Rooftop gardens and greenery can help ease some of the severe heat in cities, according
to research from climate scientists at NASA.
For several decades, researchers have promoted replacing black tar and other dark-colored
roofing materials with bright, Sun-reflecting surfaces or "green roofs" full of plant cover.
Now they have used freely available satellite data to measure how effective these changes
are.
Heat is often intensified or amplified in cities, a phenomenon known as the urban heat
island effect.
Asphalt, concrete, and similar materials absorb and retain significantly more heat than vegetation,
so temperatures in urban areas are often 10 degrees Fahrenheit hotter than surrounding
suburbs or rural regions.
In neighborhoods with fewer trees and green spaces, this heat often disproportionately
affects older adults, low-income communities and some communities of color.
Green roofs are designed to harness the cooling power of plants to lower the temperature in
city spaces.
The greenery may be extensive (shallow soil, low-maintenance plants) or intensive (deeper
soil, more diverse plants and trees).
The GISS team studied three sites in Chicago to see how green roofs affected surface temperatures
around those buildings, and whether there was a difference between those sites and others
nearby without green roofs.
Two of three green roofs in the study reduced temperatures, but results indicated that effectiveness
may depend on location and plant diversity, among other factors.
The results were recently published in the journal Sustainable Cities and Society.
The research team partnered with Chicago’s Public Health and Planning and Development
departments to study three sites that had installed green roofs in the early 2000s:
Millennium Park, City Hall, and a Walmart shopping center.
Using imagery captured by the Landsat 5 satellite between 1990 and 2011, the researchers compared
changes in land surface temperatures and vegetation abundance at the study sites as well as nearby
control sites without green roofs.
Results from the three sites were mixed.
Millennium Park, which has an intensive mix of plants and is located near Lake Michigan,
showed significantly lower average temperatures after its green roof was installed in 2004.
It was the only site where the roof fully mitigated climate warming over the study period.
City Hall, also an intensive site, had a green roof installed in 2002.
Its temperatures after green roof installation were lower than those at the control site,
but they were rising toward the end of the study period.
The Walmart site told a different story.
While the green roofs at Millennium Park and City Hall were added onto existing buildings,
the supermarket was newly built during the study period.
Even though the Walmart installed an extensive green roof, the conversion of the land from
a vacant, grassy lot to a store meant the vegetation index for the landscape decreased.
The benefits of green roofs depend on a variety of factors — from geographic region and
plant diversity to rooftop structure and the cooling efficiency of the building itself.
Studies with larger sample sizes are needed to tease apart these details, but this study
represents a promising start.
With urban heat island effects expected to intensify as Earth’s climate warms, it will
become more important to understand these variables.
The study method is designed to be used by other cities for further research.The simple
analysis, publicly available data, and model for working directly with cities could help
urban planners assess the viability of green roofs in their own areas.