Thursday, November 3, 2011

White roofs do little to stop global warming, solar panels do more, says Stanford's Mark Jacobsen

A couple years ago an idea was circulating that just painting your roof white or even silver colored would stop the heat island effect.  The urban heat island is the observation that urban areas, with lots of parking lots and buildings and very few trees and green areas, have a build-up of heat.  Go a little way out of town and temperatures cool off because the vegetation and shade and whatnot naturally acts to keep temperatures cooler.  The thinking goes that many things in urban areas, like Asphalt, are dark colored and therefore high school physics says it'll absorb heat.  The solution seemed to be that simply painting things white would cut the temperature and resolve the heat island.

Cities release more heat to the atmosphere than the rural vegetated areas around them, but how much influence these urban "heat islands" have on global warming has been a matter of debate. Now a study by Stanford researchers has quantified the contribution of the heat islands for the first time, showing that it is modest compared with what greenhouse gases contribute to global warming.  The team included Mark Jacobsen who's quoted saying "Between 2 and 4 percent of the gross global warming since the Industrial Revolution may be due to urban heat islands."

Some global warming skeptics have claimed that the urban heat island effect is so strong that it has been skewing temperature measurements that show that global warming is happening.  "This study shows that the urban heat island effect is a relatively minor contributor to warming, contrary to what climate skeptics have claimed," Jacobson said. "Greenhouse gases and particulate black carbon cause far more warming."

Although his study showed that urban heat islands are not major contributors to global warming, Jacobson said reducing the effect of heat islands is still important for slowing the rise of global temperatures.

The "paint roofs white" idea was described as "geoengineering" and found that white roofs did indeed cool urban surfaces, but that they also caused a net global warming, largely because they reduced cloudiness slightly by increasing the stability of the air, thereby reducing the vertical transport of moisture and energy to clouds.

Another idea that has been circulating is that because solar panels are dark color that they'd contribute to the heat island, and that perhaps we shouldn't use solar panels.  Jacobsens study can be taken to refute this idea, and give us a green light to using solar panels.  First, solar panels generate electricity hence offsetting electricity generated from fossil fuel powered plants.  Second, they reduce sunlight absorbed by buildings (assuming the panels are installed on the building) by shielding the building while being installed on the building.  Third, solar panels do not reflect the sunlight back into the air, meaning that light won't be re-absorbed by the atmosphere pollutants.

Jacobson is the director of Stanford's Atmosphere/Energy Program and a senior fellow at Stanford's Woods Institute for the Environment and the Precourt Institute for Energy. Graduate student John Ten Hoeve contributed to the research and is coauthor of the paper. Funding for the research was contributed by NASA and the U.S. Environmental Protection Agency.

Urban 'heat island' effect is only a small contributor to global warming, and white roofs don't help to solve the problem, say Stanford researchers