Cool Boston's hottest neighborhoods with more white roofs and trees, study says

Residents in Boston’s hottest neighborhoods could experience cooler summer temperatures if the city planted more trees and installed white roofs that reflect sunlight, according to a new study from Boston University.

Researchers used summer temperature data and computer modeling to determine what combination of the cooling strategies would bring the most relief without breaking the budget.

The study, published Tuesday in the journal Nature Communications Earth & Environment, suggests that planting about 2,500 trees and installing 3,000 “cool roofs” could ease temperatures for nearly 80,000 residents at a cost of about 1% of the city's annual budget.

The researchers hope their work will aid cities considering interventions as climate change increases the intensity and frequency of hot summer days. They plan to expand this analysis to other cities in the future, said Ian Smith, a research scientist at Boston University's Earth & Environment Department and lead researcher on the study.

“ Cities may have a finite amount of resources and a finite amount of space to implement these solutions. So where and which types of solutions can we implement?” he said.

The recommended mix of trees and white roofs could bring temperatures down in some Boston neighborhoods by a whole degree, according to researchers. Other studies have shown reductions of even tenths of degrees can lower the risk of heat-related deaths.

The study identified twice as many places to install cool roofs in Boston, as compared to planting trees. Adding a cool roof involves painting black surfaces white or installing reflective material.

They looked at a variety of factors, including how many people live near the spots where trees could be planted or white roofs could be installed.

They also considered residents’ age, socioeconomic status, access to public transportation and other social characteristics to determine where cooling strategies would have the greatest impact. Hot temperatures can affect anyone, but public health officials warn that certain groups like older adults or those without air conditioning are more at risk for heat-related illnesses.

While planting more trees in some areas would bring down temperatures, Smith said white roofs could be more effective in cooling the densest, hottest and most vulnerable neighborhoods.

“These solutions are most effective when they are close to people,” he said.

East Boston was one “hotspot” where white roofs could have more impact than trees because of the neighborhood’s high density of flat-roofed triple-deckers, the study found.

Cool roofs are also easier to install and maintain than trees.

“When these cool roofs are installed, there is an immediate benefit in the afternoon that they're installed,” Smith said. “When we first plant a tree, it's effectively providing no benefits.”

Plus, in a changing climate, trees and their cooling potential might be more vulnerable to threats like drought or rising temperatures.

Unlike trees, cool roofs aren’t sensitive to heat waves or days of consecutive hot temperatures, said Lucy Hutyra, a professor of Earth and Environment at Boston University.

“ The leaves on the trees get a little bit droopy,” she said. “And that means that they're not cooling as much. They're not evaporating as much water because they've gotten thirsty.”

But trees have benefits beyond their cooling abilities, Hutyra said. They can improve physical and mental health, beautify a neighborhood and slow climate change.

And in neighborhoods like West Roxbury, where there are more sloped, single-family homes instead of flat-roofed triple-deckers, trees would be more effective for cooling, the study found.

Hutyra said thinking through tree and cool roof placement can bring the greatest relief to the most residents.

“ We really need to be thinking about both of them and having some balance and equity in the distribution,” Hutyra said.