White or green roofs on buildings can help fight global warming, according to a new study. However, the effects of each design are dependent on where the building is located.
Heat from the sun is retained in urban areas due to dark materials, such as asphalt and tar. Buildings with green roofs, or covered in white paint, help minimize the effect, according to the study.
Many people in warm areas paint their roofs white, reducing heat absorbed by the house. Any roof designed to reflect light back from the Sun is considered a cool roof. In areas like Florida, however, cool roofs also have a negative effect on rainfall.
When the top of a building is totally or completely covered in vegetation, it is considered a "green roof." This design, full of plants, shrubs and bushes, is becoming more popular in the last few years. The City Hall in Chicago is now covered by a garden. Other cities are seeking solutions to rising temperatures in their metropolitan areas.
"[U]rban expansion across the United States imparts... a significant fraction of anticipated temperature increases resulting from greenhouse gas-induced warming... This warming is a significant fraction of the 21st century greenhouse gas-induced climate change simulated by global climate models," researchers wrote in the paper announcing the results of their study.
The research was led by Matei Georgescu, of Arizona State University. His team found that no one solution was ideal for all locations. While white roofs worked best to mitigate temperatures in Florida, cool roofs were better in the Central Valley of California. Local rainfall, energy demands and more, created challenges for each design.
Homes with white roofs in northern climates require more energy to heat during the winter, negating most energy savings gained during the summer. Although they do not keep temperatures as low in the summer as cool roofs, green roofs provide a degree of warming in colder months. Green roofs are, therefore, better suited to conditions in the north.
In Florida, cool roofs were found to reduce rainfall by almost 50 percent. Biologists are concerned that degree of change in precipitation could have wide-ranging effects on the local eco-system.
Georgescu wanted to look at different cooling systems, and see how they behaved in various climates.
"We simply wanted to get all of the technologies on a level playing field and draw out the issues associated with each one, across place and across time," the researcher said.
The authors of the study are urging city planners to be careful when choosing green or cool roofs for their urban areas.
