A N.J.-based research team found that they can cut a city block's temperature by up to 5 degrees in summer.
It sounds elementary, and a group of New Jersey researchers now has proof: Trees make cities cooler. The concentration of concrete in the Philadelphia region creates what scientists call an "urban heat island" - an area where the temperature is hotter than surrounding areas because buildings and roads soak up the sun's heat.
A study released this month by Montclair State University and NASA's Goddard Institute for Space Studies documented that Camden and Newark suffer from the same sweltering effect. But the scientists also searched for ways to confront the heat island - and found that shade trees, such as oaks and maples, can make a city block as much as 5 degrees cooler.
"Our study really highlighted the effect of trees, which not only mitigate the heat island but add to residents' quality of life," said study author William Solecki, a professor of geography at Montclair. "It was always the neighborhoods in the urban core, where there was block after block of row houses with black tar roofs and no tree coverage, where we recorded the highest temperatures."
Other researchers have quantified a tree's cooling effect in terms of air-conditioning costs. Hashem Akbari, leader of the Heat Island Group at Lawrence Berkeley National Laboratory in California, has found that a full-grown tree near a house can save the homeowner as much as $20 a year.
"This isn't rocket science. Everyone knows that on hot days, it's cooler under the shade of a tree," Akbari said. "But when you quantify the dollar savings of each tree, it's amazing what they can do." Trees cool their surroundings through evapotranspiration, in which water evaporates from leaves into the atmosphere. Laurence Kalkstein, a heat island specialist at the University of Delaware, noted that evapotranspiration could also make the air more humid, counteracting the trees' cooling benefit.
The Montclair-NASA study found that suburban sprawl is rapidly extending Camden's heat island. Newer suburbs, with large strip-mall parking lots, have a higher temperature than similarly dense older suburbs, which are more likely to have trees, Solecki said.
New Jersey's Department of Environmental Protection, which helped fund Solecki's study, is heeding the results.
Commissioner Bradley Campbell said the state was seeking $36 million from the Board of Public Utilities to plant 100,000 trees in Camden, Newark, Trenton, Elizabeth and Paterson. Separately, an air-pollution settlement with utility company Conectiv established a community program that will plant 1,500 trees in Camden in the next three years.
"We want to focus our planting in the communities that will reap the greatest benefit from counteracting the heat island," Campbell said. "The remarkable thing about trees is they not only reduce heat and energy costs, they muffle noise and enhance property values as well."
The down side is that city trees need 15 gallons of water a week and are vulnerable to drought, pollution, and salt from streets and sidewalks. Seven city trees die for each one that survives, forestry experts say.
The Energy Coordinating Agency Inc., a Philadelphia nonprofit devoted to reducing energy costs for low-income residents, has focused on another option Solecki's study champions: installing reflective white roofs on rowhouses. White roofs made of polymer and calcium carbonate repel the sun's energy, while black tar roofs soak it up, increasing temperatures inside homes.
A white roof can make the top floor of a rowhouse 5 degrees cooler and inspire residents to further cool off and beautify the neighborhood by planting trees, director Liz Robinson said.
"Residents on our white-roof blocks seemed to grasp immediately that planting trees would continue the cooling effect," she said. "The two strategies should be used together as a low-cost way to cool down Philadelphia."
Mindy Maslin of the Pennsylvania Horticultural Society discovered trees' natural air-conditioning a long time ago - and didn't need an academic study to prove it, she said. Maslin, who runs the society's Tree Tenders program, works with Philadelphia's neighborhood associations to plant 2,500 trees a year and tend to the 250,000 that are already grown.
"Trees are like magic when it comes to cooling down city streets. Drive down Lincoln Drive in the summertime, and the temperature drop is amazing," Maslin said. "A city without trees isn't just hot - it isn't liveable."
By Kaitlin Gurney, Staff Writer, Philadelphia Inquirer, April 28, 2003