Editors: Jessica Roberts, Jillian Aicher, Colt Watkiss
Contributing Researcher: Rhea Mallett[*]
Urban Heat Island and Equity: What Can Local Governments Do?
Extreme heat is a national public health emergency that kills more than any other weather-related event and will worsen as climate change increases global temperatures. Extreme heat disproportionately impacts communities that are non-white, low income or have suffered historically racist disinvestment and urban planning practices. Urban planning and zoning created UHIs, and land use regulations can provide solutions for mitigating their impacts.
UHIs experience higher air temperatures than surrounding areas. These ‘islands’ of higher temperatures all contain mostly man-made impervious surfaces that absorb and retain heat, such as buildings, rooftops, roads, sidewalks, parking lots, and courtyards. They lack vegetation such as trees, grass, and shrubs. The impervious surfaces (i.e., asphalt and concrete) capture and intensify heat during the day and slowly release the absorbed heat back into the air at night, resulting in higher nighttime temperatures that prevent residents from getting relief. The heat differential between a UHI and a more affluent neighborhood nearby can be as high as 27°F.
U.S. localities, large and small, are creating solutions through land-use regulations that control the causes and mitigate the impacts of UHIs. The focus of these strategies and links to examples follow:
Trees are “air conditioning for cities,” as air temperature under trees can be 20-45°F cooler than a nearby unshaded area. Trees block solar radiation, filter particulates, and absorb pollutant gases. Trees also provide critical “evapotranspiration” benefits, converting the sun’s energy into water vapor which cools the air and utilizes solar energy that would have otherwise created more heat. Localities that prioritize increasing tree canopy have implemented laws that force developers to preserve trees, ensure tree density, plant minimum trees per lot, install trees shading sidewalks, protect tree roots during construction and have even created enforcement provisions that require fines, bonds, and five-year waiting periods to ensure the safety of trees. Extending tree preservation to private property owners reinforces the philosophy that mitigation by one person helps everyone. Tree preservation funds for situations where compliance is difficult allow localities to redirect resources to areas where trees are most needed.
Green Roofs are 30° to 40°F cooler than conventional roofs. They also redirect solar energy through evapotranspiration and help with air pollution. Green roofs also keep buildings cooler, reducing reliance on air conditioning which increases energy demand and pollution. Requiring green roofs on large developments has been so successful that at least one locality requires 100% green roof compliance. Localities offer incentives for green roofs, such as zoning density bonuses, increased building height, and streamlined permitting.
Other cool roofing materials can be used alone or in conjunction with green roofs. Cool roofing materials combine a higher reflectivity (albedo) as well as emissivity for heat. A Solar Reflectance Index (SRI) measures the surface’s ability to reflect heat, which can be used to set minimum standards. Many localities will provide some flexibility, allowing for either 50% green roofs or 75% SRI, a combination of both for 100% of roof covering, or a scoring system that allows developers to pick and choose different green infrastructure.
Pervious or cooler materials are also mandated for non-roof hardscapes, such as paths, sidewalks, courtyards, and pedestrian right of ways. A commonly seen ordinance requires developers to ensure that 50% of their ‘non-roof hardscape’ are either shaded or utilize cooler material, such as pavers, porous concrete, or other pervious surfaces.
Parking lots and spaces are major sources of heat absorption. Heat mitigation strategies include shading percentages, minimum tree amounts based on parking lot size, or the use of impervious materials. Some localities reduce parking spaces required by allowing shared parking or reducing requirements to zero. And at least one city upended the parking paradigm for developers by changing from parking allowance minimums to maximums.
Purposeful planning will also mandate open spaces, incentivize optimal landscaping, and provide shade to promote walkability. The effectiveness of each planning measure depends on climate, landforms, and building densities. However, all planning must confront the role prior urban design has played in today’s inequities and prioritize the most heat vulnerable in its land-use solutions mitigating extreme heat.
[*] Rhea Mallett is an LLM candidate at the Elisabeth Haub School of Law and Land Use Law Center Volunteer.
Jessica Roberts is a second-year student at the Elisabeth Haub School of Law and Research Assistant to Professor Nolon.
Jillian Aicher is a second-year student at the Elisabeth Haub School of Law and Research Assistant to Professor Nolon.
Colt Watkiss is a first-year student at the Elisabeth Haub School of Law and Land Use Law Center Volunteer.
The previous blogs in the series are listed here:
- Reframing Sustainability: Introducing the Land Use, Human Health, and Equity Project
- Planning for Public Health: A New Beginning for Land Use Law
- The Role of Density in Combatting Climate Change and COVID-19
- Novel Coronavirus Claims Implicate Age-Old Property Rights Questions
- State & Local COVID-related Emergency Powers: Individual Rights
- COVID-Related Land Use Regulations and Judicial Deference
- Mediation of Eviction Disputes May Hold the Key to the Survival of Small Businesses
- Using Zoning to Help Eliminate Food Deserts: A Few Steps Forward
- Urban Heat Islands and Equity
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