STANFORD (US) — The urban ‘heat island’ effect contributes less than 5 percent to overall global warming, far less than greenhouse gas or black carbon, new research shows.
The study also finds that if all the roofs in urban areas were painted white, warming would increase, not decrease, as previously believed.
“Between 2 and 4 percent of the gross global warming since the Industrial Revolution may be due to urban heat islands,” says Mark Z. Jacobson, a professor of civil and environmental engineering at Stanford University. He and graduate student John Ten Hoeve compare the figure to 79 percent from greenhouse gas and 18 percent from black carbon. The urban heat island refers to heat that emanates from city environments.
Black carbon is a component of the soot created by burning fossil fuels and biofuels and is highly efficient at absorbing sunlight, which heats the atmosphere.
Net global warming is gross global warming minus the cooling effect of light-colored atmospheric particles that reflect sunlight back into space, which offsets about half of global warming to date. Net, or observed, global warming is what is typically reported in the media.
Responding to skeptics
Some global warming skeptics have claimed that the urban heat island effect is so strong that it has skewed temperature measurements that show that global warming is happening. They have argued 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 says. “Greenhouse gases and particulate black carbon cause far more warming.”
Prior to Jacobson’s study, reported online in the Journal of Climate, claims about the importance of heat island to global warming could not be addressed directly. The few previous modeling studies by other researchers that had examined the effect of urban heat islands on regional scales did not calculate global impact.
Jacobson’s high-resolution study was the first study of the impact of urban heat islands on global sea-surface temperatures, sea ice, atmospheric stability, aerosol concentrations, gas concentrations, clouds, and precipitation. He characterized urban surfaces around the world at a resolution of 1 kilometer, making his simulation both extremely detailed and globally comprehensive.
“This study accounted not only for local impacts of the heat island effect, but also feedbacks of the effect to the global scale,” he says.
Although the study showed that urban heat islands are not major contributors to global warming, Jacobson says reducing the effect of heat islands is still important for slowing the rise of global temperatures.
The urban heat island effect is caused mostly by replacing soil and vegetation with paved roads, sidewalks, and buildings. Paving prevents evaporation of water from the soil and plant leaves. Since evaporation is a cooling process, reducing evaporation warms cities. Additionally, the darker colors of some roads and buildings absorb more sunlight, heating a city further.
One “geoengineering” proposal for reducing the impact of urban heat islands is to paint roofs worldwide a reflective white. 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.
Jacobson’s study did not examine one potential benefit of white roofs—a reduced demand for electricity to run air conditioning in hot weather. But a recent study done at the National Center for Atmospheric Research showed that the decrease in air conditioning use, which occurs mostly in the summer, might be more than offset by increases in heating during winter months.
“There does not seem to be a benefit from investing in white roofs,” says Jacobson. “The most important thing is to reduce emissions of the pollutants that contribute to global warming.”
Photovoltaic panels helpful
One way to reduce emissions while simultaneously reducing summer air conditioning demand is to install photovoltaic panels on roofs. Such panels not only generate electricity, reducing emissions of fossil fuels from electricity-producing power plants, but they also reduce sunlight absorbed by buildings because they convert sunlight to electricity.
Because photovoltaic 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, creating heat.
“Cooling your house with white roofs at the expense of warming the planet is not a very desirable trade-off,” Jacobson says. “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.”
Funding for the research was contributed by NASA and the U.S. Environmental Protection Agency.
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