-people-per-square-mile population density.

Another issue is acclimation. "What is called a heat wave varies from city to city," Kalkstein said. The denizens of sunny Miami tolerate balmy days better than rain-soaked Seattleites, even if the heat surges to lower temperatures in the Pacific Northwest.

Under climate change, people in urban areas may get used to warmer temperatures, though they would still be vulnerable to heat surges, especially when they strike in April rather than August, Kalkstein observed. "We find that early-season heat waves are much more problematic than those later in the year," he said.

Groups like the Global Cool Cities Alliance are now trying to get cities to adopt these adaptation strategies, pitching them as a way to protect public health. However, it's slow going, given that cities around the country address heat vulnerability differently, if at all.

Stone said he now wants to investigate the economics behind planting trees and reflective coatings so cash-strapped cities can see the benefits of investing in these projects. "The major emphasis here is that cities should be undertaking heat management planning," he said. "There are steps they can take to actually slow the rate at which they're warming."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500

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