How We Can Use Climate Action Planning to Beat the Heat

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Flickr (CC) | Vasilios Sfinarolakis

What is extreme heat?

People are exposed to extreme heat when summertime weather is hot and humid relative to historic baselines. Dramatic temperature spikes can be a driver of heat-related morbidity and mortality, as can small differentials between day and nighttime temperatures; when temperatures remain unusually high while we are sleeping and our bodies trying to recover from the heat of the day. Because these events are extreme relative to normal conditions, individuals, households and communities are often unprepared to cope with their impacts.

Cities are hotter than their country cousins.

Cities experience higher daytime temperatures and less nighttime cooling than surrounding (peri-urban) areas — a phenomenon known as the urban heat island effect. A number of different factors account for this urban/peri-urban difference. For one, cities tend to consist of larger amounts of concrete, asphalt, and other materials which have thermal properties that facilitate concentrations of heat. Secondly, rural and peri-urban areas tend to have more trees, vegetation and soil than do relatively urbanized areas. Through processes of evaporation and evapotranspiration, these landscape elements provide a cooling effect. Water evaporates from surface waters and from water that has infiltrated into soil (evaporation), cooling the air as it does. Similarly, vegetation such as trees absorb water through subsurface roots and release it into the air through their leaves (evapotranspiration). Impervious surfaces like asphalt roads — which do not allow water to filtrate into the soil and be taken up by roots of plants — inhibit these processes of evaporation and evapotranspiration. Third, the shape of the urban built environment, or what is known as “urban geometry,” can create concentrations of heat. High rise buildings reduce wind flow, and urban canyons can cause air stagnation, creating concentrations of both high temperatures and pollution.

What threats are posed by urban heat island?

In the United States, heat is the leading weather-related killer (Klinenberg, 2002; U.S. Environmental Protection Agency, 2014). High temperatures become dangerous to human health when people have difficulty maintaining their internal body temperatures. This condition can lead to heat cramps, heat exhaustion or heat stroke, and can impact other health problems, such as circulatory or respiratory diseases.

Within urban heat islands, there are more islands.

So on average, cities tend to be hotter than the less dense and built-up places that surround them. However, not everyone who lives in a city experiences the urban heat island effect in the same way. Temperature variation within a given city can be even greater than the average temperature difference between that city and its surrounds. Exposure to extreme heat is largely driven by conditions of the built environment and these conditions vary considerably across urban landscapes. Midtown and lower Manhattan are characterized by dense high-rise buildings which, in some areas, form urban canyons. Neighborhoods like Forest Hills, Queens have single family residential homes and substantial tree canopy. Sections of Park Slope, Brooklyn are comprised of midrise buildings adjacent to green, open spaces and urban forests of Prospect Park. The various compositions of landscapes and built environments that we find throughout New York City have different temperature signatures (Hamstead et al., 2015). Areas of the city that are hot relative to the city as a whole are known as micro-urban heat islands. People who live in micro-urban heat islands are disproportionately exposed to heat-related health risks.

Mapping illustrates distributions of risk.

City governments including New York have begun to recognize that extreme heat is an increasingly dangerous threat to urban residents. Yet, unlike hazards such as floods and storm surges which are commonly studied by agencies like the Federal Emergency Management Agency in the U.S., communities often lack basic knowledge of where extreme heat threats are likely to have the most impact, and who is most likely to be affected. This is why WE ACT for Environmental Justice and the Natural Resources Defense Council are working to map the geographic distributions of heat exposure and heat sensitivity in Northern Manhattan. Temperatures in Northern Manhattan tend to be cooler in such areas as those surrounding Inwood Hill Park and Fort Tryon Park, and hotter in areas like the 207th Street rail yard where built infrastructure dominates and green space is lacking.

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Fig. 1: Distribution of exposure to heat across Northern Manhattan reflected by land cover, percent tree canopy (at the Census block group-level) and surface temperature.
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Fig. 2: Distribution of Blacks/African Americans in Northern Manhattan as a whole, and for each community district — East Harlem, Central Harlem, Washington Heights and Inwood.
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Fig. 3: Distribution of individuals earning incomes below the poverty line in Northern Manhattan as a whole, and for each community district — East Harlem, Central Harlem, Washington Heights and Inwood.
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Fig. 3: Distribution of overall vulnerability measured by exposure to surface temperature, percent tree canopy and sensitivity based on age (>65 years) and income (below poverty level) in Northern Manhattan as a whole, and for each community district — East Harlem, Central Harlem, Washington Heights and Inwood.

How can we beat the extreme heat?

How can we adapt to rising temperature and the increasing frequency and intensity of extreme heat events? For one, we can incorporate landscape features which reduce exposure to extreme heat into the urban built environment. Urban greening — planting trees and vegetation — as well as ‘blue-ing’ and ‘turquoise-ing’ (Childers et al., 2015) — increasing access to surface waters, restoring wetlands and daylighting underground streams to above-ground channels — will help to cool air temperatures through processes of evaporation. Increasing water infiltration through green roofs, porous paving and other green infrastructure will promote cooling through evapotranspiration. These kinds of public investments have numerous benefits beyond moderating temperatures — particularly if they are designed in such a way that enable people to access and use them for recreation or other kinds of activities.

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AUTHOR

Dr. Zoé Hamstead is an urban ecologist and Assistant Professor of environmental planning at the University at Buffalo School of Architecture & Planning. Her research and practice focus on urban resilience and sustainability planning in the context of climate change.

Works Cited

Childers, D., Cadenasso, M., Grove, J., Marshall, V., McGrath, B., Pickett, S., 2015. An Ecology for Cities: A Transformational Nexus of Design and Ecology to Advance Climate Change Resilience and Urban Sustainability. Sustainability 7, 3774–3791. doi:10.3390/su7043774

WE ACT for Environmental Justice

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