A Data-Driven Approach to Cooling a City
Urban heat islands threaten public health. Dallas is turning to a smart growth strategy -- and lots of trees -- to deal with the problem.
City governments face critical challenges as more and more people move into urban areas, including rising housing costs, traffic congestion and waste generation. For fast-growing cities, particularly those in warmer climates, there is growing awareness of another environmental issue that impacts public health: heat islands.
In large cities, the urban heat island effect results in annual average temperatures that can range from 1.8 to 5.4 degrees higher than the surrounding areas; at night, cities can be up to 22 degrees hotter. This happens because cities are landscapes of asphalt and have buildings of concrete and steel that stretch into the skies. These materials absorb heat, hold it during the day and release it when the sun goes down. Urban traffic congestion aggravates this problem as carbon dioxide emissions from vehicles trap heat.
Hotter temperatures have serious implications for public health. According to the Environmental Protection Agency, exposure to extreme heat can lead not only to heatstroke and dehydration but also to cardiovascular, respiratory and cerebrovascular disease. Prolonged periods of extreme heat now kill more people in the United States every year than all other weather-related events combined, according to the Centers for Disease Control and Prevention.
Growing rapidly in the Sun Belt, Dallas is warming faster than all but two large U.S. cities, Louisville and Phoenix, according to one study. Knowing they had to act to stave off a public health crisis, Dallas officials and environmental groups partnered with the Trust for Public Land (TPL), the Texas Tree Foundation (TTF) and several other organizations to develop Smart Growth for Dallas, an initiative to improve the city's social, economic and environmental resilience.
Smart Growth for Dallas is intensely data-driven. In 2017, a report from TTF found that "greening strategies" to preserve and expand the tree canopy can provide as much as 15 degrees of cooling on hot summer days. TTF also found that tree planting and preservation are three and a half times as effective in lowering temperatures as "cool materials strategies" such as the use of paving and roofing materials that reflect light.
Data provided by TTF and TPL helped craft the five pillars of Dallas' smart growth strategy. In addition to increasing the tree canopy, it calls for improved carbon-free transportation, such as trails and transit lines; better water management to absorb rainfall, reduce flooding and recharge drinking water supplies; the use of green infrastructure to benefit people with health conditions; and locating green infrastructure to benefit underserved and disadvantaged populations.
These five priorities were mapped and analyzed, neighborhood by neighborhood, to identify areas that would benefit most from a combination of green infrastructure interventions. In addition to canopy coverage, the analysis incorporated income levels; public health information such as the prevalence of diabetes, asthma and cardiovascular disease; temperatures in areas with the most foot traffic; and levels of vulnerability to floods. The analysis emphasized areas with greater health inequality, focusing on the elderly and child populations. Street safety was also included in the analysis, identifying places where "green streets" could improve pedestrian and bicycle safety while also improving water quality and cooling urban heat islands.
Among all of Dallas' neighborhoods, the mapped analysis found that Oak Cliff had some of the greatest need for green intervention. Oak Cliff is an area southwest of downtown with 91,000 households and a population of 275,000, of whom 28 percent live below the poverty line. The Nature Conservancy (TNC) joined forces with TTF and TPL for a project that became known as Cool and Connected Oak Cliff.
TNC, TTF and TPL mobilized volunteers, most of them from the community itself, and armed them with shovels and saplings. So far, the coalition has planted more than 800 trees, with a target of 1,000 in the ground by spring 2019. Over the next 40 years, the organizations estimate, the additional trees will generate $2.9 million in environmental benefits, including removing 248 tons of carbon dioxide from the air and intercepting four million gallons of stormwater. (You can see a comparative map of tree-planting locations in Oak Cliff here.)
While planting 1,000 trees is a good start, much more needs to be done. To have a real impact on urban heat island effects, TTF reported, Dallas needs to add about 250,000 trees to its canopy, spread throughout the city but concentrated in neighborhoods where the health benefits of would be highest -- neighborhoods like Oak Cliff.
Oak Cliff's 1,000 new trees will be just the beginning of the effort centered on the neighborhood. TTF, TPL and TNC plan to update the map every year and monitor temperatures and public-health data over the next five. Citywide, data and GIS technology will provide the critical information Dallas' leadership needs going forward to ensure that the city's most vulnerable continue to have priority for green infrastructure investment. Most importantly, continuous monitoring will maintain accountability and ensure that these efforts are truly making an impact as the city continues to grow and heat up.