Urban Trees or Cool Roofs: Which Cools Cities Best?

The Challenge of Urban Heat

As summer brings rising temperatures, cities often become like ovens, with buildings and pavement trapping the sun’s heat while vehicles and air conditioners add to the warmth. In urban areas with limited greenery, temperatures can be more than 10 degrees Fahrenheit (5.5 Celsius) higher than in nearby suburbs. This increased heat forces air conditioning systems to work harder, putting pressure on the electrical grid and increasing the risk of power outages.

To combat this issue, cities have several strategies at their disposal. Planting trees that provide shade and release moisture into the air is one effective method. Similarly, creating cool roofs that reflect sunlight instead of absorbing it can also help reduce temperatures. However, the effectiveness of these solutions varies depending on the city and its specific conditions.

The Power of Trees

Urban trees offer a natural defense against rising temperatures. They provide shade and release water vapor through their leaves, a process similar to human sweating, which cools the surrounding air and reduces afternoon heat. Adding trees to city streets, parks, and residential yards can make a meaningful difference in how hot a neighborhood feels, with tree-covered blocks being nearly 3 F (1.7 C) cooler than those without trees.

However, planting trees isn’t always straightforward. In hot, dry cities, trees often require irrigation, which can strain already limited water resources. Additionally, trees must survive for decades to grow large enough to provide adequate shade and moisture. Annual maintenance costs can also be high, reaching about $900 per tree per year in some cities. Moreover, dense urban neighborhoods where heat is most intense may lack the space needed to plant more trees.

Cool Roofs as an Alternative

Another option for cooling urban areas is the use of “cool roofs.” These are rooftops coated with reflective paint or made from light-colored materials that reflect more sunlight back into the atmosphere rather than absorbing it as heat. Cool roofs can lower indoor temperatures by about 2 to 6 F (1 to 3.3 C) without air conditioning and can cut peak cooling demand by up to 27% in air-conditioned buildings. They also provide immediate relief by reducing outdoor temperatures in densely populated areas.

Maintenance costs for cool roofs are generally lower than for expanding urban forests. However, they come with their own limitations. Cool roofs work better on flat roofs than sloped ones with shingles, as flat roofs are often covered by heat-trapping materials and exposed to more direct sunlight. Cities also have a finite number of rooftops that can be retrofitted. In areas with many light-colored roofs, additional cool roofs may not significantly impact neighborhood temperatures.

Choosing the Right Mix of Solutions

Many cities around the world have implemented measures to adapt to extreme heat, including tree planting and cool roof programs. For example, nonprofit organizations in Detroit have planted over 166,000 trees since 1989, while Los Angeles has updated building codes to require new residential roofs to meet specific reflectivity standards.

A recent study analyzed Boston’s potential to lower heat in vulnerable neighborhoods. The results showed that a balanced, budget-conscious strategy could deliver significant cooling benefits. For instance, planting trees was found to be more effective than installing cool roofs in places where trees could actually be planted. However, in neighborhoods that needed help, reflective roofs were the better choice.

By investing less than 1% of the city’s annual operating budget, Boston could reduce heat exposure for nearly 80,000 residents. This would result in a reduction of summertime afternoon air temperatures by over 1 F (0.6 C) in those neighborhoods. While this might seem modest, such reductions have been shown to significantly decrease heat-related illness and death, increase labor productivity, and reduce energy costs associated with building cooling.

Tailoring Strategies to Local Conditions

Not every city will benefit from the same mix of solutions. Boston’s urban landscape includes many flat, black rooftops that reflect only about 12% of sunlight, making cool roofs that reflect over 65% of sunlight an especially effective intervention. Boston also has a relatively moist growing season that supports a thriving urban tree canopy, making both solutions viable.

In places with fewer flat, dark rooftops suitable for cool roof conversion, tree planting may offer more value. Conversely, in cities with little room left for new trees or where extreme heat and drought limit tree survival, cool roofs may be the better option. For example, Phoenix already has many light-colored roofs, so trees may be an option there, but they will require irrigation.

Implementing Effective Solutions

Adding shade along sidewalks can serve a dual purpose by providing pedestrians with a place to get out of the sun and cooling buildings. In New York City, street trees account for an estimated 25% of the entire urban forest.

Cool roofs can be more difficult for governments to implement because they require working with building owners. Incentives are often necessary to encourage adoption. For example, Louisville, Kentucky, offers rebates of up to $2,000 for homeowners who install reflective roofing materials and up to $5,000 for commercial businesses with flat roofs that use reflective coatings.

Efforts like these can help spread the benefits of cool roofs across densely populated neighborhoods that need cooling the most. As climate change leads to more frequent and intense urban heat, cities have powerful tools for lowering temperatures. With careful consideration of what already exists and what is feasible, they can find the right budget-conscious strategy to deliver cooling benefits for everyone.