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Debunking the Impact of City Heat on Global Warming

Debunking the Impact of City Heat on Global Warming

It's no surprise that cities get a lot hotter than rural areas. Metro areas have more roads and taller buildings than the countryside, the better to collect sunlight by day and prevent the release of warmth by night. These distinguishing elements can cause city temperatures to reach greater relative heights — a phenomenon known as the urban "heat island" effect. That's certainly been the case in Tokyo, where temperatures have risen much more quickly compared to surrounding rural areas of Japan over the years. Here's a look at Tokyo temps against the global average since 1900:

Some climate change skeptics have wondered if urban heat islands may be contaminating global warming data. Does the existence of hot urban islands like Tokyo make it look like the whole world is getting warmer? After all, cities appear to be overrepresented when it comes to temperature measuring stations; although urban regions make up just about 1 percent of the Earth's land, roughly 27 percent of collection sites are located in areas with a population above 50,000 people.

Well non-believers will have look elsewhere for support, because two new studies provide clear evidence that city heat has very little to do with the overall rise in the planet's temperatures. The first paper, set for publication in the Journal of Climate, comes from Stanford University researchers Mark Jacobson and John Ten Hoeve, who report that the urban heat island effect "may contribute to 2 to 4 percent of gross global warming." The rest comes from greenhouse gases (79 percent) and black carbon, or soot produced from burning fossil fuels (18 percent).

The researchers reached their conclusion after modeling climate changes over a period from 2005 to 2025 using a detailed, comprehensive simulation. "This study accounted not only for local impacts of the heat island effect, but also feedbacks of the effect to the global scale," Jacobson told the Stanford News Service. The pair also examined whether white roofs — a technique of painting city roofs white to reflect heat — truly serve to cool city temperatures. In fact, they found that white roofs provide little benefit for urban heating and may even cause a net warming effect.

The second paper, still in its preliminary phase and awaiting peer-review, compared temperature trends at thousands of rural and urban sites around the world. The study team found that the trends match well; the "opposite" of what one should expect "if the urban heat island effect was adding anomalous warming to the record." They conclude that "urban warming does not unduly bias estimates of recent global temperature change."

This second paper, in particular, may go a long way toward removing any lingering shred of climate change doubt. It was one of several studies conducted as part of the Berkeley Earth Surface Temperature project. Two of the researchers in the B.E.S.T. study group are noted critics of global warming literature, and the Koch brothers (oil company owners who have a history of supporting Republican causes) own Koch Industries, even donated a reported $150,000 to the project — presumably with the hopes it would bolster the skeptic position.

Figure via "Influence of Urban Heating on the Global Temperature Land Average Using Rural Sites Identified from MODIS Classifications."

Eric Jaffe is a contributing writer to The Atlantic Cities and the author of A Curious Madness (2014) and The King's Best Highway (2010). He lives in New York. All posts »

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