Unlock Cooler Cities Now:Essential Tips for Tackling Urban Heat Islands
New England is America’s second-most urbanized area, with more than 80% of its people living in cities. While cities provide New England residents with many benefits, they also experience higher-than-average temperatures and more intense heat extremes. A 2020 study, for example, showed that average temperatures in Providence’s less-vegetated neighborhoods could be more than 12 degrees hotter than well-vegetated areas like the city’s East Side. The increase is likely the result of the urban heat island effect (UHI).
Climate change often worsens the UHI effect, creating hot spots in cities lacking sufficient tree canopy density. As global temperatures rise because of climate change, a city’s baseline temperature increases, intensifying the already elevated conditions. Climate change also increases the frequency of heat waves, further intensifying the effect. Trees, however, are a natural solution to mitigating UHI. They help combat UHI by increasing a city’s canopy density, creating a cooler, more comfortable environment.
Factors Impacting the UHI Effect
Cities often become hotter than surrounding rural areas due to replacing natural land cover with heat-absorbing surfaces like pavement and buildings. These surfaces soak up solar radiation and release it as heat during the day, raising urban temperatures. Other factors influencing UHI include reduced vegetation/green spaces, city traffic, air conditioning, tall buildings, and narrow streets.
These factors can have profound effects on cities and urban residents, including:
• Increased energy consumption
• Elevated air pollution levels
• Higher heat-related illnesses
• Increased respiratory issues
• Decreased quality of life
• Increased infrastructure risk
• Increased ground-level ozone (smog)
UHI impacts those with the least access to financial resources and those dealing with additional health conditions more than other groups. For instance, seniors, children, and those with per-existing conditions are more susceptible to heat-related illnesses. It also impacts poorer communities, often with less green space and more heat-absorbing surfaces.
Keep in mind that average temperatures in some areas, however, vary from year to year. Even within a city, temperatures vary depending on proximity to the coast, green spaces, and the urban heat island effect. Plus, the Northeast can be pretty humid in the summer. Meanwhile, climate change isn’t helping.
Combating Urban Heating Island Effect
Combining UHI starts with having sufficient tree canopy—the layer of leaves, branches, and stems of trees covering the ground when viewed from above. It provides various cooling mechanisms that help reduce UHI, including evapotranspiration. This process triggers water uptake by roots and releases it through leaves, cooling the surrounding air and lowering ambient temperatures. A sufficient tree canopy also blocks solar radiation, reduces the heat absorbed by surfaces, alters wind patterns to provide cooling breezes, and reduces the surface temperature of pavements and roofs, among other things.
Tree canopy is a crucial component of urban planning. Some cities, however, lack sufficient canopy density. Tree canopy density in Warwick, Pawtucket, and Cranston, for instance, ranges from about 20% to 30%. Providence’s is more than 27%, with 3,221 acres of land and more than 415,000 trees. What’s the ideal canopy density? In the past, it was thought that 40% was the ideal goal density. Today, sufficient tree canopy depends on the context. What’s ideal for a desert city will differ from that of a city in a forested region.
Strategic Steps in Confronting UHI
The combined effects of UHI and climate change generate more intense heat waves and higher average temperatures in cities, boosting health risks and increasing energy consumption. As in Providence, increased investment in urban forestry initiatives is a key strategy for creating more resilient and sustainable cities in the face of climate change. Below are some steps cities can take to combat UHI.
• Species selection—Choose tree species that adapt well to the urban environment (e.g., drought-tolerant and pollution-resistant). Selecting tree species with large canopies and high evapotranspiration rates is critical. Key species include white oak, red maple, river birch, Gingko biloba, American hornbeam, Sweetbay magnolia, and Serviceberry.
• Tree Placement and Planting Design—This step requires strategic planting near buildings, streets, and other heat-generating surfaces. The goal is to create “urban forests” or green corridors to maximize cooling benefits.
• Urban Forest Management—Proper tree care, including watering, pruning, and pest management, ensures tree health and longevity. Community involvement in urban forestry initiatives also helps. See RI Tree’s website for more information on tree care.
The Urban Heat Island effect is a real challenge for some cities. Climate change isn’t making things any easier for these cities. Now is a good time to start thinking about combating these phenomena by increasing tree canopy. However, how much tree canopy you’ll need depends on where you live. Don’t focus on a specific percentage for tree canopy. Instead, assess your city’s needs, set realistic goals, focus on equitable tree distribution, and prioritize tree health.
Understanding the UHI effect, however, can lessen its impact on urban residents and the surrounding environment. Mitigating UHI is crucial for creating more sustainable and comfortable cities. Strategies like planting trees, using reflective materials, and designing buildings for better ventilation can help combat this growing problem.
For more information on how to boost tree canopy, feel free to contact one of our consultants at RI Tree.org.