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2 ways cities can beat the heat: Which is best, urban trees or cool roofs?

Beachgoer Sergei lazovskiy cools off under the shower after enjoying the sunny and hot day at the Haulover Park Beach at on Tuesday, May 28, 2024, in Miami Beach, Florida.
David Santiago
/
The Miami Herald
Beachgoer Sergei lazovskiy cools off under the shower after enjoying the sunny and hot day at the Haulover Park Beach at on Tuesday, May 28, 2024, in Miami Beach, Florida.

This article is republished from The Conversation, an independent and nonprofit source of news, analysis and commentary from academic experts. Ian Smith is a Research Scientist in Earth & Environment at Boston University. Lucy Hutyra is a Distinguished Professor & Chair of Earth and Environment at Boston University.

When summer turns up the heat, cities can start to feel like an oven, as buildings and pavement trap the sun’s warmth and vehicles and air conditioners release more heat into the air.

The temperature in an urban neighborhood with few trees can be more than 10 degrees Fahrenheit (5.5 Celsius) higher than in nearby suburbs. That means air conditioning works harder, straining the electrical grid and leaving communities vulnerable to power outages.

There are some proven steps that cities can take to help cool the air – planting trees that provide shade and moisture, for example, or creating cool roofs that reflect solar energy away from the neighborhood rather than absorbing it.

But do these steps pay off everywhere?

We study heat risk in cities as urban ecologists and have been exploring the impact of tree-planting and reflective roofs in different cities and different neighborhoods across cities. What we’re learning can help cities and homeowners be more targeted in their efforts to beat the heat.

The wonder of trees

Urban trees offer a natural defense against rising temperatures. They cast shade and release water vapor through their leaves, a process akin to human sweating. That 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 blocks that have tree canopies nearly 3 F (1.7 C) cooler than blocks without trees.

But planting trees isn’t always simple.

READ MORE: City of Miami opens cooling centers for this year’s extreme heat season

In hot, dry cities, trees often require irrigation to survive, which can strain already limited water resources. Trees must survive for decades to grow large enough to provide shade and release enough water vapor to reduce air temperatures.

Annual maintenance costs – about US$900 per tree per year in Boston – can surpass the initial planting investment.

Most challenging of all, dense urban neighborhoods where heat is most intense are often too packed with buildings and roads to grow more trees.

How cool roofs can help on hot days

Another option is “cool roofs.” Coating rooftops with reflective paint or using light-colored materials allows buildings to reflect more sunlight back into the atmosphere rather than absorbing it as heat.

These roofs can lower the temperature inside an apartment building without air conditioning by about 2 to 6 F (1 to 3.3 C), and can cut peak cooling demand by as much as 27% in air-conditioned buildings, one study found. They can also provide immediate relief by reducing outdoor temperatures in densely populated areas. The maintenance costs are also lower than expanding urban forests.

However, like trees, cool roofs come with limits. Cool roofs work better on flat roofs than sloped roofs with shingles, as flat roofs are often covered by heat-trapping rubber and are exposed to more direct sunlight over the course of an afternoon.

Cities also have a finite number of rooftops that can be retrofitted. And in cities that already have many light-colored roofs, a few more might help lower cooling costs in those buildings, but they won’t do much more for the neighborhood.

By weighing the trade-offs of both strategies, cities can design location-specific plans to beat the heat.

Choosing the right mix of cooling solutions

Many cities around the world have taken steps to adapt to extreme heat, with tree planting and cool roof programs that implement reflectivity requirements or incentivize cool roof adoption.

In Detroit, nonprofit organizations have planted more than 166,000 trees since 1989. In Los Angeles, building codes now require new residential roofs to meet specific reflectivity standards.

In a recent study, we analyzed Boston’s potential to lower heat in vulnerable neighborhoods across the city. The results demonstrate how a balanced, budget-conscious strategy could deliver significant cooling benefits.

For example, we found that planting trees can cool the air 35% more than installing cool roofs in places where trees can actually be planted.

However, many of the best places for new trees in Boston aren’t in the neighborhoods that need help. In these neighborhoods, we found that reflective roofs were the better choice.

By investing less than 1% of the city’s annual operating budget, about US$34 million, in 2,500 new trees and 3,000 cool roofs targeting the most at-risk areas, we found that Boston could reduce heat exposure for nearly 80,000 residents. The results would reduce summertime afternoon air temperatures by over 1 F (0.6 C) in those neighborhoods.

While that reduction might seem modest, reductions of this magnitude have been found to dramatically reduce heat-related illness and death, increase labor productivity and reduce energy costs associated with building cooling.

Not every city will benefit from the same mix. 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.

Efforts like these can help spread cool roof benefits across densely populated neighborhoods that need cooling help most.

As climate change drives more frequent and intense urban heat, cities have powerful tools for lowering the temperature. With some attention to what already exists and what’s feasible, they can find the right budget-conscious strategy that will deliver cooling benefits for everyone.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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