Rising temperatures are threatening urban areas, but efforts to cool them may not work as planned.
The greenhouses that sprawl across the coastline of southeastern Spain are so bright that they gleam in satellite photos. Since the 1970s, farmers have been expanding this patchwork of buildings in Almería province to grow produce such as tomatoes, peppers and watermelons for export. To keep the plants from overheating in the summer, they paint the roofs with white lime to reflect the sunlight.
That does more than just cool the crops. Over the past 30 years, the surrounding region has warmed by 1 °C, but the average air temperature in the greenhouse area has dropped by 0.7 °C. It’s an effect that cities around the world would like to mimic. As Earth’s climate changes over the coming decades, global warming will hit metropolitan areas especially hard because their buildings and pavements readily absorb sunlight and raise local temperatures, a phenomenon known as the urban heat island effect. Cities, as a result, stand a greater chance of extreme hot spells that can kill. “Heat-related deaths in the United States outpace — over the last 30 years — all other types of mortality from extreme weather causes,” says Kim Knowlton, a health scientist at Columbia University in New York. “This is not an issue that is going away.”
Some cities hope to stave off that sizzling future. Many are planting trees and building parks, but they have focused the most attention on rooftops — vast areas of unused space that absorb heat from the Sun. In 2009, Toronto, Canada, became the first city in North America to adopt a green-roof policy. It requires new buildings above a certain size to be topped with plants in the hope that they will retain storm water and keep temperatures down. Los Angeles, California, mandated in 2014 that new and renovated homes install ‘cool roofs’ made of light-coloured materials that reflect sunlight. A French law approved in March calls for the rooftops of new buildings in commercial zones to be partially covered in plants or solar panels.
But the rush to act is speeding ahead of the science. Although cool roofs and green roofs can strongly curb temperatures at the tops of buildings, they do not always yield benefits at the street level, and they may trigger unwanted effects, such as reducing rainfall in some places. “There was a notion that the community had reached a conclusion and there was a one-size-fits-all solution,” says Matei Georgescu, a sustainability scientist at Arizona State University in Tempe. “But that is not the case.”
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