A new study published in the Journal of Climate claims that painting rooftops white—a method championed by energy secretary Steven Chu and others to combat climate change—only minimally reduces local cooling, and actually causes a slight increase in overall global warming. How the Heck:
The researchers used a global climate model called GATOR-GCMOM [PDF], which incorporates a host of data from satellites and weather stations worldwide. It models how relationships between various environmental conditions, like the presence of clouds or pollutants, will affect local and global climate.
The model found that more white roofs means less surface heat in cities (which is obvious enough to anyone who's sat in a car with a black interior in the sun). Lower local temperature means less water evaporates and rises up to eventually form clouds, says lead author and Stanford University researcher Mark Jacobson. The decrease in clouds allows more sunlight to reach the Earth's surface, leading to higher temperatures overall.
The model also predicts that much of the light reflected by rooftops will eventually be absorbed by dark carbon soot and particulates that are especially prevalent in the air above urban areas. This could limit local cooling and cause warming elsewhere as the particles drift away.
Not so Fast:
One possible benefit to white roofs is the reduction of cooling costs for the buildings painted white, which isn't explicitly addressed in this paper. This could make the practice useful in warmer climates, but at least one study has found that a switch to white roofs wouldn't lead to energy savings on a global scale.
The study didn't calculate how the change would impact energy use, or how such a change could impact emissions and their effect on climate.
Even with switch to 100% white roofs, the predicted increase in global temperature (0.13 F over 20 years) is quite small, and dwarfed by expected effects of greenhouse gases and carbon soot.
Data generated by this model are preliminary, as with any computer simulation of a system as vastly complicated as Earth's climate, and the matter is far from settled. Other researchers stand by their own calculations that white roofs can provide energy savings in a variety of climates and reduce heating of the Earth's surface.
To Paint or Not to Paint:
Jacobson speculates that any energy savings in white-roofed buildings would be eaten up by increased energy use elsewhere (i.e., for cooling) from overall warming caused by white roofs. So it's probably not a great measure for widespread use, he says. But in a warm, sunny climate, a white roof almost certainly doesn't hurt on an individual basis and may help reduce the need for air-conditioning (as inhabitants of sultry climes have known for a long time).
Conversion to white roofs is a bad idea globally, Jacobson says. Instead, if you want to make a difference, install a photovoltaic system or solar panels on your roof, which reflects light and also generates clean electricity.
Reference: Mark Z. Jacobson and John E. Ten Hoeve. Effects of Urban Surfaces and White Roofs on Global and Regional Climate. Journal of Climate. 2011. DOI: 10.1175/JCLI-D-11-00032.1
Image: Christopher Dick / Flickr