A Stanfoord University Professor of civil and environmental engineering has suggested that installing solar photo-voltaic panels on your house would be a better contribution to reducing global temperatures than the geo-engineering solution of painting vast numbers of urban roofs white to combat the urban heat island contribution to global warming. "There does not seem to be a benefit from investing in white roofs," said Professor Jacobson. "The most important thing is to reduce emissions of the pollutants that contribute to global warming."
Cities and urban areas release more heat to the atmosphere than rural areas - this is known as the Urban Heat Island effect. Climate skeptics have used this to argue that surface temperature data is contaminated by the location of data collection in urban areas. A new scientific study by Stanford University researchers has quantified the contribution of the heat islands on a global basis for the first time, showing that the contribution to global warming from urban heat islands is modest compared with what greenhouse gas emissions contribute.
Related: Skeptical Science: Does Urban Heat Island effect exaggerate global warming trends? | RealClimate: The Surface Temperature Record and the Urban Heat Island
"Between 2 and 4 percent of the gross global warming since the Industrial Revolution may be due to urban heat islands," said Mark Z. Jacobson, a professor of civil and environmental engineering who led the study. He and graduate student John Ten Hoeve compare this with the greenhouse gas contribution to gross warming of about 79 percent and the black carbon contribution of about 18 percent.
The study also modeled the geo-engineering theory of painting urban roofs and other surfaces white to increase the albedo or reflectivity to moderate local and global temperatures. This has been postulated widely as a temporary solution to combating global temperature increase, including by USA Energy Secretary Chu. Researchers at the Lawrence Berkeley National Laboratory lead by Hashem Akbari put forward the proposal initially in September 2008 of White Roofs Cooling the World, Offsetting CO2, and Delaying Global Warming. Professor Jacobson and Ten Hoeve contend in their study that the Akbari study "did not use a global model to simulate whether the conversion caused a net cooling or warming of global climate."
According to the white roof simulation results in the Jacobson study, model results "show that conversion to white roofs cooled population weighted ground and air temperatures over the simulation. However, feedbacks of the local
changes to the large scale resulted in a gross global warming, but smaller in magnitude than the UHI. Whereas, the population-weighted air temperature decrease due to white roofs was ~0.02 K, the global temperature increase was ~0.07 K."
Urban heat islands are caused by replacing soil and vegetation with paved roads, sidewalks and buildings. Paving and buildings reduce the amount of evapo-transpiration of water in urban areas. Since evaporation is a cooling process, reducing evaporation warms cities. In addition, darker colours of roads, buildings and paved surfaces absorb more sunlight, heating a city further. The effect is more prominent at night when atmospheric convection decreases, which can result in an inversion layer trapping heat in.
The study authors refute the climate skeptic arguments that the urban heat island effect is so strong that it has been skewing temperature measurements that show that global warming is happening, and that urban areas are a larger contributor to global warming than the greenhouse gases produced by human activity, and thus drastic measures to reduce greenhouse gases are not needed.
"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."
There are no previous studies that have examined the effect of urban heat islands on a global impact level, while several have addressed the effect on regional levels. Jacobson's high-resolution study used satellite data to examine the impact of urban heat islands on global sea-surface temperatures, sea ice, atmospheric stability, aerosol concentrations, gas concentrations, clouds and precipitation. The simulation was extremely detailed and globally comprehensive examining urban surfaces around the world at a resolution of one kilometer.
"This study accounted not only for local impacts of the heat island effect, but also feedbacks of the effect to the global scale," he said.
While the study showed that heat islands are not major contributors to global warming Jacobson said that reducing the effect of heat islands is still important for slowing the rise of global temperatures.
Jacobson's computer modeling concluded that white roofs did indeed cool urban surfaces. However, they 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. In Jacobson's modeling, the reduction in cloudiness allowed more sunlight to reach the surface. The increased sunlight reflected back into the atmosphere by white roofs in turn increased absorption of light by dark pollutants such as black carbon, which further increased heating of the atmosphere.
The study did not examine a potential benefit of white roofs in reducing air conditioning use during hot weather which the Akbari paper - Global cooling: increasing world-wide urban albedos to offset CO2 - addressed. But this slight advantage may be more than offset by increased heating required during winter, as indicated in a study done at the National Center for Atmospheric Research.
The Jacobson study stated that:
"White roofs reduce summer air conditioning energy demand and change surface albedo. A conversion of rooftops worldwide to white roofs, accounting for their albedo effect only, was calculated to cool population-weighted global temperatures by ~0.02 K but to warm the Earth overall by ~0.07 K. Local ground cooling stabilized surface air, reducing sensible and latent heat fluxes and local cloudiness, increasing local surface solar radiation, resulting in local cooling smaller in magnitude than without the cloud reduction. Higher reflection also increased air heating by black and brown carbon in soot. Feedbacks of local changes to the global scale were magnified over high-latitude snow and sea ice, causing a net but highly-uncertain warming effect on global climate. The local cooling due to white roofs may reduce or increase energy demand and thus other emissions as well, a factor not accounted for in these simulations. This feedback should be considered in any final assessment of the effects of white roofs on climate."
Solar panels reduce warming and emissions
While white roofs may not be of much use in reducing global warming, Jacobson did identify that installing photo-voltaic solar panels were of use as they reduce emissions of fossil fuels from greenhouse gas electricity-producing power plants, but they also reduce sunlight absorbed by buildings because they convert sunlight to electricity.
As the photo-voltaic panels do not reflect the sunlight back to the air, unlike white roofs, reflected light is not available to be absorbed again by pollutants in the air, thus preventing the creation of more atmospheric heat.
"Cooling your house with white roofs at the expense of warming the planet is not a very desirable trade-off," Jacobson said. "A warmer planet will melt the sea ice and glaciers faster, triggering feedbacks that will lead to even greater overall warming. There are more effective methods of reducing global warming." he said.
So, instead of painting your roof white, look at installing solar panels to turn that sunlight into electricity to combat increasing atmospheric temperatures and global warming.
- Adapted from Media release, Stanford University, October 18, 2011 - Urban 'heat island' effect is a small part of global warming; white roofs don't reduce it
- Mark Z. Jacobson, John E. Ten Hoeve - Effects of Urban Surfaces and White Roofs on Global and Regional Climate (PDF)
- Akbari, H., S. Menon, and A. Rosenfeld (2009) Global cooling: Increasing world-wide urban
albedos to offset CO2, Climatic Change, 94, 3-4, 275-286, doi:10.1007/s10584-008-9515-9. Full Paper (PDF) | See media release (PDF)
- Wikipedia - Urban heat island
- Image of Solar Panel by pixor / Flickr used under Creative Commons (CC BY-NC-SA 2.0)