January 01, 2018

Mitigation of Urban Heat Islands: Greening Cities with Mandates versus Incentives

Jillian C. Kirn

The term “urban heat island effect” refers to a phenomenon that impacts numerous metropolitan centers throughout the United States and the world. Urban heat islands occur where cities are significantly warmer than their surrounding suburban and rural areas. The urban heat island effect is a result of how human beings build and utilize their cities. Cities that are classified as urban heat islands usually experience lower air quality and an increase in the production of pollutants, including ground-level ozone. These consequences, coupled with a continued focus on climate change, have spurred cities around the world to implement programs to tackle and mitigate the phenomenon.

The urban heat island effect is caused primarily by paving over natural land surfaces, particularly with nonreflective paving materials that absorb and retain heat. Also, when a natural surface is covered with nonreflective materials, storm water is no longer retained in soil and the quantity of water available for evapotranspiration (which has a cooling effect) is reduced. As a result, solar energy that would otherwise be consumed in evaporating water is instead retained as additional heat. Secondarily, waste heat generated from sources such as car exhaust and air conditioning units also contributes to the heat effect. Population increases often expand metropolitan areas, which leads to further increases in a city’s average temperature.

According to the U.S. Environmental Protection Agency (EPA), urban air temperatures can be up to 5.6o Celsius (C) (approximately 10o Fahrenheit (F)) warmer than the surrounding suburban and rural areas. EPA, Cooling Summertime Temperatures: Strategies to Reduce Urban Heat Islands (Sept. 2003), www.epa.gov/sites/production/files/2014-06/documents/hiribrochure.pdf. In an urban heat island, even night air temperatures are warmer, because built surfaces that absorb heat during the daytime radiate it back during the evening hours. This temperature discrepancy is most pronounced in winter and summer. While the heat island effect might result in a warmer winter and longer growing season within the urban area, it also turns cities into muggy saunas in the summer.

According to EPA, the excessive hot air temperatures caused by urban heat islands can result in physiological disruptions, organ damage, and death. Id. In the early 2000s, citing these potential health effects, EPA developed a Heat Island Reduction Initiative. Through this program, EPA works with researchers, community groups, and public officials to identify and implement methods that reduce heat islands. EPA, Reducing Urban Heat Islands: Compendium of Strategies, Draft (2008), www.epa.gov/heat-islands/heat-island-compendium.

Green building, particularly green roof, legislation has been implemented outside of the United States for decades as one means of mitigating urban heat. Germany’s green roof industry has been supported and subsidized by its government since the 1970s. Christian Werthmann, Green Roof: A Case Study 24 (2007). In 2001, Tokyo mandated new buildings over 1,000 square meters (m) to have at least 20 percent green roofing. The Worldwatch Institute, Can a City Be Sustainable? State of the World (2016). In 2009, Toronto mandated green roofs on industrial and residential buildings. Jackie Snow, Green Roofs Take Root around the World, Nat’l Geographic (Oct. 27, 2016). In March 2015, France proposed a piece of legislation that, had it passed, would have required that all new commercial buildings be built with green and/or solar roofs. Agence France-Presse, France Decrees New Rooftops Must Be Covered in Plants or Solar Panel (Mar. 15, 2015), www.theguardian.com/world/2015/mar/20/france-decrees-new-rooftops-must-be-covered-in-plants-or-solar-panels. In the past decade, local and state governments in the United States have joined in with a marked and steady uptick in the enactment of their own green building legislation.

In the wake of the June 2017 announcement that the United States intends to withdraw from the Paris Climate Accord, representatives of many United States cities (including Los Angeles, Atlanta, and Salt Lake City), states, and companies have pledged to meet the United States’ greenhouse gas emissions targets under the Paris agreement. As a result, green building and other measures to mitigate air emissions, temperature increases, and environmental impacts within cities are an even hotter topic of municipal discussions.

Approaches to Mitigate the Urban Heat Island Effect

The three primary strategies that are used to mitigate urban heat islands are (1) green roofs, (2) light-colored “cool pavement,” and (3) trees and vegetation. Each of these methods has the potential to make an impact on the urban heat island effect, and most cities include all three approaches in crafting their green building policies.

A green roof is a roof of a building that is either completely or partially covered with vegetation and a growing medium, planted over a waterproof membrane. Sophisticated green roofs, most frequently used on skyscrapers, generally include additional layers such as a root barrier and drainage and irrigation systems. Rooftop ponds are occasionally used and may be helpful for treating greywater. Green roofs serve several purposes for a building, including absorbing storm water, providing insulation to the building, and lowering urban air temperatures. According to EPA, “[o]n hot summer days, the surface temperature of a green roof can be cooler than the air temperature, whereas the surface of a conventional rooftop can be up to 90oF (50oC) warmer.” Using Green Roofs to Reduce Heat Islands, EPA, Heat Islands (2017), www.epa.gov/heat-islands/using-green-roofs-reduce-heat-islands (citing Karen Liu & Bas Baskaran, Thermal Performance of Green Roofs through Field Evaluation, National Research Council of Canada, Report No. NRCC-46412 (2003)). The term green roof also may refer to roofs that use some form of green technology, such as a cool roof or a roof with photovoltaic panels.

Some suggest that the efficacy of green roofs is not fully established. While there is significant evidence demonstrating that green roofs can reduce heat from rooftops, in practical application this is not always the case. Among the various green building technologies, green roofs can be particularly temperamental. A study from Columbia University found that green roofs have distinct fungal communities that help plants to thrive in harsh, polluted environments, but the fungal communities are not sufficient to build an entire rooftop ecosystem. Krista L. McGuire, et al., Digging the New York City Skyline: Soil Fungal Communities in Green Roofs and City Parks. PLOS One 8(3): e58020 (2013), https://doi.org/10.1371/journal.pone.0058020. The current iterations of green roofs often rely heavily on a limited number of plant species and generally lack the biodiversity necessary to be optimally effective. A case study of New York’s green roofs noted that “rooftop vegetation has to be able to survive the high winds, prolonged UV radiation and unpredictable fluctuations in water availability. To resist these harsh environments, most green roofs are planted with sedum, a nonnative species that can survive wind and long periods without rainfall. A roof planted with sedum, however, is no greener, from the standpoint of sustainability, than is ordinary tar or asphalt.” Amy Kraft, Why Manhattan’s Green Roofs Don’t Work and How to Fix Them, Scientific American (May 17, 2013), www.scientificamerican.com/article/why-manhattans-green-roofs-dont-work-how-to-fix-them. A green roof can be highly effective but must be engineered to include a variety of plant species and maintained in such a way as to fulfill the sustainability potential of the roof.

Many cities include distributed solar photovoltaics in green roof incentives. The two technologies can be symbiotic, with solar photovoltaics providing shade for some plants and solar photovoltaics working more effectively when a roof is cooled to optimal operational temperature by vegetation. Mathew Frith & Dusty Gedge, Green Roofs and Solar Power—Biosolar Roofs Are Smart Green Infrastructure (2017), https://livingroofs.org/green-roofs-solar-power. As an added benefit to including solar panels on green roofs, powering one’s property with clean energy generated from the solar panels and selling the excess energy to the utility appeals to a public increasingly attuned to environmental, energy-efficiency, and self-sufficiency considerations.

Another type of green building technology used to mitigate urban heat is “cool pavement.” Cool pavement has increased solar reflectance and, therefore, stays cooler in the sun than traditional pavement. The Lawrence Berkeley National Laboratory Heat Island Group’s research found that “like conventional tar or dark roofs, dark, asphalt pavements get hot in the sun because they absorb 80–95% of sunlight.” Heat Island Group, Lawrence Berkeley National Laboratory, Cool Pavements (2017), https://heatisland.lbl.gov/coolscience/cool-pavements. These hot pavements contribute to the urban heat island effect by increasing the temperature of the local air and contribute to global warming by increasing pollution and radiating heat into the atmosphere. Pavements have added potential to make a significant contribution to the urban heat island effect because they comprise nearly one-third of urban surfaces. Hot pavements also have the potential to raise the temperature of storm water runoff, thereby affecting evapotranspiration as well as ecological stability. By contrast, cool pavements are solar reflective and stay cooler in the sun than traditional pavements. Cool pavements can be made from traditional paving materials, such as concrete. Other alternatives for cool pavement include coating asphalt with cool-colored material or utilizing clear binding agents in the manufacture of a highly reflective aggregate paving material.

Tree- and vegetation-planting projects are self-explanatory. According to EPA, “[t]rees and vegetation lower surface and air temperatures by providing shade and through evapotranspiration. Shaded surfaces, for example, may be 20–45oF (11–25oC) cooler than the peak temperatures of unshaded surfaces. Evapotranspiration, alone or in combination with shading, can help reduce peak summer temperatures by 2–9oF (1–5oC).” H. Akbari et al., Peak Power and Cooling Energy Savings of Shade Trees, Energy and Buildings 25:139–148(1997); D. Kurn, S. Bretz, B. Huang, & H. Akbari, The Potential for Reducing Urban Air Temperatures and Energy Consumption through Vegetative Cooling, ACEEE Summer Study on Energy Efficiency in Buildings, American Council for an Energy Efficient Economy, Pacific Grove, California (1994). Trees and vegetation may be beneficial in all locations but are most effective as a mitigation strategy when strategically planted around buildings or to shade parking lots and streets. J. Huang, H. Akbari, & H. Taha, The Wind-Shielding and Shading Effects of Trees on Residential Heating and Cooling Requirements, ASHRAE Winter Meeting, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Atlanta, Georgia (1990). Many U.S. cities have implemented a tree-planting program for beautification purposes with the added benefit of combatting urban heat island effects.

Mandates versus Incentives

Even cities that are resolute in their commitment to reducing greenhouse gas emissions grapple with how best to structure programs and policies aimed at combatting the urban heat island effect. Negotiating the terms of a potential policy involves balancing the interests of building owners and real estate developers with environmentalists and other public interests. The two primary legal approaches to implement the various technological alternatives to mitigate heat islands are mandates and incentives.

American cities have been eager to experiment with “green building” regulations designed to achieve energy efficiency. Most U.S. cities that aim to mitigate urban heat also look at it as a component of larger green building policies and initiatives. A city may enact legislation that includes vegetation requirements, green roof incentives, and cool pavement requirements among myriad other requirements under the zoning codes. However, while many cities provide incentives to the private sector for green buildings, or require that city-owned buildings follow green building principles, few cities require private developers to comply with green building standards.

Mandates for green building—either for city-owned buildings or for private developers—may reduce long-term energy costs and create positive externalities. The United States General Services Administration estimates that green roofs should last 50 years as opposed to the more traditional 15-year life span of a standard roof. U.S. General Services Administration, The Benefits and Challenges of Green Roofs on Public and Commercial Buildings (2011), https://app_gsagov_prod_rdcgwaajp7wr.s3.amazonaws.com/The_Benefits_and_Challenges_of_Green_Roofs_on_Public_and_Commercial_Buildings.pdf. Many studies, including a recent article from the United States Green Building Council, propose that a green building may consume a quarter less energy than a conventional building, and reductions in energy use also lead to reductions in pollution. U.S. Green Building Council, Benefits of Green Building (Oct. 2017), www.usgbc.org/articles/green-building-facts.

To that end, one of the primary reasons that cities often choose to implement incentives rather than mandates is that while they may require green building in their zoning code, codification of incentives often presents fewer public policy obstacles than the codification of mandates. For instance, in France, building owners refused to bear the additional construction expenses posed by a green roofing ordinance, threatening to pass those costs along to consumers if the green building policy were enacted. Ruby Lyon & Alan Burchell, France’s Eco-Roof Law—Too Green to Be True? (Feb. 29, 2016), www.urbanstrong.com/frances-famous-eco-roof-law-too-green-to-be-true. Furthermore, a case has been made by certain detractors that mandates for green certification may impose short-run costs on developers and taxpayers, pushing development out to the suburbs. Some also have raised questions regarding the efficacy of green roofs (as noted above) and whether it would make more sense to focus on cool pavement than green roofing (since pavement comprises more of a city’s surface than rooftops).

In states that engage in retail net metering (allowing solar distributed generation properties to gain full or mostly full retail credit for energy produced on their property that is not consumed there but sold back to the grid), providing incentives for green roofs with solar photovoltaics may be a controversial approach because it subsidizes what some consider an already overvalued commodity. Some argue that retail net metering overvalues both the energy and capacity of solar-distributed generation and imposes cross-subsidies on nonsolar residential customers. As a result, critics of net metering may be outspoken detractors of tax credits or grant monies for green buildings that further decrease the cost of energy for rooftop solar photovoltaic owners.

Generally, incentive policies tend to face less popular opposition but are only optional. It should be noted, however, that green infrastructure incentives are not a monolith and there are several different types of incentive programs that may be deployed in the interest of facilitating green building. These incentives include grants, awards and recognition programs, rebates and installation financing, development incentives, and storm water fee discounts. To achieve the greatest impact on urban heat, cities would need significant buy-in from developers on any combination of incentives that they choose to implement. If the incentives are not attractive enough to get most developers engaged, then the incentives remain largely ineffective.

National Examples

San Francisco has implemented possibly the most aggressive green building mandates in the country. As of January 1, 2017, San Francisco has the distinction of being the first (and so far, only) U.S. city to mandate solar and living roofs on most new construction. City and County of San Francisco, San Francisco Better Roofs (Jan. 2017), http://sf-planning.org/san-francisco-better-roofs. The city passed new legislation requiring 15 to 30 percent of roof space on most new construction to incorporate solar panels, living roofs, or a combination of the two. In San Francisco, rooftops constitute 30 percent of the city’s land area. The new mandate allows for developers to select a combination of solar photovoltaic, solar water heating, and living roof systems to maximize benefits based on location and building programs.

New York and Philadelphia are less aggressive but have implemented a more conventional hybrid approach to green building, utilizing both mandates and incentives to encourage green building initiatives. For instance, in 2008, New York City enacted a building code that requires most new buildings to cover 75 percent of their roof area with a reflective white coating or to be rated as highly reflective using the international Energy Star standard energy-efficient consumer products. Local Laws of the City of New York for the Year 2011, No. 21 (2011), www1.nyc.gov/assets/buildings/local_laws/ll21of2011.pdf. Since January 2012, existing buildings that replace or renovate 50 percent or more also are required to add reflective materials to their rooftop. New York’s “CoolRoofs” program addresses roofs on buildings that were built before the mandate took effect. NYC Business, NYC CoolRoofs (2017), www1.nyc.gov/nycbusiness/article/nyc-coolroofs. In addition to that mandate, New York also provides property tax abatement incentives for green roofs. The 2013 amendment to New York’s property tax abatement (New York State S.B. S4802) expands the definition of a green roof, slightly increases the value of the tax abatement, and extended the application deadline by five years, to March 15, 2018. See New York Mayor’s Office of Sustainability, Green Buildings and Energy Efficiency (2017), www.nyc.gov/html/gbee/html/incentives/roof.shtml.

On June 14, 2017, following an extreme three-day heat wave in New York, Mayor Bill de Blasio’s office announced the launch of a $106 million initiative to “mitigate the threat to public health from the urban heat island effect exacerbated during summer months.” Named “Cool Neighborhoods NYC,” the program expands on the Cool Roofs program referenced above and is focused on bringing relief to neighborhoods most susceptible to heat-related health risks. To that end, about 80 percent of the $106 million investment will go toward planting trees in Northern Manhattan, the South Bronx, and Central Brooklyn. In addition to the $82 million going to those three neighborhoods, $16 million will support tree planting in parks, and another $7 million has been allocated for reforestation efforts across all five boroughs. The city also has prioritized 2.7 million square feet of public and private rooftops to be fit with greenery and painted white over the next few years. The Official Website of the City of New York, Mayor Announces Program to Help Curb Effects of Extreme Summer Heat (June 14, 2017), www1.nyc.gov/office-of-the-mayor/news/411-17/mayor-program-help-curb-effects-extreme-summer-heat.

Like New York City, since May 2010, Philadelphia has implemented a cool roof mandate. Its legislation requires all new construction in the city to use white roof coverings or those that are Energy Star-rated as highly reflective, just as New York does. The law exempts certain projects, including green roofs and those with rooftop photovoltaic solar equipment. In addition to the cool roof mandate, Philadelphia legislation provides for incentives such as its Green Roof Tax Credit, which provides businesses a rebate for 50 percent of green roof costs up to $100,000 and density bonuses/zoning incentives. Philadelphia Water, Green Roof Tax Credit Fact Sheet (2016), http://phillywatersheds.org/doc/Green%20Roof%20Tax%20Credit_2016%20Fact%20Sheet.pdf. In 2016, Philadelphia wrote green roofs into its local sustainability plan with a slightly different approach: local officials are using data to assess which areas in the city are the hottest, with the intention of installing green roofs in those areas. Mayor Jim Kenney, City of Philadelphia, Office of Sustainability, Greenworks: A Vision for a Sustainable Philadelphia (2017), https://beta.phila.gov/media/20161101174249/2016-Greenworks-Vision_Office-of-Sustainability.pdf.

Seattle adopted minimum landscaping requirements in 2007, known as the Seattle Green Factor, which require that certain new developments in neighborhood business districts must provide for vegetative cover on the equivalent of 30 percent of the applicable property. The regulations apply to developments with over 4,000 square feet (370 m2) of commercial space, more than four residential units, or more than 20 new parking spaces. Developers can select among a variety of green building methods to meet this requirement, including planting trees, preserving trees, and installing green roofs and green walls. Seattle Department of Public Works, The Seattle Green Factor (Feb. 1, 2007), www.seattle.gov/dpd/cs/groups/pan/@pan/documents/web_informational/dpds021348.pdf. The regulations are part of Seattle’s Commercial Code and encourage planting of larger trees and vegetation not just on roofs but in areas visible to the public, providing beneficial urban green space. The regulations also include bonuses for harvesting rainwater and xeriscaping. Seattle’s regulations are accompanied by a worksheet that helps applicants calculate a score and assists in determining whether various combinations of landscaping and development methods meet the requirements.

Denver has the dubious distinction of being one of the top five most intense summer urban heat islands in the United States, based on average daily urban-rural temperature differences over the past 10 years. Doyce Rice, Which U.S. Cities Have the Worst Urban Heat Islands?, USA Today, Aug. 21, 2014. In the past year, the Denver Green Roof Initiative succeeded in getting a new initiative (Initiative 300) on the November 2017 ballot that would require many new buildings to install green roofs to mitigate the heat. Jon Murray, Voters Will Get to Decide whether Large New Denver Buildings Must Sprout Rooftop Gardens, Denver Post, Sept. 5, 2017. Critics argued that the city should incentivize rather than mandate the green roofs, and the legislation was opposed by the Downtown Denver Partnership, Denver Mayor Michael Hancock, and the organization Citizens for a Responsible Denver. Blair Miller, Denver Mayor Opposes Green Roof Ballot Initiative, Says It ‘Goes Too Far Too Fast,’ Denver Channel, Oct. 11, 2017. Nevertheless, the initiative passed. As drafted, the initiative is modeled after San Francisco’s recent mandate and would modify the local building code to require a green roofing or solar photovoltaics on a portion of the rooftops of new buildings over 25,000 square feet. The rooftop requirements increase 10 percent every 50,000 square feet and are capped at 60 percent dedicated coverage for buildings of 200,000 square feet or more. Chris Walker, Green Roof Initiative Organizer on I-300’s Big Win, Westword, Nov. 10, 2017. Despite the passage of the initiative, however, it remains to be seen how Initiative 300 will be implemented. Initiative 300 requires the creation of a technical advisory board to guide the development of policies and procedures to make the initiative a reality. The Denver City Council also can change the ordinance with a two-thirds majority after a six-month period has elapsed. To that end, the next six months likely will include robust discussions regarding implementation of green infrastructure mandates that may inform other municipalities considering similar initiatives.

Incentives and Risks for Green Development

Based on the current political and regulatory climate, we can anticipate that many municipalities will look for ways to incentivize green infrastructure programs in the next few years. Even after municipalities have overcome the initial obstacles posed by developing local green infrastructure programs that balance a variety of interests, the increase in green roofing and green building will likely also lead to an increase in claims resulting from errors in the design, installation, and maintenance of green roofs. To that end, developers, owners, contractors, and green roof architects should consider ways to mitigate potential risks involved with green development.

To mitigate liability, the stakeholders in a development that features green roofing should clearly outline performance requirements and expectations in their contracts, even if that means diverging from the customary boilerplate. It likely will not be possible to eliminate all risk, but parties should consider how the contracts deal with the unanticipated results of green building including, potentially, the failure to deliver claimed energy-efficiency levels, failure to deliver claimed Leadership in Energy and Environmental Design (LEED) credits, and allocation of risk for mold or other environmental hazards arising from a green roof itself. As cities look to cut down on heat, developers, owners, and their attorneys should take the opportunity not only to join the conversation, but also to insulate against potential liability and risks posed by the ongoing development of green building technologies.

Jillian C. Kirn

Ms. Kirn is an associate in the Philadelphia, Pennsylvania, office of Greenberg Traurig LLP. She may be reached at kirnj@gtlaw.com.