Forecast for the Future's Cities: Hot!
Sometimes a giant ice block can help make hot days more manageable. But permeable surfaces - like that pictured above - can also help cool us down. Photo via Houston Marsh.

Sometimes a giant ice block is all you need to make a hot day cool. But permeable surfaces - like that pictured above - can also help mitigate rising temperatures in the world's cities. Photo via Houston Marsh.

On the hottest days of summer, AC-less city-folk can be found sticking their heads in refrigerators and sinks full of ice water, putting their faces in front of fans — or open fire hydrants — and guzzling ice cold beverages — even iced coffee!

They may not know it, but part of the problem is that they live on a “heat island.”


When the temperature rises, city-dwellers suffer more than people in rural areas. This is because of the urban heat island (UHI) effect , which keeps cities’ average temperatures higher: 1.8–5.4 degrees Fahrenheit higher during the day, and up to 22 degrees Fahrenheit higher at night, according to the U.S. Environmental Protection Agency (EPA).

Cities are heat islands in part because buildings block breezes, and are made of materials that store heat and emit it at night. Similarly, heat-retaining materials like concrete and steel are laid over previously permeable, moist surfaces as cities develop. So instead of going into plants or soil, heat is absorbed by concrete or asphalt and re-radiated into the atmosphere at night.

These built environment factors keep cities sizzling-hot in the summertime.


But, as we are witnessing again this summer, the UHI phenomenon can be deadly: according to the Centers for Disease Control (CDC), between 1979 and 2003, excessive heat caused 8,015 deaths in the United States. The extra demand for energy can cause blackouts, which makes the situation even more dangerous.

The heat is deadlier than the cold. The U.S. Global Change Research Program (quoted here) reports that cold snaps push death rates up by 1.6 percent, compared with a 5.7 percent increase during heat waves.

Exacerbating the situation, heat islands keep growing. The world’s population has become predominantly urban, and according to the United Nations Population Division, the urban population is expected to nearly double in the next forty years. This means that by 2050, approximately 6.3 billion people will live in cities — more than 68 percent of the projected population. As ClimateWire reported recently, in certain parts of the Middle East, central Asia and western Africa — regions that are more sensitive to UHI because of their geography — the population is expected to triple, in certain cases:

World Urbanization Prospects, from the United Nations Population Division.

World Urbanization Prospects, from the United Nations Population Division.

Adding urgency to the situation, new research suggests that cities are more sensitive to climate change and will experience proportionately higher increases in average temperatures as atmospheric carbon dioxide increases.


A recent report by Mark McCarthy, Richard Betts, and Martin Best of the Climate Impacts team at the U.K.’s weather service office,  the Met, shows how cities are warming faster than rural areas. Their models predict that urban daytime temperatures around the world will rise by more than 5 degrees Fahrenheit when atmospheric carbon dioxide levels reach 645 parts per million, which could happen as early as 2050. And in regions more sensitive to UHI, the nighttime temperature could rise by an additional 5 degrees.

To put these predictions into perspective, the Met warned that this means a five-fold increase in the number of dangerously hot nights over the next thirty years.

Some of the world’s most vulnerable —  including the elderly, pregnant women, children, poor people and people working outdoors — are sure to be hit the hardest by these temperature increases.

So are cities working to mitigate the UHI effect?


Stuart Gaffin, an associate research scientist with Columbia University’s Center for Climate Systems Research, told the New York Times recently that urban areas are notoriously difficult for climate change modelers, but that fighting urban heat islands “is a proxy for fighting climate change.”

Gaffin is working with New York City Mayor Michael Bloomberg on developing UHI mitigation projects, including the NYC Cool Roofs Program, which even has a Do It Yourself Cool Roof Kit.

Cool roofs are painted with a reflective white coating, which reflects sunlight and also has high emissivity, which means that it releases heat more easily than conventional materials. According to the EPA, cool roofs stay up to 60 degrees cooler than conventional roofs during peak summer weather. They thus reduce energy use – and in turn, greenhouse gas emissions –  and cool buildings by around 10 to 20 degrees Fahrenheit.

A green roof at Con Edison Training Center in Long Island City, Queens, keeps heat in the building in the winter and keeps heat out in the summer. Photo via

A green roof at Con Edison Training Center in Long Island City, Queens is covered in 21,000 plants, which keep heat in during the winter and keep heat out during the summer. Photo via

New York City is not alone. Gaffin says cities around the world are beginning to realize the importance of what he calls “sunlight management.”  (In April we wrote about Abu Dhabi’s efforts to combat UHIThis Indian company produces reflective white coating for cool roofs, and Chicago’s green alleys use permeable pavement technology.)

Other methods for managing sunlight include green roofs — which are covered with sunlight-absorbing plants, like this one at Con Edison — and cool pavements, which reflect more solar energy, enhance water evaporation, or otherwise stay cooler than conventional pavements.

And of course,  one of the best ways to fight the UHI effect is also the simplest solution: planting trees and vegetation is a sure way to cool cities, since trees and vegetation provide shade and evapotranspiration.

Cool roof coating reflects sunlight away from buildings and also has high infrared emissivity, which means that it doesn't store as much heat as conventional roofing materials. Photo via

Cool roof coating reflects sunlight away from buildings and also has high infrared emissivity, which means that it doesn't store as much heat as conventional roofing materials. Photo via

Needless to say, all of these cooling methods have additional benefits, which include enhanced air and water quality, improved storm water management (a main goal of Chicago’s cooling permeable pavement), lower maintenance costs and lower noise pollution.


The EPA’s Urban Heat Island Community Action Database allows you to search UHI mitigation efforts by location (Washington, D.C. is listed for increases in trees and vegetation with its Plan to Improve Air Quality), by initiative type (e.g. ordinance, building code, outreach program), and by strategy (e.g. trees and vegetation, cool roofs, green roofs, and cool pavements). They also include a cool, clickable map that allows you to click on any area of the United States to see what’s being done to cool cities down and mitigate climate change.

Right Menu Icon