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The Future’s So Bright, You Need a Green Roof

Heat Islands in a Sea of Rising Temperatures

Sun beating down on an NYC street SOURCE: In the general absence of defined heat island policies, more environmental construction enables heat island mitigation, but often as a byproduct. A look at how urban areas bake and how green building technologies can cool them.

Summers as a kid, I scampered barefoot across the blistering asphalt parking lot of the local swimming pool to make it through the gate and onto the grass. That difference between pavement and vegetation is at the heart of urban heat islands, the phenomenon that causes cities to reach higher temperatures than surrounding rural areas. Heat islands have been observed since the early nineteenth century and they tend to expand and intensify as city size increases. Especially at night, temperatures in London and New York can sometimes exceed nearby locales by almost 10°C—nearly 18°F—with average summer differences around 2°C–4°C.[1] Excessive heat itself causes health problems and triggers a cascade of secondary effects that increase other health risks for city dwellers and inflate energy consumption and greenhouse gas emissions.

The Environmental Protection Agency, the federal agency most focused on heat island mitigation, walks a tightrope between promoting environmental practices and not offending certain industries or social groups. One way to reduce heat islands, after all, would be to pave, build, and drive less. For instance, when Purdue University studied its home Tippecanoe County, it found almost three parking spaces—not counting the multiple floors of parking garages or lots on private property—for every one resident.[2] (Sprawl smears rather than vitiates heat islands.) But EPA focuses less controversially on the materials with which we urbanize, not how or why, though this approach still helps cool urban areas and reduce energy consumption.

Heat islands increase ground-level ozone (smog) by evaporating more volatile organic compounds from gas tanks and accelerating chemical reactions that contribute to ozone. Many cities, especially in the eastern United States and in southern California, violate National Ambient Air Quality Standards for ozone. Their heat islands aggravate the problem. Heat islands also increase air conditioning use, which means cars or power plants burn more fossil fuels. This, in turn, raises building energy costs by consuming more energy at the most expensive peak times, exacerbates climate change, and releases more pollutants like mercury or nitrogen oxide, a precursor to both smog and acid rain. The cumulative acute consequences can kill thousands of people, as did heat waves in the Midwest in 1995 or Europe in 2003. As atmospheric carbon dioxide concentrations are currently increasing at the fastest rates yet,[3] heat islands presage what life may be like in the future outside urban areas. Nonetheless, few cities have taken concerted action to address heat islands. Ameliorating heat islands now, though, could save energy, reduce greenhouse gas emissions, and help locally offset global temperature increases.

Heat Islands 101

Heat islands draw energy from two main sources: the sun and human activity. Compared to trees, grass, and dirt, surfaces like asphalt roads and tarpaper shingles absorb and retain more solar radiation and trap much less water, which could evaporate and cool the surfaces. The net effect means the urban infrastructure gets hotter—sometimes 30 to 50°C hotter for components like black roofs[4]—and stays hotter than vegetated areas. This surface heat bleeds into the air, even long after sunset, to form the heat island.

At the same time that they soak up heat, cities produce it. All those residents maintain a healthy internal temperature of 37°C (roughly 98.6°F). Computers, blenders, and light bulbs emit heat, as do the automobiles, air conditioners, and bakeries. This anthropogenic heat often contributes less to the heat island than solar radiation, but it can be significant, as David Sailor at Portland State University has argued. “While it is true that anthropogenic heating is small compared with summertime mid-day solar insolation,” he wrote, “it plays a major role in the surface energy balance at times when the urban heat island effect is at its maximum (night time and winter).”[5] The resultant heat island is further modulated by airflow through the city, nearby bodies of water, and other regional characteristics. For example, irrigated lawns in a desert city like Phoenix may actually keep some neighborhoods cooler during the day.

The Environmental Protection Agency, within its current constraints, tries to encourage heat island mitigation. Of the agency’s five broad goals, “Clean Air and Global Climate Change” in recent years received about 13 percent of the agency’s $7 billion budget, 3 percent more than the last-place priority, “Compliance and Environmental Stewardship.”[6] Some of that 13 percent trickles down to the Climate Protection Partnerships, mostly voluntary ways for industry and consumers to reduce greenhouse gas emissions. Much of that trickle goes to Energy Star, the joint Department of Energy-EPA program to promote energy efficient appliances and, more recently, buildings. State and Local Programs, meanwhile, houses the Partnerships’ work on heat islands, which has produced public meetings, a website, and on-line calculator after an initial study of five urban heat islands from 1998-2002; a guidebook in progress was originally slated for publication in 2007.[7] (In full disclosure, I was involved in the outreach involved for about a year as an EPA contractor.)

Many of studies have shown heat waves increase mortality from the following factors and perhaps other causes:

Heat-related effect [8]


Heat exhaustion and heat stroke

Body temperature rises from excessive heat. Heat exhaustion can cause disorientation, nausea, or vomiting. Untreated, severe heat stroke can eventually cause death.

Cardiac and circulatory problems

The heart works harder to circulate blood to lower body heat, while the blood is thickened by dehydration. The net strain can induce heart attacks or other problems.

Pulmonary problems

Particularly children, the elderly, and those with asthma or other breathing difficulties can suffer from increased ozone levels and other pollution associated with higher temperatures.

Cool Construction

As mentioned previously, EPA policy revolves around building construction methods, and zeroes in on three generally accepted options: increased vegetation, cool roofs, and green roofs, with cool pavements emerging as a fourth choice. Other efforts, such as New York or Houston’s heat island initiatives or Arizona State’s National Center of Excellence,[9] likewise focus on these technologies.

These technical approaches operate similarly and often have benefits beyond heat islands. The first option, planting more trees and other vegetation in cities, reduces heat islands because vegetation reflects more light back to space than common construction materials, shades surfaces and buildings, and stores water that then evaporates and takes heat with it. But that’s not all. By shading roads, for example, trees reduce the pavement’s temperature fluctuations, prolonging its service life. Trees also raise property values, sequester carbon dioxide, and filter pollutants from the air and water. These benefits almost always outweigh the cost of planting and maintaining urban forests.[10]

Green roof in Toronto

Place vegetation on a roof instead of alongside sidewalks, and you create a green roof. Modern green roofs began catching on in Germany and Switzerland in the 1970s. This one in is in Toronto.

Place vegetation on a roof instead of alongside sidewalks, and you create a green roof. Modern green roofs began catching on in Germany and Switzerland in the 1970s. They are spreading in the United States because they boost insulation and enable evaporative cooling, the process that wicks heat away with water into the air, which keeps the roof cooler and reduces the heat island. The living insulation reduces energy costs associated with buildings and also protects the roof membrane underneath the plants and soil from exposure to the elements, thus extending the roof’s life. In addition, like vegetation elsewhere, vegetated roofs can expand habitats for wildlife and beekeepers, filter pollutants, and also reduce runoff by keeping rainwater on the roof or slowing its rate of discharge. Portland, Oregon, already has 9 acres of green roofs and hopes to quintuple that number soon, primarily to keep pollution out of local waterways and the ocean. Chicago, meanwhile, has focused on green roofs to save energy and keep the city cooler, since a grassy expanse absorbs less heat than sheets of rubber.

A cheaper, albeit less beneficial, alternative to green roofs is the cool roof. Cool roofs utilize the principle that white surfaces reflect more light than dark ones and thus stay cooler. Consequently, white plastic sheets or sprays coat many roofs, including California’s state capitol. Certain clay tiles can also reduce temperatures. Cool roof products have Energy Star certification and are more popular in southern states like Florida, Arizona, and Texas. But even Chicago, Baltimore, and Philadelphia benefit from them because they can lower heat-wave mortality and typically save more money from decreased air conditioning than they cost in increased heating.[11] One study estimated that 5 to 10 percent of a city’s electricity demand compensates for the increased temperatures from the heat island alone.[12] That is one reason California began to require cool roofs for certain types of construction after the soaring electricity prices and rolling blackouts of 2000 and 2001.

Cool pavements, finally, take advantage of reflectivity, evaporative cooling, or both. Some pavements, like concrete with lighter additives, have a higher albedo (reflect more light.) Others, like porous asphalt, allow water to drain through them into the ground, which replenishes groundwater and also cools the pavement when that water evaporates from the soil. But given all the pavement and roofs in the world, and with a planetary population that is increasingly urban, widespread changes in net albedo could make a huge difference. Hashem Akbari, a prolific heat island researcher, and his collaborators at Lawrence Berkeley National Laboratory estimated that increasing the reflectance of roofs 40 percent and of pavements 15 percent would reduce energy consumption enough to cut carbon dioxide emissions by 44 metric gigatons, much more than a year’s worth of global greenhouse gas emissions.[13] Simulations elsewhere have suggested widespread green roofs and cool roofs could also lower average city temperatures, which may become increasingly worthwhile as overall global temperatures rise.[14]

Add It Up

In the general absence of defined heat island policies, the drive toward more environmental construction currently enables heat island mitigation, but often as a byproduct. Residential and commercial buildings each account for about 17 percent of America’s greenhouse gas emissions when you include the electricity they consume.[15] Making them more energy-efficient saves their owners money and typically cuts their contribution to the local heat island. The large-scale effects of such efforts, supported by programs like LEED and GreenGlobes and increasingly written into city and state codes, will be known only later. Meanwhile, the benefits of other projects, like replacing roadways with cooler pavements, contribute less directly to the contractor or owner and are instead diffused across the entire public.

Heat islands reveal what life may be like in a world a few degrees hotter. They also reveal the tragedy of the commons that plagues efforts to slow or adapt to global climate change: some individuals benefit greatly from business as usual while everyone loses in the end. However, even the constrained, initial attempts to address heat islands show both individuals and societies can win, but that much more collective will and political coordination is needed for the winnings to increase and spread.

Mark Meier is a freelance writer with a particular interest in ethics, identity, and social structure.


[1] Greater London Authority, “London’s Urban Heat Island: A Summary Guide for Decision Makers” (London: Greater London Authority, 2006), available at New York State Energy Research and Development Authority, “Mitigating New York City’s Heat Island with Urban Forestry, Living Roofs, and Light Surfaces” (2006), available at

[2] Douglas M. Main, “Parking spaces outnumber drivers 3-to-1, drive pollution and warming,” Purdue University News (2007), available at

[3] Global Carbon Projectm “Carbon Budget 2007,” available at

[4] S. Konopacki, L. Gartland, H. Akbari, and I. Rainer. 1998. “Demonstration of Energy Savings of Cool Roofs.” Paper LBNL-40673. Lawrence Berkeley National Laboratory, Berkeley, CA. S. Konopacki, and H. Akbari. 2001. “Measured Energy Savings and Demand Reduction from a Reflective Roof Membrane on a Large Retail Store in Austin.” Paper LBNL-47149. Lawrence Berkeley National Laboratory, Berkeley, CA.

[5] David J. Sailor and Hongli Fan, “The Important of Including Anthropogenic Heating in Mesoscale Modeling of the Urban Heat Island,” (American Meteorological Society conference, 2004), available at

[6] FY2009 EPA Budget in Brief, available at


[8] The information in this table is drawn from EPA’s “Excessive Heat Event Guidebook” available at and Health Effects of Ozone in Patients with Asthma at

[9],, and

[10] See for two examples: Portland Parks and Recreation. 2007. “Portland’s Urban Forest Canopy: Assessment and Public Tree Evaluation” available at image.cfm?id=171829. And also E.G., McPherson, J.R. Simpson, P.J. Peper, S.E. Maco, and Q. Xiao. 2005. “Municipal Forest Benefits and Costs in Five US Cities.” Journal of Forestry 103(8):411-416.

[11] S. Konopacki, H. Akbari, M. Pomerantz, S. Gabersek, and L. Gartland. 1997. “Cooling Energy Savings Potential of Light-Colored Roofs for Residential and Commercial Buildings in 11 U.S. Metropolitan Areas.” Paper LBNL-39433. Lawrence Berkeley National Laboratory, Berkeley, CA.

[12] H. Akbari, “Energy Savings Potentials and Air Quality Benefits of Urban Heat Island Mitigation” (2005), available at

[13] Akbari, H., S. Menon, and A. Rosenfeld, “Global Cooling: Increasing Solar Reflectance of Urban Areas to Offset CO2,” (2008). In press, Climatic Change. Reported in Research Highlights. “White Roofs Cool the World, Offset CO2, and Delay Global Warming.”

[14] 1) K. Liu. and B. Bass. 2005. “Performance of Green Roof Systems.” National Research Council Canada, Report No. NRCC-47705, Toronto, Canada. 2) C. Rosenzweig, W. Solecki et al. 2006.” Mitigating New York City’s Heat Island with Urban Forestry, Living Roofs, and Light Surfaces.” Sixth Symposium on the Urban Environment and Forum on Managing our Physical

[15] The 1990-2006 Inventory of U.S. Greenhouse Gas Emissions and Sinks is available from

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