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The “urban heat island effect” is the effect that cities tend to be hotter and release more heat to the atmosphere than do the surrounding natural areas. As urban areas develop, changes occur in their landscape. Buildings, roads, and other infrastructure replace open land and vegetation. Surfaces that were once permeable and moist become impermeable and dry. These changes cause urban regions to become warmer than their rural surroundings, forming an “island” of higher temperatures in the landscape.

Hot hot hot

While the reality of this effect is not in question, climate “skeptics” have said that this effect is what is really causing global warming, not the pollution of the atmosphere by the equivalent of 8 gulf oil spills per day worth of carbon dioxide. Never mind the fact that cities only make up a fraction of one percent of the Earth’s surface, many opponents to anthropogenic global warming (AGW) want to deny the carbon dioxide hypothesis. They claim that the urban heat island effect is the main driver of AGW, therefore emissions do not have to be reduced or regulated.

The Study

To the detriment of climate deniers, a new 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.

One of the authors of the study, Mark Z. Jacobson, a professor of civil and environmental engineering, quantified the urban heat island effect:

Between 2 and 4 percent of the gross global warming since the Industrial Revolution may be due to urban heat islands.

Compare this with the greenhouse gas contribution to gross warming of about 79 percent and the black carbon contribution of about 18 percent, the authors estimated. Black carbon is a component of the soot created by burning fossil fuels and biofuels and is highly efficient at absorbing sunlight.

As to the veracity of the study:

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 one kilometer, making his simulation both extremely detailed and globally comprehensive.

This indicates, opposed to denier’s claims, a staggering 97 percent of contributions to global warming are based on carbon emissions, dwarfing any urban heat island effect. As before stated, this makes sense from a quantity standpoint: we pour orders of magnitude more greenhouse gas and carbon into the atmosphere than do cities produce additional heat. To make up this difference and prove deniers correct, the percentage of cities covering the Earth would have to explode exponentially.

One more time quoting Jacobson:

The most important thing [to mitigate climate change] is to reduce emissions of the pollutants that contribute to global warming.

The new journal article has been released in the Journal of Climate.

Conclusions

We are continuously finding that carbon emissions from human beings since the industrial revolution, and not some confounding variable, are the root cause of global warming. If we can get beyond all the rhetoric, the politics, and the misdirection, we can begin to solve this problem.

Sources:

If you are feeling technical, you can find the study here.

If you aren’t, you can read the press release here.