Green Cities Can Be Cooler

By Margot Le Guen, Climate and Society ’13

Cities all over the world are getting hotter and more polluted as a result of climate change and urban heat island effect. Is this phenomenon irremediable? Certainly not in view of the burgeoning of innovative adaptation measures across the world.

A Health Issue

Beyond thermal comfort concerns, managing heat-related events falls in the field of public health policy as they are associated to sometimes devastating mortality and morbidity rates. A few examples: Europe accounted for about 35,000 excess deaths in 2003. India also regularly reports dreadful consequences of heat waves, notably in 1998 with 1,300 excess deaths in the only state of Orissa due to temperatures above 120°F. Dense cities tend to be 2°- 6°F hotter than rural areas: indeed, dense building configurations with dark pavement trap air and store heat preventing cities to cool especially at night, and exposing vulnerable populations (the elderly, socially isolated, chronically ill, homeless) to major health issues like heat strokes, heat stress, respiratory and cardiovascular diseases.

Identifying no-regret and low-cost adaptation measures to prevent such devastating impacts appears to be compelling. Among those: urban green spaces and rooftop adjustments provide numerous co-benefits. Interestingly, students and children are usually the best testers for these innovative initiatives.

Young Solutions

L’escuela de Jornada Completa in Buenos Aires, Argentina, has been hosting a green roof since 2011: in addition to the goal of demonstrating the benefits of green roofs, the project provided educational purposes as the green roof became an outdoor classroom ideal for learning about ecology and agriculture.

Students in the Philippines won the World Bank competition “Youth Climate Change Project” with a practical project aiming to compare temperature reductions before and after all roofs of a neighborhood in the city of Cebu were painted in white in only one day.

Another inspiring case study can be found in Dhaka where slum dwellers built a bamboo platform on the lake nearby one of the densest informal urban settlements in Bangladesh. “Ashar Macha”, the name of the initiative, means “platform for hope”. And indeed it brought a clean and cooler space physically separated from the filthy and suffocating air of the slum, dedicated to children who can play and read books in the small library.

Bigger Picture

How do those measures reduce climate risks? Urban vegetation reduces the urban heat island effect by providing shade, removing heat from the air through evapotranspiration, absorbing heat in plant thermal mass and thus reducing ambient temperatures. The surface of a conventional rooftop can be up to 90°F warmer in hot summer days in typical American cities! But green spaces offer considerably more advantages: they improve air quality, reduce runoff by limiting the amount of hard surfaces and through the absorption of water in soils and plant roots, and generate energy savings by reducing needs of AC and electricity. Last but not least, they bring esthetic value, valorize landscapes and offer recreational areas for local populations.

Painted white roofs on the other hand, increase indoor thermal comfort and reduce the urban heat island effect thanks to two properties: high solar reflectivity and high infrared emissivity. Global Cool Cities Alliance shows how city-level ambient air temperatures can be reduced from 92°F to 85°F and how cool roofs offer carbon offset opportunities.

All in all, cooling urban environments may not be more complicated than grabbing secateurs or paintbrushes!

Check it out: New York City also launched its own initiative to paint all roofs in white and offers volunteering opportunities.