The Earth is warming, and by extension so are our cities. Managing the effects of climate change requires us to think creatively about our existing resources, like underutilized or contaminated land, and how those resources can contribute to a cooler, greener collective future.

In 2014, the United Nations Department of Economic and Social Affairs announced that the majority of humans were residing in urban environments for the first time in recorded history. As the world’s population centers become more densely urbanized, average temperatures in these areas are on the rise. The Urban Heat Island (UHI) effect can be felt on any typical hot and sunny day in cities like Los Angeles or Washington, D.C. In the middle of the city, concrete highways and structures absorb UV rays from the sun and radiate heat into the surrounding area. If you were to venture outside of these cities to less densely populated rural areas, you may find temperatures up to 27℉ lower.

This vast increase in temperature isn’t only an issue while the sun is out. Nighttime temperatures in urban areas have been found to be as much as 22℉  higher than air temperatures in neighboring, less developed areas. The UHI effect is exacerbated by removing green spaces, which leads to an increase in greenhouse gas emissions and air pollution levels. Removing green spaces puts a strain on other critical urban infrastructure such as the energy grid, water quality and public health systems. Redeveloping underutilized land, such as vacant lots or former industrial and commercial sites, presents excellent opportunities to rethink UHI mitigating factors such as the urban tree canopy, green roofs, and other issues related to site design and building materials. Land recycling presents an opportunity to design from the ground up in anticipation of the our changing climate and the demands it will place on all of us.

Green Roof, Cool Roof

“Green” roofing and “cool” roofing are two ways that densely populated cities are combatting increased temperatures in an effort to mitigate the UHI effect. Green roofs absorb stormwater, provide space to grow food and habitat for pollinators, and reduce ambient heat through evapotranspiration (the process through which plants release water into the surrounding atmosphere). As the city of Vancouver, British Columbia discovered when it developed its Olympic Village in 2010, green roofs moderate interior temperatures, and even on hot summer days green roofed buildings can be cooler than ambient air temperatures, whereas conventional roofs can be up to 50% warmer. When developing green roofs in urban environments, special attention should be given to choosing vegetation that is native to the local environment and tolerant to local conditions.

Five Elements of Green Roofs:

• Root Resistant Layer: a barrier that deters roots from penetrating or damaging the structure.
• Drainage Layer: A layer used to channel excess water to drains.
• Filtration Layer: A layer of material that prevents fine particles from draining out of the system, limiting impacts to stormwater quality.
• Vegetative Layer: A layer of plants that can sustain themselves in a relatively shallow growing medium.
• Growing Medium: A mineral-based medium up to six inches deep, which supports plant growth.

Can Your Roof Be Cool?

Cool roofs are designed to mitigate the UHI effect by reflecting more sunlight and absorbing less heat than a standard roof. This is accomplished by using highly reflective shingles, tiles, or paint. Under the heat of the summer sun, standard roofs can reach temperatures of up to 150℉ while their cool roof counterparts stay up to 50℉ cooler, decreasing the need for air conditioning and increasing energy savings. In October of 2014, Los Angeles became the first major city to unanimously pass a building code update requiring all new and refurbished homes to have cool roofs, which use sunlight-reflecting materials. Thanks to these coatings, virtually any roof can be cool. These coatings are typically a white, paint-like substance that reflects heat, can offer water protection, surface restoration, and are available for most types of roofs.

Cool Pavements Produce Cold, Hard Benefits

By definition, a “cool” pavement stays cooler than standard pavements because reflectivity is enhanced by using surface coatings and reflective material aggregates. Rethinking which materials are used in critical infrastructure is crucial to mitigating the Urban Heat Island effect. Much like standard roofs, dark pavements and asphalts absorb up to 95% of the sunlight that they are exposed to and radiate that heat out into the immediate surroundings. Hot pavements can aggravate the UHI effect because they make up roughly one third of urban surfaces and also increase the temperature of stormwater runoff. Canoga Park, CA put cool pavements to the test by using light grey surface coating to reduce pavement temperatures. Locations with “cool pavement” read a morning temperature of just 73℉ compared to previous temperatures of 93℉ on dark asphalt. Los Angeles is at the forefront for pioneering “cool pavements” in hopes of mitigating heat UHI effects and hopes to test in 14 other council districts Summer 2017.

Other measurable benefits of cool pavements include:

• Comfort and health: cool pavements can reduce temperatures and associated health issues for communities and provide a more comfortable environment.
• Increased energy savings: reflective pavement can save energy by reducing energy needs for lighting and by offering increased durability (discussed below).
• Durability: reduced pavement temperature can slow the aging effects sunlight and heat can have on asphalt materials.
• Water quality: cool pavements can lower runoff temperatures, thus reducing thermal shock to life in aquatic ecosystems.
• Safety: cool pavement can enhance nighttime visibility, which can increase the effectiveness of street lamps and headlights, increase pedestrian and vehicle visibility, as well and reduce energy costs associated with lighting.

The Urban Tree Canopy

The Urban Tree Canopy (UTC) is the layer of tree cover seen from a bird’s eye view of a city. An increase in the UTC layer provides communities with two important benefits. First, the vegetative cover intercepts more rainwater, limiting the amount of pollutants entering the stormwater drainage system through runoff channels. Second, the UTC layer limits the Urban Heat Island (UHI) effect by reducing heating and cooling costs, lowering air temperatures, reducing air pollution and providing wildlife habitats. Urban tree canopies can be used as an alternative to cooler pavement materials.

In 2014, The City of Phoenix launched its Cool Urban Spaces Project, with the goal of increasing the tree canopy in order to mitigate UHI. Two significant results of this tree and shade initiative were that tree canopy cover increased by 25% and led to a temperature reduction of 4.3°F when compared with a treeless neighborhood. Second, other planting activities such as adding grass patches in residential backyards reduced neighborhood temperatures by about 0.5°F.

UHI, UTC, and Land Recycling

Land recycling and decreasing the Urban Heat Island effect share a common goal: to remediate and reuse land to create an abundance of economic, environmental, and community benefits. For each dollar spent by the EPA on land recycling projects, $16 are leveraged throughout the community. Some of the most environmentally contaminated sites are frequently located in densely urbanized and lower income inner-city areas that are most susceptible to the negative impacts of the UHI effect. Brownfields sites often require remediation, and it makes sense to have that remediation benefit the entire community by limiting the existing problem of the Urban Heat Island effect. Per the 2016 Congressional testimony of Mathy Stanislaus, former EPA Assistant Administrator, roughly 33% of the U.S. population lives within three miles a brownfield site. Stanislaus adds: “While there is no single way to characterize communities located near our sites, this population is more minority, low income, linguistically isolated, and less likely to have a high school education than the U.S. population as a whole.”

Cools roofs, reflective pavements, and urban tree canopies are methods that can address and mitigate UHI. These strategies offer a myriad of community benefits, including improved environmental health, reduced energy needs and increased ecosystem health, and can be especially beneficial to communities when implemented on underutilized land. We don’t know exactly what the climate will look like 25-50 years from today, but we generally know that it will be more extreme. As we look for ways to build cities for the future, UHI mitigation measures offer impactful tactics that can help cities stay cooler even as summers heat up.