In Dallas as in Phoenix, People Look to Trees for Relief from Urban Heat Islands
DALLAS — Few neighborhoods in America need trees more than South Oak Cliff, a sun-blasted shade desert in this rapidly warming Texas city.
Like much of Phoenix, the south-side Dallas district lacks sufficient shade for comfort during summer and safety during heat waves, when temperatures can climb as much as 11 degrees higher than parts of the city where the shade cover is greater.
Despite South Oak Cliff's leafy name, big shade trees are relatively scarce along its streets. In this part of Dallas, the canopy, a measure of areas covered by trees, is one-third as dense as it is in the city overall, and neighbors say the trees that do exist are aging and falling over in storms.
Dallas ranks just behind Phoenix on the list of fastest-warming urban heat islands, metropolitan areas where heat-absorbing concrete and asphalt push temperatures higher during the hottest months.
These islands of urban development contribute to a steady rise in temperatures and can intensify heat's deadly effects, focusing the worst of it on inner cities and neighborhoods where residents can least afford to seek relief.
Although it sounds simplistic, the common prescription for such overheated areas is more trees, which create shade to block the heat that city streets and buildings gather and hold well into night.
So when volunteers peeled off their sweatshirts to plant curbside trees a block east of a freeway here on an unseasonably warm 75-degree day in December, Josephine McGee had to know if there were more trees where those came from.
She pulled over on her drive home and asked the crew from the Texas Trees Foundation how she could get a tree in the corner of her yard down the street. They gave her a paper to sign and said they would be back in the spring with a tree for her.
“It’s a filter for good air, and I like the shade,” she said. “If I’ve got good shade, it keeps me from burning up so much energy on the inside.”
Rising temperatures in urban areas
McGee lives smack in the middle of Dallas' urban heat island, where researchers say heat-absorbing concrete and asphalt raise summer temperatures about three-quarters of a degree Fahrenheit every decade.
In a study of urban heat, only two cities posted higher rates of warming compared with surrounding rural areas: Louisville, Kentucky, where the effect is amplified by a much greener countryside, and Phoenix. The study looked at rising temperatures attributed to the urban heat island.
In Phoenix, researchers found that urban temperatures are rising close to a degree per decade, adding to the effects of global warming. That’s an average, meaning some days in summer are much higher. And that means shading streets and sidewalks to cool the air is an ever-increasing imperative.
“Phoenix is warming at three times the rate of the planet,” said Brian Stone, director of the Georgia Institute of Technology’s Urban Climate Lab, where the urban heat study was conducted.
As the heat worsens, more people die. Heat was a factor in at least 150 deaths in Maricopa County during 2016, according to the county health department, and, in 2017, public health officials confirmed 129 heat-associated deaths, with 51 still under investigation. In Dallas, where experts say record-keeping is less robust, about 50 to 60 people are thought to have died in the exceptionally hot summer of 2011.
Trees are an immediate way to address heat at a local level. Stone produced a detailed study of heat by neighborhood for Texas Trees, and he said Phoenix and other cities need similar data to target their tree-planting efforts. It models the precise number of trees a neighborhood needs to maximize the cooling effect. The work costs about $100,000.
While Phoenix’s roughly 11 percent shade canopy is far skimpier than Dallas’, it is a percentage point or two better than the canopy in the South Oak Cliff neighborhood. Dallas benefits from the cooling respiration of more grass and leafier trees, and even its hottest neighborhoods appear greener than Phoenix's.
By blocking heat and preventing its absorption during the day, trees can help keep temperatures lower at night, when a heat island's effects are most profound. After the sun sets, the baked roads and structures are slow to radiate energy back into space, keeping temperatures from falling.
Phoenix now routinely experiences 90 degree summer nights, which were recorded only twice before 1970.
That’s also deadly: Maricopa County statistics show the daily rate of heat-related deaths nearly doubles to 1.8 from 1 when the night warms to the lower 90s from the upper 80s.
Planting trees to temper the heat
Scientists, city planners and neighborhood activists increasingly seek to plant trees as the best way to turn down the dangerous heat. The Texas Trees Foundation is heeding the call in an unprecedented campaign, aerially and precisely mapping suitable locations for 1.8 million new Dallas trees and focusing its efforts in areas that science shows will benefit the most.
The foundation paid Georgia Tech’s Stone to study the city’s heat zones in rare detail this year, using temperature gauges to measure near-ground air temperatures on a grid of 5,000 cells of about a square kilometer each.
This yields clearer information about a neighborhood’s actual feel than do the more common satellite images measuring surface temperatures, Stone said.
Tree-planting efforts in Dallas
The sun rises over full grown trees in the South Oak Cliff neighborhood in Dallas on Dec. 1, 2017. Patrick Breen/The Republic