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How Trees Can Help Cool Urban Heat Islands
Impact, Innovation, Smart Cities August 1, 2021

2020 tied for the hottest year on record with temperatures shattered around the globe. The recent heat wave in the Pacific Northwest has no precedent. Heat kills more people than extreme weather events like hurricanes or tornadoes in the United States, contributing to 12,000 deaths per year.

Nowhere is this heat felt more than in cities. Heat is amplified in “urban heat islands” where roads, pavement and buildings absorb and retain heat, and then release it back into the air, well into the night. Some urban neighborhoods can have mid-afternoon temperatures that are 15°F to 20°F hotter than nearby tree-lined communities or rural areas.

Heat islands temperature profiles

Source: Hot Zones, Urban Heat Islands, Climate Central

This effect increases energy costs for air conditioning, strains the electricity grid, increases air pollution levels, and heat-related illness and deaths.

Urban heat islands worsen the impact of heat waves, particularly for vulnerable populations, such as children, older adults, and those with health conditions. Many cities aren’t prepared for the risk of power failures during a heat wave. A recent study estimated that a combined blackout and heat wave would expose at least two-thirds of residents in those cities to heat exhaustion or heat stroke.

Trees can save lives

Planting trees is one of the most effective strategies to protect people from heat in cities, especially in low-income neighborhoods that have fewer shade trees and higher temperatures than wealthier neighborhoods with tree-lined streets.

Trees can lower the air temperature in city neighborhoods by 10 degrees. The less the pavement is exposed to the sun, the less it will absorb its energy. Trees not only provide shade, their leaves release moisture to cool the environment. This reduces the demand for air conditioning, helping to avoid power failures during heat waves.

Cooling is not the only benefit that trees bring to urban environments. They also reduce air pollution, absorb storm water, store carbon, attract wildlife and improve our mental and physical health.

Many cities across the US and around the world are launching initiatives to harness the power of trees to cool their cities. Nashville’s Root Nashville campaign is to plant 500,000 trees by 2050, focusing on hotter neighborhoods with high concentrations of vulnerable populations. Pittsburgh recently announced plans to plant 100,000 trees over the next 10 years, focusing on lower income and minority communities.

Over the past decade, Paris has led the way in urban greening through multiple actions including planting small forests, reducing the amount of area occupied by cars, creating green spaces, urban agriculture, and transforming asphalt schoolyards into green community assets.

There are efforts in Europe, India and other countries to plant “tiny forests” in city neighborhoods. These forests, sometimes just the size of a basketball court, grow quickly and provide shade, release moisture, attract plants and animals, and store carbon.

Urban trees offer broad economic benefits with an estimated $18.3 billion in air pollution removal, carbon sequestration, lowered energy use in buildings and reduced emissions from power plants.

In addition to planting trees, there are many things cities can do with urban design to reduce the impact of extreme heat:

  • Add vegetation and water to urban spaces
  • Plant green roofs
  • Whitewash any large area of black asphalt like roofs, roads, and parking lots
  • Provide more access to public air-conditioned spaces
  • Vary the height of new buildings to increase airflow and create shade canyons
  • Adapt building design to use more natural ventilation in buildings

Learn more: Using Trees and Vegetation to Reduce Heat Islands

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