A new report from Climate Central lays out how U.S. cities can mitigate urban heat islands (UHIs) and adapt to the increasing frequency of extreme heat events.
“Neighbourhoods in a highly developed city can experience peak temperatures that are 15° to 20°F [8.3°C to 11.1°C] hotter than nearby areas with more trees and less pavement,” explains an introduction to the report. Especially during summer months, the paved roads and minimal greenery common in urban areas retain heat that would otherwise be reflected or evaporated. Other major factors influencing UHIs are building height, human-created heat emissions, and narrow “urban canyons”.
Such extreme heat threatens the health of vulnerable populations such as children, older adults, and those with existing conditions, says Climate Central. It also increases the risk of heat stress for those who work outside. Hotter air also becomes stagnant and traps pollutants in the lower atmosphere, worsening air quality.
To measure how the built environment creates these UHIs, Climate Central created an index based on research led by Valentino Sangiorgio and published last year in the journal Scientific Reports. By measuring and comparing the influence of different urban factors that influence temperature, the team found that the amount of greenery present contributes to 21% of the total UHI effect, while building height contributes just 8%. Albedo, or the amount of sunlight reflected from surfaces, contributes 29%.
The new report adapts Sangiorgio’s model to estimate how UHI intensity varies across U.S. cities, finding that the five cities with the highest index are New Orleans, Newark, New York City, Houston, and San Francisco. Cities that are most influenced by having compact, taller buildings are found mostly in the northeastern and midwestern United States, while other cities were influenced by a large percentage of impermeable surfaces, such as Houston.
The researchers stress in their study that the index score is an average, and different areas within a city will score higher or lower based on the specific surroundings. The report notes that low-income neighbourhoods and communities of colour often see disproportionate impacts from the UHI effect due to discriminatory, race-based housing practices such as redlining. An interactive map from the Science Museum of Virginia outlines the increased exposure to heat in redlined neighbourhoods.
The report authors call on city governments to adapt and prepare as climate change increases the frequency and intensity of heat waves. Short-term solutions include better public education, transportation to take citizens to public areas with air conditioning, public strategies for monitoring vulnerable residents during extreme heat, and financial assistance for energy bills for cooling. Longer-term solutions include planting more trees along paved areas, whitewashing roads and sidewalks, and adding green roofs and cooling roofs to buildings.