Putting the freeze on heat islands
Summertime, and the livin’ should be easy. But, for a growing number of urban planners, the season hovers over their heads like so much smog. On a hot, sunny day, temperatures rise, energy use soars, ozone production accelerates, and air quality plummets. It is the agony of the heat. And it is caused, in part, by the presence of urban heat islands.
Heat islands are created when trees, grass and shrubbery are removed and replaced by dark, horizontal, impervious materials — such as asphalt or roofing. The dark surfaces absorb heat during the day, raising temperatures and triggering the chemical reactions that produce smog. Additionally, the heat that is absorbed during the day is released into the atmosphere at night, sometimes creating wind and isolated thunderstorms.
While heat islands are not the only cause of air pollution, they do exacerbate the problem, says David Dunagan, water conservation specialist for Fulton County, Ga. “Ozone is formed by the combination of volatile organic compounds (VOCs) and nitrous oxides in the presence of heat,” he explains. “So one of the parts of that triangle is heat. And the fact that we have these urban heat sinks has made the ozone problem worse than it already would be.” Numerous government and nonprofit organizations have undertaken studies to examine the effect of heat islands on urban environments.
They have identified various methods for mitigation, and they are testing those methods in cities across the nation. From using reflective paving and roofing materials to planting trees and implementing “cool” ordinances, cities are attacking heat islands and quantifying their results. And, as never before, that data is available to local planners who are sweating the island heat.
Made without shade
Studies by Berkeley, Calif.-based Lawrence Berkeley National Laboratory (LBNL) and the National Aeronautics and Space Administration (NASA), Washington, D.C., suggest that tree loss is the major contributor to heat islands. As cities scramble to accommodate booming populations and attendant development, they are clearing out more and more of nature’s cooling agents.
“Trees not only shade the surface but also give off water,” explains Dale Quattrochi, a geographer and senior research scientist for NASA’s Global Hydrology and Climate Center in Huntsville, Ala. “Of course, water vapor helps cool the overall environment.”
In 1989, in an effort to examine the connection between tree loss and heat, American Forests, a Washington, D.C.-based nonprofit conservation group, began looking at data for one of America’s fastest-growing cities — Atlanta. Using weather station data and thermal satellite images, the group identified a correlation between land use and temperature.
“Working with hurricane scientists in Atlanta, we looked at about 20 years of satellite data and found that there was a considerable loss of vegetation — about 60 percent of the heavy forest canopy — and there was an increase in the temperature, especially in rapidly developing areas,” says Gary Moll, vice president for American Forests’ Urban Forest Center. The study showed that, in 1972, summer surface temperatures in downtown Atlanta and at Hartsfield International Airport were 6 degrees higher than those in the rural surroundings. By 1993, that difference had reached 9 to 12 degrees. “In Gwinnett County, where all the development had taken place, a huge heat island had grown from nothing,” Moll says. “Twenty years ago, it was 10 degrees cooler.”
According to NASA, satellite data show that the Atlanta metropolitan area has lost nearly 350,000 acres of tree cover since 1973. Quattrochi notes that the temperature differential between Atlanta’s urbanized areas and suburbs has now reached 10 to 12 degrees in the summer.
Why is that important? In a case study of Los Angeles, LBNL found that a 1-degree rise in temperature can increase the city’s smog risk by 3 percent and boost its energy demand by 2 percent. (Just in terms of energy, that 1-degree rise would cost Los Angeles about $25 million in electricity over the course of a year.) Conversely, LBNL models suggest that, by lowering the temperature, ozone production can be slowed, and air quality can be improved.
Greening the hot spots
In 1997, EPA instituted the Heat Island Reduction Initiative, a program that seeks to quantify the effects of heat island mitigation on pollution prevention, and energy and dollar savings. The agency has designated five cities — Sacramento, Calif.; Salt Lake City; Baton Rouge, La.; Chicago; and Houston — as demonstration sites, and each of the cities is implementing a variety of mitigation strategies.
Given the dramatic connection between trees, heat islands and air quality, it is little wonder that many heat island mitigation projects (including those that are not affiliated with EPA) incorporate tree planting. The projects range from implementing and updating tree-planting ordinances to targeted plantings.
Sacramento is among the cities attempting to mitigate urban heat islands by strengthening and enforcing an existing tree ordinance. The city’s parking lot tree-shading ordinance has always required that developers plant trees that, in 15 years, will achieve 50 percent shading of any lots they build. However, a recent survey by the U.S. Forest Service showed that many lots were only 25 percent to 45 percent shaded after 15 years, partially because some developers were granted variances.
As the city has learned more about heat islands, new attention has been given to the ordinance. “In the last couple of years, we’ve had several [heat island-related] presentations to our public works department, our planning staff, our planning commission, our design review board, and soon to our city council,” says Jim McDonald, associate planner for Sacramento. “It’s made a noticeable difference in actions by the design review board and the planning commission, in that they have not granted variances since the presentations were made to them.”
In addition to shoring up staff knowledge of heat islands, however, the city has developed design guidelines to assist developers in choosing, planting and maintaining the trees to ensure that the 50 percent cover is achieved. “We’ve revised our list of recommended trees, and we also require that [the developers] maintain the trees for the life of the project, pursuant to national landscaping standards,” McDonald says. “Also, if any of the trees are damaged or die, they have to replace those trees, and we provide some guidance for that.”
While tree planting is an important part of mitigating heat islands, it must be done in a targeted manner, Quattrochi warns. “One of the big things is planting trees, but [they have to be] trees that are not high VOC emitters (e.g., oaks, maples),” he explains. Additionally, they have to be placed strategically.
To assist cities in modeling their tree planting — determining where to plant trees and how many to plant — American Forests has developed CityGreen, a GIS-based program that calculates the impact of planting. Using satellite imagery for base maps, the system incorporates user-defined variables to determine energy savings, as well as impacts on stormwater and air quality. “You can get a measurement of how trees will make a difference,” Moll says. The user “plants” a tree, and the system can determine “whether moving that tree 5 feet can make a big difference.”
The value of strategically placed vegetation on heat, energy use, stormwater and air quality also is being examined in Chicago, where the city is planting a rooftop garden at City Hall. The $1 million project, begun last year, will be fully implemented by the end of May, says William Abolt, Chicago’s commissioner of environment.
“It’s about how the reduction of urban heat islands can improve air quality by making individual buildings more energy-efficient and by making the city as a whole cooler,” Abolt explains. “We’re testing [the rooftop garden concept] at City Hall first and then rolling it out and testing it — along with some other [heat island mitigation] strategies — in neighborhoods throughout the city and in other locations downtown.”
Atop the building’s 38,000-square-foot roof, the city is installing 21,000 plants, including shrubbery, prairie grasses and flowers. “We’re focusing on low-maintenance, native plants,” Abolt says. “We’ve selected plants that are essentially drought-resistant, that provide good coverage and color, and that can withstand cold temperatures and high winds.”
Soil depths on the roof will range from 3 inches to a few feet, and some structural changes had to be made to accommodate the soil load. However, Abolt expects the city to recoup those expenses. “We expect to save $4,000 a year on operations costs,” he says, noting that the garden will produce those savings by lowering roof temperatures in the summer, reducing City Hall’s overall energy demand. “Additionally, it will provide substantial stormwater benefits,” Abolt says. “Virtually all of the water that hits the roof will be absorbed rather than going down into the sewer system.”
Reflecting the rays
Roofing is, in fact, the subject of numerous pilot projects across the country. NASA has identified dark roofing as a heat island contributor, and it advocates reflective (i.e., light-colored) roofing materials as a mitigation strategy.
“In major cities, most air-conditioning systems are located on roofs, which means they have to work twice as hard and use more energy to cool the air going into the building because it’s so hot on the roof,” Quattrochi says. “[Reflective roofing] can reflect from 60 to 80 percent of the incoming solar radiation.”
That reflectivity can go a long way toward reducing heat, as evidenced in Salt Lake City. Using a heat spy (a hand-held heat-sensing device), officials compared temperatures on top of the Delta Center (home to the NBA’s Utah Jazz) to those atop the state courthouse. “The Delta Center has a white membrane roof,” notes Camille Russell, a project coordinator for the Utah Office of Energy Services and coordinator of EPA’s Salt Lake City-based pilot projects. “We took temperatures on a very hot summer day, and we were getting temperatures around 90 degrees Fahrenheit. The courthouse has a black membrane roof — same material, different color — and we were getting temperatures about 160 degrees Fahrenheit.”
Those numbers applied to LBNL’s case study of Los Angeles imply that reflective roofing can reduce smog risk and energy costs significantly. In fact, in 1997, LBNL modeled the impact of reflective roofing — in different amounts and with different percentages of reflectivity — on energy costs for 11 cities.
(Calculations incorporated variables such as cost of power, type of building stock, type of heating and cooling system, and level of insulation.) “The numbers were impressive,” notes Virginia Gorsevski, program manager for EPA’s Heat Island Reduction Initiative. “In Phoenix alone, they showed $37 million in annual savings if a substantialportion of the city implemented reflective roofs.”
But how feasible is it to require developers and property owners to use reflective roofing? Highland, Utah, is finding that such an order produces little objection.
Situated 30 miles south of Salt Lake City, the city has prepared a master plan for a 50-acre overlay zone that will be privately developed as a town center. For that project, the city has established design guidelines that require the developers to use reflective roofing, reflective parking lot surfaces and landscaping in their construction. According to Russell, Highland is one of only a few cities that have mandated the full menu of heat island reduction strategies.
The Highland design guidelines for the overlay zone require 75-percent reflectivity for materials used on low-sloped or flat roofs. Additionally, roofs exceeding 10,000 square feet must be designed with breaks — e.g., skylights or clear stories or interior courtyards — equalling 5 percent of the total roof surface.
Of all the heat island reduction strategies, reflective roofing probably requires the least amount of adjustment for developers, says Soren Simonsen, a principal for Salt Lake City-based Cooper/Roberts Architects. (The firm assisted Highland in establishing its overlay ordinance and design guidelines.) “For most large roofs that are using a single-ply membrane, typically the cost differences [between light-colored and dark roofs] are almost negligible,” he explains.
However, he notes that the use of reflective roofing can prompt the need for structural amendments or mechanical system upgrades. “The 70- to 80-degree temperature difference between a very light-colored roof and a very dark roof means that snow does not melt as fast,” Simonsen says. “[In cities other than Highland], some developers have indicated that the additional snow load requires additional sizing of the structural members. The real expense is in upgrading mechanical systems to handle the extra heat load.”
Simonsen is describing what has come to be known as the “winter penalty” — the light-colored roofing that makes a building easier to cool in the summer also makes that building harder to heat in the winter. However, according to EPA, the energy cost savings in the summer offset the additional costs of heating in the winter.
A concrete answer
The final strategy for mitigating heat islands takes the concept of reflective roofing and employs it on the ground. Reflective paving surfaces, such as concrete, are being tested in several cities, but Ogden, Utah, recently garnered publicity as the test site for a pilot project by the Utah Department of Transportation.
At an intersection in downtown Ogden, UDOT overlaid a portion of an existing asphalt roadway with a few inches of concrete. Known as whitetopping, the process has been tested by other transportation departments as a means of extending pavement life.
While the Ogden project is primarily a test of the concrete’s durability, it has produced some encouraging statistics with regard to its cooling abilities. “We go and measure the asphalt and then walk over to the whitetopping and find up to a 20-degree reduction in surface temperature,” Russell says.
Despite numbers such as those, whitetopping is unlikely to be easy to promote, primarily because it is expensive. “Depending on [the project size] and the availability of materials, it costs probably two to two-and- a-half times as much to pave with concrete as it does with asphalt,” Simonsen says.
He also notes that, because of the additional costs, Highland has given developers an alternative in the town center project. “For any public ways and public parking lots — i.e., projects that would be paid for out of city funds — those would all be concrete paved,” Simonsen explains. “For private developers that are doing large parking areas, we gave them the option of doing concrete paving with 10 percent interior landscaping or doing asphalt and 20 percent interior landscaping. And we also described the types of landscaping that would be used.”
Lighting a fire
As cities, government agencies and other organizations continue to study heat islands, their goal is to quantify the results of their testing and make that information available to urban planners. Already, there are efforts under way to give planners the technical assistance and tools to apply the data for mitigation of existing heat islands and prevention of future ones. (See the resource list on page 38.)
“One of the things we really want to do is get this information down to the local and governmental levels, because that’s basically where all of this is going to have the most impact and make the most sense,” Quattrochi says. “We’re trying to work with cities and show EPA, as well as the cities themselves, that, by mitigating this urban heat island effect, they can actually influence their environment and help [themselves] reach compliance with EPA standards.”
Currently, EPA does not assign Clean Air Act credits to cities implementing heat island mitigation projects. Quattrochi, Abolt and McDonald hope that day will come.
For now, EPA is focusing on distributing the data from its pilot projects and educating planners on the potential benefits of trees, reflective roofing and reflective pavement. “We’d like to provide everybody with information on whether it’s worthwhile to employ these strategies,” Gorsevski says. “Some cities are more interested in the financial benefits, while others are more interested in smog or energy savings. Some are simply interested in smart growth and smart development, and they can put this to use in their land-use planning.”