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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.
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. On top of that, it is unclear whether the limited programmes currently in place will have a measurable effect on temperature—and citizen health—and whether cities will expand their efforts enough to produce results. “If you're just putting green roofs on city hall and schools, it's not going to move the needle,” says Brian Stone Jr, an urban scientist at the Georgia Institute of Technology in Atlanta.
Despite all of the heat-related risks that cities face in the future, few have put heat-management plans in place. Louisville in Kentucky is one: it will soon become the first major US city to develop an urban heat-adaptation plan, says Stone, who is leading the project. The effort is driven by necessity. Louisville has the fastest warming urban heat island in the United States, and temperatures there have climbed by more than 4 °C since 1961. Part of the problem is that the city has lost 54,000 trees per year to insects, ice storms and lack of care.
Stone is now collecting the baseline data that most cities lacked before embracing cooling steps. He is travelling around Louisville measuring tree cover, finding hot spots and identifying areas with vulnerable residents. The next step is to create a blueprint that combines cool roofs, green roofs, tree plantings and cool paving materials that could change the fate of the city's most at-risk residents. Stone is starting with modest but realistic assumptions in his modelling: the conversion of just 100 buildings to green roofs, for example. At the same time, the city hopes to increase its number of trees.
