Long lauded as a solution to urban heat islands, green and cool roofs have become commonplace in many cities the world over. But far from being a phenomenon of the Global North, they have cropped up in Southern cities looking to combat food insecurity as well. With an estimated 66% of the world’s population – more than six billion people – living in urban areas by 2050, and with climate change disproportionately affecting cities, it is no wonder that mitigating the effects of urban heat islands is a priority for urban planners today.

Urban heat islands – the result of urban areas absorbing more solar energy than nearby rural areas and radiating that heat back into the atmosphere – make cities hotter than surrounding exurban and rural areas. As the study reports, this can result in a city of one million people having temperatures that are 1-3°C (33.8-37.4°F) than surrounding areas. In addition to increasing energy consumption (the EPA indicates that electricity demand for cooling increases 1.5-2.0% for every 0.6 °C (1 °F) increase in air temperatures), urban heat islands (UHIs) affect human health, increase water usage and air pollution and impair water quality. When coupled with heat waves, urban heat islands can raise the risk of mortality among sensitive populations like children, the elderly and those with health conditions. According to one study, city-dwellers in low- and middle-income countries are particularly vulnerable to the health impacts of climate change.

In spite of that, and while there has been evidence that green and cool roofs can reduce the urban heat island effect, there is much that is unknown about the exact effect that they have on a city’s overall climate. A recent study, titled “Green and cool roofs to mitigate urban heat island effects in the Chicago metropolitan area: evaluation with a regional climate model” by the Environmental Change Initiative at the University of Notre Dame, attempts to understand just how green and cool roofs affect urban heat island – with some surprising results.

Cooling Cities

While planners often look to increase the amount of green space in a city in order to mitigate the effects of urban heat islands, recent studies have found that not all green spaces are created equal, and that how large they are and how they are distributed plays a large role in their impact on urban heat. A study conducted in London found that average urban temperatures decreased more with the spread of smaller parks throughout the city rather than the building of a few large parks.

Green roofs are roofs that are completely or partially covered with vegetation, and while some are used to grow edible plants, others are used to grow ornamental plants. Cool roofs, on the other hand, are made of reflective materials that limit solar absorption, strongly reflecting sunlight and cooling them instead, thereby reducing the amount of heat conducted to the building below. Their effect is often likened to the difference between wearing a white t-shirt and a black t-shirt on a hot sunny day. While cool roofs decrease the need for cooling during warm months, they may increase the need for heating during cooler months.

The study, which was conducted in Chicago in August 2013, reported some interesting, if surprising, results. The team reported that, while cool and green roofs did reduce the urban heat island effect, resulting in an overall cooling of the city, regional and local side effects were dependent on a number of additional factors.

urban heat islands

Chicago City Hall Green Roof by TonyTheTiger, CC BY-SA 3.0.

The overall impact of installing green and cool roofs was the reduction of peak urban roof temperatures during the daytime by 2-3°C (35.6-37.4°F). Green and cool roofing strategies had a significantly larger impact in the downtown Chicago area, however, where they lowered roof surface temperatures by 7-8°C (44.6-46.4°F), indicating that mitigation is more effective in dense, highly urbanized environments.

Not all side effects improved the quality of air in the city however. Due to the reduction of the relative temperature difference between the air over the city and air over the nearby Lake Michigan, the breezes that blow from the lake into the shore were weakened, making shoreline neighborhoods warmer rather than cooler.

In addition to reducing temperatures above the urban rooftops, the roofs cooled the lower atmosphere. The team also found that green and cool roofs decreased horizontal wind speeds and reduced the mixing of hot and cold vertical air. This caused air to stagnate near ground level, potentially reducing the quality of air in the city.

To address some of the unintended consequences of green and cool roofs, the researchers encourage urban planners to reconsider the design and positioning of streets to better channel the breezes flowing inland in cities flanked by bodies of water. They also note that, in order to effectively mitigate the effect of urban heat island, careful studies should be conducted before selecting sites for green and cool rooftops.

Studies like this could help urban planners understand the impact of green and cool roofs on air quality, allowing them to position them in a way that would maximize their benefits. An article by Ashish Sharma, Research Assistant Professor at the University of Notre Dame and one of the researchers who worked on the study, elucidates some of the factors that should be weighed in making decisions about how to integrate green and cool roofs into a city. He notes that environmental justice should play an important role in decisions about where to install green and cool roofs, particularly since low-income neighborhoods are often disproportionately affected by hot weather. He also notes that, while green roofs add substantial green space, provide habitats for plants and insects and improve air quality, they cost more to install and maintain than cool roofs, which require no maintenance. Accounting for a city’s water resources and average temperatures should also play a role in determining whether cities opt for green roofs or cool roofs – with northern, water-rich cities benefiting more from green roofs, and southern, arid cities benefiting more from cool roofs.