An Urban Land Institute study on urban heat islands suggested creating a half-mile “cooling corridor” in Lower Roxbury including green infrastructure, water features and heat-minimizing paving materials. Image courtesy of Arrowstreet

Everyone knows that the world is getting warmer. And while the discussion is often about global scale issues, like moving away from fossil fuels, the health issues created by extreme heat are local and in particular impact the most vulnerable segments of our population.

The way we build and regulate our cities has been multiplying the effects of hotter days and longer summers for decades and it is time we take a holistic view of our cities, from transit and roads to building codes and zoning, to make relatively easy changes to reduce heat impacts for all of us but in particular low-income, urban communities.

Often called the “heat island effect,” buildings and roads absorb and re-emit the sun’s heat more than natural features and built-up areas become “islands” of higher temperatures. Heat islands can form during the day or night, in small or large cities, in suburban areas, and in any season. Heat islands form because of many factors, not just the amount of solar radiation a surface reflects or absorbs. As we make our buildings smarter, the electronics and cooling systems we put in them to protect us from the hotter outside temperatures shed heat back into the environment, further exacerbating the impacts on what little natural environment is left in the city.

Of course, there is a lot we can learn from how nature and plants have evolved to deal with solar radiation. While plants may have dark leaves, they breathe and produce water vapor cooling the air and surfaces around themselves. They provide shade to the harder, darker surfaces underneath them, preventing those surfaces from heating and extended the heat of the day into the night. Large areas of natural vegetation create their own microclimates, while bodies of water absorb environmental heat and create cooling breezes. By finding ways to leverage those natural strategies into the built environment, we can easily and effectively improve the lives of our neighbors.

Pros and Cons of White Roofs

The most common strategy to reduce is heat in buildings is using highly reflective white roofs. While a good start, all this does is reflect the solar radiation somewhere else, and if you’ve ever lived above a building with a white roof, you know how unpleasant looking down on it can be and how much heat it can add to your home or office.

That isn’t to say changing the way we handle roofs isn’t important. Arrowstreet and ARUP helped the Massachusetts Institute of Technology examine their entire portfolio of roofs to determine the best sustainable use for each one.

Some roofs were in bright sunlight and prefect for solar panels. Others were in shade all the time but can be used to store rainfall, both reducing peak flow into stormwater systems and providing local cooling. Some roofs got good sun but were not oriented well for solar systems, and so a green roof that absorbs water and provides local cooling was the better option.

A tool like this, used on a neighborhood or city-wide scale, could have a significantly positive impact on existing, dense neighborhoods that have few other options for impacting their environment.

Scott Pollack

Better Streetscapes Can Help

On the street, many studies have shown that planting trees that provide shade over existing, hard surfaces can reduce the air temperature by more than 7 degrees.

Alta, working for the Southern California Association of Governments (SCAG) has developed a toolkit for Urban Heat Island reduction strategies. The Boston chapter of the Urban Land Institute issued its Living with Heat report to help local real estate developers, designers and policymakers understand the consequences of extreme heat and to seek solutions to make buildings, neighborhoods, parks and outdoor spaces more adaptable and comfortable for people.

Some strategies, such as using existing structures to create new, covered parks and establishing a network of cooling stations along existing pedestrian patterns, are modest in scale. Others require a longer-term approach, such as creating highly landscaped, pedestrian cooling corridors that take advantage of prevailing winds to transform existing streets into micro-climates that use nature’s way of cooling the air to “air condition” the city.

What all of these ideas have in common is the need to change existing policies so that instead of considering buildings, mechanical systems and public streets as independent things to be regulated separately, but all part of a holistic whole whose goal is to improve living conditions for its citizens. We must change the rules so the way we regulate one thing, like rainwater, can have a positive impact on something that may seem entirely separate, like how hot are streets are.

Scott Pollack is a principal at Arrowstreet in Boston.

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