by Veronica Westendorff, PLA, ASLA, SITES AP

Part 2: A Review of Policies and Programs Addressing UHI Across the US

To learn more about the impacts of climate change on our growing cities, I began to research some of the challenges that urban areas are experiencing as they grow. In addition to housing, offices, and shops for consumer goods and services, roads and other infrastructure are needed to support these communities. This brings more heat, and more consumption of energy, goods, and services in a way that is not sustainable. Last week, I took a look at urban trees as a means of reducing the urban heat island effect (UHI) within cities. Here, I’ll be exploring the question: what policies or programs are in place across the United States to reduce UHI in cities using trees?

Resilient Cities

The American Council for an Energy-Efficient Economy (ACEEE) created a list of cities in the United States with ordinances that address urban heat island and enhance cities’ energy efficiency, which is an integral part of reducing UHI. I reviewed the 50 cities below, looking at their programs and policies to see which were designed specifically to use trees to mitigate UHI.

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by Veronica Westendorff, PLA, ASLA, SITES AP

Part 1: Urban Trees as a Means of Reducing UHI Within Cities

Charlotte, North Carolina, is one of the fastest growing areas in the U.S. The largest city in North Carolina, and 22nd largest in the country, Charlotte has an average of 44 new people moving into the metro area each day (Peterson, 2017). Construction within the city and in surrounding towns continues to put pressure on the existing land and ecosystems. This is not unique to Charlotte—all over the United States, development and growth are increasing the size and scale of urban areas, with both beneficial and detrimental effects.

While urbanization increases density, reduces the need for additional infrastructure, creates more efficiencies, and provides jobs, education, and resources, the exchange of land from forests or plains to built surfaces causes a loss of urban ecosystem services. One result is increased heat in urban areas, known as the urban heat island effect (UHI), caused by impervious areas that absorb heat during daylight hours and holds it into the night, releasing it slowly so that the next day starts with higher surface temperatures than the surrounding, less built-up areas. More built areas bring more heat, creating a positive feedback loop that is one of the great challenges cities face.

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by Veronica Westendorff, PLA, ASLA, SITES AP

It doesn’t take a scientist to know. In the middle of summer, walking down the streets of almost any city, there is a notable wave of heat rising off the sidewalks where old trees have deteriorated or been removed and either no replacements or new, young trees which barely cast a shadow across the surface of the walkway are in their place. In contrast, sidewalks and streets lined with mature trees offer respite for pedestrians and cyclists. We cross the street to stand in the shade of a building or under the cooling canopy of the trees around us.

While this change in temperature, referred to as Urban Heat Island (UHI), is noticeable during the day, the real impact of UHI is felt at night, when the sun has set and the impervious surfaces around us hold and slowly release the heat of the day (Norton et al., 2015). This heat begins to compound, and the following day begins at a higher temperature, increasing the overall heat in these areas. Differences in temperature may vary by as much as 22 degrees Fahrenheit (Urban ReLeaf, 2016) and are markedly higher in urban areas with more impervious surfaces and less green space. Land cover type plays a large role in moderating these effects. Impervious areas and sealed soil act almost the same in the creation of UHI, while greened areas that include shrub cover and areas with trees and urban forests lessen the effects of UHI (Norton et al., 2015).

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