Urban Heat And Urban Design — An Opportunity To Transform In NYC
By: Joyce Klein-Rosenthal + Jeffrey Raven
July 18, 2017
How can planners and designers work with large urban centers to prepare for the adverse impacts of climate change, while also adapting to current climate variability and extreme weather events? With a substantial expected increase in daily average temperatures in coming years, mid-latitude cities such as New York are also expected to experience more frequent and intense heat waves (NYCPCC, 2015; USGCRP, 2016).
Part 1 of this article describes the urban climate phenomenon that amplifies the harmful effects of urban heat — the urban heat island (UHI) effect — and presents concepts for adaptive urban design strategies to improve the urban environment and mitigate the UHI effect, as the first of a two-part discussion. Part 2 (to be posted in August 2017), will describe an important case study now in the works; NYC's rezoning of the East Midtown business district in Manhattan, and how that dense core district may be redeveloped to respond and adapt to climate risks.
The redevelopment of East Midtown (EM) provides an opportunity for urban designers and planners to consider climate-informed guidelines for land-use development in New York City, by applying the best practices of adaptive design during the initial planning stages of redevelopment, to transform the city's built environment, reduce the urban heat island effect, while creating the framework for a healthier and more sustainable central business district.
Designing for uncertainty — Climate risk and the urban heat island effect
Confronting the challenges of climate change in global cities requires expanding the agency of urban planning and design within urban development, and the integration and application of knowledge of climate science and ecology to the planning and design of climate-resilient communities. Applying knowledge of urban climate to design dynamic, desirable and healthy communities begins with an understanding of climate risk, within an embedded understanding of the political, economic, social, health and ecological priorities of community residents.
The problem: Hot days and nights in the urban heat island
The urban heat island effect refers to the higher temperatures that can be found in city centers, which makes them hotter than their surrounding suburban and rural areas, due to their human-made surfaces, lack of vegetation, and lack of natural land-cover. Heat islands are created principally by human-made surfaces, including asphalt or concrete roofs, parking lots and roads, which absorb sunlight and re-radiate that energy as heat. The concrete, metal and stone of buildings and street surfaces store and conduct heat, and act as multiple reflectors of this energy (Clarke, 1972). Since city streets typically have fewer trees and vegetation to shade buildings and cool the air by evapotranspiration, urbanized land-cover tends to retain less surface water from precipitation than natural land-cover, and moisture is less available for evaporative cooling (Hart & Sailor, 2009). Urban areas generally have lower wind speeds, and less building heat is lost to the atmosphere by convection (Clarke, 1972) due to the reduced sky-view factor.
Other factors that contribute to warmer urban streets include: increased storage of heat by urban materials; city morphology, urban design, such as the orientation and form of buildings and roads; building massing; the creation of anthropogenic heat; the relative lack of water in urban environments; and lower heat loss and altered wind patterns in narrow urban canyons (Chow & Roth, 2006; Hough, 2000). Synoptic weather conditions such as wind speed, cloud cover and height, can enhance the magnitude of the heat island effect (Oke, 1973; Gedzelman et al., 2003). Additional anthropogenic sources of heat, such as air-conditioning systems that dump waste heat outside of buildings, add heat to the pedestrian environment during the hottest times of the year. Lower surface albedo, a measure of reflectivity, results in greater absorption of incoming solar radiation.
Policy makers and planners must be alert to UHI magnitude; this is the temperature difference between a city and its surrounding region, which is greatest during dry, clear, low-wind nights, as the surfaces that comprise the built environment retain and re-radiate more heat into the air at night than vegetation and non-urban land (Clarke, 1972; Gaffin et al., 2008). This is a problem for public health, as those without air conditioning at home can suffer from continued exposure to very hot temperatures at night. The surface geometry and thermal properties of the built environment significantly impact the heat island magnitude (Voogt, 2002). Building density, design and height determine the sky-view factor and a city's canyon geometry, which along with building materials, determine the absorption and storage of incoming solar radiation (Hart & Sailor, 2009). The result is a large reservoir of heat and radiation between buildings.
In addition to the local heat island effect, New York City is growing warmer due to global climate change. The city's climate risk analysis (2015) projects that average temperatures are expected to increase by 4.1 to 5.7℉ by the 2050s, with a longer warm season and an expected increase in the frequency and intensity of heat waves (NYCPCC, 2015, p. 10).
Extreme heat, high daily temperatures and heat waves are major health stressors currently in American cities; rates of heat-related injury and natural cause mortality increase on hot summer days, and these climate-health impacts are likely to increase in a warming climate (USGCRP, 2016). The indirect effects of extreme heat include electrical power outages and utility and transit disruptions.
In light of these UHI trends and impacts, identifying those places, communities and individuals most vulnerable to climate-health impacts is a top priority for public health and planners and designers active in climate adaptation. Climate-health outcomes can vary by neighborhood within a city and vary at the intra-urban, regional, national and international scale. Certain populations, because of their age, socioeconomic status and income, demographic factors, race and ethnicity, physical, environmental, and neighborhood are more vulnerable to climate-related health consequences (Klein Rosenthal et al., 2014; Madrigano et al., 2015; NIEHS, 2016; USGCRP, 2016) than the overall population. For heat-health impacts, those groups at higher risk include older adults (aged sixty-five and over); low-income groups and marginalized communities; children; persons of all ages with chronic health conditions and disabilities; and those in certain occupational groups, including outdoor workers, farmers and agricultural workers, construction workers, first responders, and laborers in hot environments lacking access to cooling (NIEHS, 2016, p.3; Watts et al., 2015). Heat must be understood as an unevenly distributed urban pollutant, and this means how climate adaptation is addressed is a social equity and climate justice issue (Carmin et al., 2013; Klein Rosenthal et al., 2014; Shi et al., 2016).
The US Global Change Research Program (USGCRP, 2016) health report notes that based on the current population tolerance to heat "an increase of thousands to tens of thousands of premature heat-related deaths in the summer" are projected each year due to climate change by 2100 in the US (very likely, high confidence), although adaptation is "very likely to reduce these impacts (USGCRP, 2016, p.8)." Although decreases in cold-related deaths are also expected, these decreases are likely to be "smaller than the increase in heat-related deaths in most regions (Ibid)."
High temperatures also increase the formation and levels of ozone and other air pollutants; these also contribute directly to illness and increased death rates on days with poor air quality, and "exacerbate cardiovascular and respiratory disease. Pollen and other aeroallergen levels are also higher in extreme heat (WHO, 2016, p.2)." These air pollutants can trigger asthma, and urban temperature increases are likely to increase this burden (Ibid).
As temperatures rise, cities that understand and grapple with urban climate impacts to cool urban districts will be better prepared for extreme heat, while also reducing energy consumption. Recent research underscores both the threats posed by increased extreme heat for cities, and the economic savings and opportunities that can be gained by heat mitigation. The use of UHI mitigation strategies in densely populated urban areas such as midtown Manhattan can reduce the health impacts of extreme heat, reduce energy costs, increase environmental quality for residents and commuters, and help support the carbon mitigation global goals of the Paris Accord (COP21) (Estrada et al., 2017).
Today, New York City is actively combatting climate change by way of its 80 x 50 plan, which calls for an 80% reduction in carbon emissions by 2050. Already, its new design guidelines are spurring adaptation of homes and neighborhoods to increase the resilience of the built environment to sea-level rise and storm risks.
Guidelines for climate-informed design and heat island mitigation
Although the science of urban climate has developed over decades, urban planning and design is at an early stage of incorporating and operationalizing this knowledge into urban development in American cities (Mills et al., 2010). These disciplines are effective platforms for creating cooler and climate-resilient neighborhoods (Mills et al., 2010; Watts et al., 2016; Raven et al., in press). Cost-effective measures that bring adaptive benefits and mitigate the heat island effect, while also reducing energy use and carbon emissions must be prioritized. Some of these measures also provide benefits for urban ecology, such as reducing polluting storm-water runoff and supporting biodiversity.
Intensely urban, impervious and dense central business district can be designed and adapted to create a healthier urban environment through appropriate, climate-informed design. Four main adaptive design strategies for urban areas are highlighted here: improving the efficiency of urban systems, both in energy and transportation; optimizing the form and layout of urban districts to enhance ventilation; promoting appropriate building materials with high reflectivity; and increasing the use of green and blue urban infrastructure (Raven et al., in press).
- Sustainable and efficient urban systems
Research is needed to assess the sources of waste heat from urban functions and systems in East Midtown, and to prioritize the best approaches for mitigating them. In Tokyo, the waste heat from air conditioning and traffic can increase air temperature by 2℉ on weekdays in summer (Ohashi et al., 2007), and the higher air temperatures have a significant impact on building energy performance, increasing the need for cooling.
Graphic Credit: Jeffrey Raven, 2016
Waste heat from mechanical systems can be recovered to produce energy or hot water. Long-term, the feasibility of using district cooling and energy systems should be assessed to increase energy efficiency, reduce electricity use, and for UHI mitigation. Car and truck traffic also contributes waste heat into Manhattan streets; ideas for reducing travel by cars into midtown include congestion pricing policies, the creation of protected bike lanes and wider sidewalks, and ultimately, convenient rapid transit alternatives.
- Urban form and layout
The orientation and the height of buildings have a large impact on their surrounding microclimates. These morphologic factors control the wind path through the city's streetscape; during hot days, summer breezes are often blocked by buildings. District-scale passive cooling strategies to enhance ventilation through use of linear parks and wind corridors can improve the quality of life during the hot season. In the East Midtown district, research by the New York Institute of Technology (NYIT) proposed to link together a series of Privately Owned Public Spaces (POPS), with their landscapes redeveloped into blue-green infrastructure, into a linear park to open a corridor for the prevailing summer winds. The proposed assembly, or "banking" of POPS to create a linear open space system should be achieved without losing the district's zoning requirements for density.
Graphic Credit: Jeffrey Raven, 2016
POPS are the privately-owned outdoor or indoor places that are open to the public in NYC, numbering roughly 525. They generally take the form of plazas or arcades; "urban plazas, residential plazas, public plazas, elevated plazas, arcades, through block arcades, through block gallerias, through block connections, covered pedestrian spaces, sidewalk widenings, open air concourses, or other privately owned public spaces" per the "Privately Owned Public Space in New York City" website managed by The Municipal Arts Society of NYC, and Advocates for Privately Owned Public Space (apops).
The effect of NYC's street canyons in trapping the heat from solar radiation can be evaluated through calculating their sky-view factor, which is measured as the portion of the sky that is visible from the ground (Svensson, 2004). Narrow urban canyons, where the buildings are taller relative to street width, are poorly configured to release the city's trapped heat into the cool night sky. As urban redevelopment proceeds, the creation of varied building heights and massing, rather than a monolithic wall of high-rise buildings, could help to mitigate the build-up of daytime heat in East Midtown's high-rise buildings.
- Construction materials
The thermal behavior of materials used for building envelopes and urban streets influences the magnitude of the heat island effect. Asphalt and concrete tend to absorb the sun's heat during daytime, and reradiate that heat during night. In contrast, cool roofing materials are reflective and emissive; these as well as other heat-resistant materials can be inexpensive compared to traditional materials and relatively easy to apply to the urban context. The heat mitigation strategies most often used by cities based on new materials include high-albedo reflective roof coatings, green roofs, and reflective or permeable pavements (Rosenthal et al., 2008).Graphic Credit: Jeffrey Raven, 2016
- Green infrastructure
Cities around the world are implementing new programs to work with engineered natural systems — green infrastructure — for multiple environmental benefits in their communities. One of the most important design approaches for urban cooling is the use of "blue and green" infrastructure. Due to evaporative cooling and their ability to shade sidewalks, buildings and the intake of air into buildings, trees and other vegetation decrease the heating of buildings, pedestrians, and the urban landscape. Other green infrastructure innovations, including bioswales, green roofs, and urban forestry, are increasingly deployed by cities to reduce polluted storm runoff and flooding, to filter air pollutants, and to benefit public health and quality of life. In Manhattan's dense midtown districts, vegetation could take multiple forms: green roofs and walls, street trees, and newly created linear parks using privately-owned public space (POPS).Graphic Credit: Jeffrey Raven, 2016
Manhattan's East Midtown district: An opportunity for climate-informed design
Zoning is among the most powerful tools available to New York City government. Currently, the City is undertaking a rezoning process for East Midtown (EM), one of NYC's densest neighborhoods and central business districts, to permit and promote the development of new commercial buildings in the 73-block area around Grand Central Terminal. The net impact of the proposed redevelopment of East Midtown, with new and large high-rise office buildings, would be to enhance and increase the heat island effect, with a potential net increase in energy use for mechanical systems, despite better energy efficiency. In Part II of this article, we will present a case study and analysis of the proposed East Midtown district redevelopment, undertaken by NYIT, as an opportunity to apply urban climate knowledge in urban redevelopment, with the following recommendations:
− Cool roofing
− Heat-resistant construction materials
− Urban climate-impact analysis included in environmental review and EIS'
− Urban climate mapping
− Protected bicycle lanes and pedestrian accessibility
− Drainage systems using green infrastructure
− Use of green roofs and green façades
− Decreased vehicle emissions and traffic
− Use of district cooling systems
− Creation of linear parks through networked POPS
− Increased Sky-View Factor through transfer of development rights (TDR)
− Consideration of wind patterns and the sun's path in the orientation of new buildings
− Diversity of building forms and heights
− Access to upgraded transit systems
Many steps towards the adaptive city
The transition to low-carbon and carbon-free urbanism for New York and other American cities must include many strategies with co-benefits that may reduce rates of major chronic diseases and improve mental health, while preventing air pollution and enhancing quality of life (Watts et al., 2015, p.1862). As we were completing this article, NYC released its new plan to protect vulnerable New Yorkers from the impacts of extreme heat. Cool Neighborhoods is a smart and highly collaborative set of proposals to help create needed biophysical and social resilience to heat-health impacts within the communities and places most at risk, and to undertake the empirical field research on the city's microclimates needed to develop our understanding of how New Yorkers can best adapt to a warmer climate. Each of the Cool Neighborhoods projects and objectives are supported by collaborative partnerships between city agencies and the Mayor's Office of Recovery and Resiliency, with faculty and researchers from local universities, community-based organizations such as Sustainable South Bronx, home healthcare workers' organizations, and a national environmental organization, The Trust for Public Land.
The Cool Neighborhoods program builds on the city's successful initiatives and adopts techniques that have helped create social resilience in other large cities; NYC's CoolRoofs program, that has coated over 6.7 million sq. ft. of rooftop space since 2009 (NYC, 2017) is expanded and targeted to at-risk buildings. A new two-year pilot program, Be a Buddy NYC, is "a community-led preparedness model that promotes social cohesion," similar in objectives to Philadelphia's Block Captain tradition, which aims to protect the most vulnerable residents in neighborhoods at risk through increased social connections between them, community organizations, and local trusted messengers (NYC, 2017). The Cool Neighborhoods package of programs should help save lives, reduce heat-related injuries, and preserve well-being during extreme heat events, and serve on all days to improve the quality of life in the city, with renewed investment in green infrastructure and urban forestry, workforce training for people who work with those at risk for heat-health impacts, by advancing the urban climate analyses that could enable better understanding of how adaptive design functions, and by emphasizing community partners in their implementation.
Urban design — the next frontier for climate adaptation
There is an urgency with which climate adaptation and carbon mitigation must be addressed for future urban viability. In the transition to low-carbon and no-carbon urbanism, reshaping the built environment that characterizes New York and many other cities is both daunting and necessary. Transforming that existing urban landscape while designing new buildings and infrastructure to the highest standards informed by climate science requires a new agency and ambition of urban design and planning to experiment with reshaping the built environment at the site, block group, block and neighborhood-level.
Despite the challenges of climate-resilient re-development, the adverse impacts of not preparing for climate change are very likely to be more harmful to urban health and ecology, while the benefits of adaptive urban transformation present new opportunities for healthy urban development and economic savings. These issues are discussed in greater detail in the forthcoming Climate Change and Cities (Cambridge University Press), which embeds climate science in urban design and urban planning to deliver co-benefits across multiple sectors and spatial scales (Raven et al., in press).
Jeffrey Raven, FAIA, LEED BD+C is Principal of RAVEN A+U / Director & Assoc. Professor of the Graduate Program in Urban + Regional Design at New York Institute of Technology (Manhattan). Jeffrey is a specialist in sustainable and resilient urban design whose research is applied in professional practice and disseminated throughout the profession, government and allied disciplines. His publications include coordinating lead author of Climate Change and Cities; Planning and Design chapter (Cambridge University Press 2017); Shaping Resilient Cities in China, India and the United States (P. Lang 2014) and Climate Resilient Urban Design, Resilient Cities (Springer 2011). Jeffrey is co-chair of the AIANY Planning & Urban Design Committee.
Joyce Klein-Rosenthal, PhD, MPH is a 2017 Ford Foundation Postdoctoral Fellow, urban planner and environmental health scientist. She recently participated in the NYC Mayor's Office of Recovery and Resiliency's Urban Heat Island Task Force and the City of Cambridge (MA) Expert Advisory Committee for their Climate Change Vulnerability Assessment. Her current research focuses on the equity and economic effects of climate adaptation policies and programs, spatial analysis of environmental hazards in American communities, and multi-level governance and policies for risk reduction and health promotion in the built environment. She is an adjunct Associate Research Scholar with the Center for Sustainable Urban Development (CSUD) in Columbia University's Earth Institute, and an affiliate of the new City University of New York (CUNY) Urban Collaboratory. Klein-Rosenthal received her PhD (2010) in Environmental Planning, MS in Urban Planning (2000) and MPH (2001) in Environmental Health Sciences from Columbia University.
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 Per the "Privately Owned Public Space in New York City" website managed by The Municipal Arts Society of NYC, and Advocates for Privately Owned Public Space (apops), POPS are: "1. a plaza, arcade, or other outdoor or indoor space provided for public use by a private office or residential building owner in return for a zoning concession; 2. a type of public space characterized by the combination of private ownership and zoning-specified public use; 3. one of 525 or so plazas, urban plazas, residential plazas, public plazas, elevated plazas, arcades, through block arcades, through block gallerias, through block connections, covered pedestrian spaces, sidewalk widenings, open air concourses, or other privately owned public spaces specifically defined by New York City's Zoning Resolution and accompanying legal instruments." Accessed June 24, 2017 at the apops website
 NYC CoolRoofs website, 2017. Accessed on June 23, 2017 at NYC CoolRoofs
 Disclosure — one of the authors, Klein-Rosenthal, participated in the NYC Urban Heat Island task force, organized by the Mayor's Office of Recovery and Resiliency. Opinions are hers alone.