Cool roofs can offset carbon dioxide emissions

Can light-coloured rooftops and roads really curb carbon emissions and combat global climate change? The idea has been around for years. A new study by researchers at Lawrence Berkeley National Laboratory - the first to use a global model to study the question - has found that implementing cool roofs and cool pavements in cities around the world can not only help cities stay cooler, they can also cool the world, with the potential of cancelling the heating effect of up to two years of worldwide carbon dioxide emissions.

Because white roofs reflect far more of the sun’s heat than black ones, buildings with white roofs will stay cooler. If the building is air conditioned, less air conditioning will be required, thus saving energy. Even if there is no air conditioning, the heat absorbed by a black roof both heats the space below, making the space less comfortable, and is also carried into the city air by wind - raising the ambient temperature in what is known as the urban heat island effect. Additionally, there’s a third, less familiar way in which a black roof heats the world: it radiates energy directly into the atmosphere, which is then absorbed by the nearest clouds and ends up trapped by the greenhouse effect, contributing to global warming.

US Energy Secretary Steven Chu has announced a series of initiatives at the Department of Energy to more broadly implement cool roof technologies on DOE facilities and buildings across the federal government. As part of the effort to make the federal government more energy efficient, Chu has directed all DOE offices to install cool roofs, whenever cost effective over the lifetime of the roof, when constructing new roofs or replacing old ones at DOE facilities. Additionally, the Secretary has also issued a letter to the heads of other federal agencies, encouraging them to take similar steps at their facilities.

“Cool roofs are one of the quickest and lowest cost ways we can reduce our global carbon emissions and begin the hard work of slowing climate change,” said Chu. “By demonstrating the benefits of cool roofs on our facilities, the federal government can lead the nation toward more sustainable building practices, while reducing the federal carbon footprint and saving money for taxpayers.”

In the latest study, the Berkeley Lab researchers and their collaborators used a detailed global land surface model from NASA Goddard Space Flight Center, which contained regional information on surface variables, such as topography, evaporation, radiation and temperature, as well as on cloud cover. For the Northern Hemisphere summer, they found that increasing the reflectivity of roof and pavement materials in cities with a population greater than 1 million would achieve a one-time offset of 57 gigatons (1 gigaton equals 1 billion metric tons) of CO₂ emissions (31 Gt from roofs and 26 Gt from pavements). That’s double the worldwide CO₂ emissions in 2006 of 28 gigatons. Their results were published online in the journal Environmental Research Letters.

“These offsets help delay warming that would otherwise take place if actual CO₂ emissions are not reduced,” says Surabi Menon, Staff Scientist at Berkeley Lab and lead author of the paper.

Co-author Hashem Akbari emphasises that cool roofs and pavements are only a part of the solution: “Two years' worth of emissions is huge, but compared to what we need to do, it’s just a dent in the problem,” says Akbari, the former head of the Berkeley Lab Heat Island Group and now Hydro-Quebec Industrial Research Professor at Concordia University in Montreal. “We’ve been dumping CO₂ into the atmosphere for the last 200 years as if there’s no future.”

This study is a follow-up to a 2008 paper published in the journal Climate Change, which calculated the CO₂ offset from cool surfaces by using a simplified model that assumed a global average for cloud cover. The earlier paper, co-authored by Akbari, Menon and Art Rosenfeld, a Berkeley Lab physicist who was then a member of the California Energy Commission, found that implementing cool roofs and pavements worldwide could offset 44 gigatons of CO₂ (24 Gt from roofs and 20 Gt from pavements).