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22.09.2014 admin
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. 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. Gross global warming is the total amount of warming that has taken place from all sources, mainly greenhouse gases, black carbon particles and heat islands. Jacobson and Ten Hoeve are authors of a paper describing the research that will be published in Journal of Climate. 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.
Prior to Jacobson's study, claims about the importance of heat island to global warming could not be addressed directly. 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 urban heat island effect is caused mostly by replacing soil and vegetation with paved roads, sidewalks and buildings.


One "geoengineering" proposal for reducing the impact of urban heat islands is to paint roofs worldwide a reflective white. 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. One way to reduce emissions while simultaneously reducing summer air conditioning demand is to install photovoltaic panels on roofs.
They also concluded that if all the roofs in urban areas were painted white, it would increase, not decrease, global warming.
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 few previous modeling studies by other researchers that had examined the effect of urban heat islands on regional scales did not calculate global impacts. 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.


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.
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. 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.
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. Additionally, the darker colors of some roads and buildings absorb more sunlight, heating a city further.
In Jacobson's modeling, the reduction in cloudiness allowed more sunlight to reach the surface.



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