Cooler Cities Save Lives

Need to Know – Stephen Leahy

Cities and towns need to go green to survive a hotter world

Key Points

• Over 3 million heat-related deaths (2000–2019)
• 30% urban greenery prevents 1.16M deaths
• Green areas reduce temperature by 3–5°C
• Urban heat island caused by traffic, buildings
• Traffic is #2 source of urban heat
• Green space lowers A/C use and flooding

Heat is a killer in many urban areas, with more than 3 million heat-related deaths globally between 2000 and 2019.*

Increasing vegetation in urban areas by 30% would have prevented 1.16 million of those deaths, according to a study in The Lancet.*

Urban greenery can cool cities by 3°C on average—a life-saving difference during heatwaves.*

Botanical gardens, wetlands, tree-lined streets, and rain gardens can cool city air by 4–5°C according to a global study.

Parks, playgrounds, and urban farming also deliver about 3°C of cooling.

How trees, shrubs, and grasses cool cities:

1. Shade reduces heat absorption and radiation from surfaces.
2. Plants absorb heat and release water vapour, cooling the air.

Need-to-Know: Cities Are Hotter Than Rural Areas

Urban areas absorb and trap heat due to asphalt, concrete, and traffic emissions. This phenomenon is called the urban heat island effect.*

Note: 70–75% of fuel burned by vehicles becomes heat, intensifying city temperatures.

Need-to-Know: Road Traffic is the Second-Largest Source of Urban Heat

Even a 10–20% increase in vegetation could have prevented 27–32% of heat-related deaths.*

“Now is the time to start preparing cities for a much hotter world, while doing everything we can to slash emissions.”
— Rogier van den Berg, WRI Ross Center for Sustainable Cities

Vision of a future green city

Benefits of Greener Cities

• Trees can lower air pollution as they can absorb harmful gases and provide oxygen.
• Reduce noise levels.
• Provide critical habitat for pollinators and many bird species, both of which are in sharp decline.
• Enhance outdoor physical activity, social engagement, and psychological wellbeing.
• Lower rates of chronic disease.

Singapore is one of the world’s greenest cities (Source: Getty Images)

Green spaces reduce the need for air conditioning and improve flood management. China’s “sponge cities” use rain gardens, permeable surfaces, and increased greenery to address heat and water challenges.* (More here)

What Can We Do?

• Plant trees, rewild gardens, and grow food locally.
• Protect public parks and mature urban trees.
• Advocate for urban planning that includes green infrastructure.

Footnotes

1. * Source: Need to Know by Stephen Leahy.
2. * The Lancet Planetary Health, 2025. View study.
3. * Global urban cooling research via ScienceDirect.
4. * Urban heat island information sourced from WRI research.
5. * Study findings on vegetation impact from The Lancet.
6. * Source: WRI and Substack article on sponge cities. Full story here.