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Wednesday, April 23rd, 2014

Scientists Confirm Urban Heat Islands' Far Reach, While NYC Tries To Keep Cool

By Nicolas Rochon


Aerial view of New York City, 2006.

Densely populated metropolises of coastal Eurasia and North America have the potential to affect weather at a great distance, according to a Nature Climate Change publication released last week.

Scientists at the University of California-San Diego, using computer-generated climate modelling, identified 86 major cities in the Northern Hemisphere, representing “urban heat islands” (UHI), that generated enough heat to alter local jet streams during the winter months, potentially harvesting implications on a global scale.

First discussed in the early 19th century by British chemist and meteorologist, Luke Howard, the UHI effect describes the difference in surface air temperature of urban areas compared to the surrounding rural area. More specifically, the black pavement, sizeable buildings and traffic associated with a major city—in replace of open vegetated and soil surfaces of rural areas—collectively act to retain thermal heat quite efficiently.

Quoted by NASA scientist Ping Zhang, the degree to which an urban area retains this heat depends on multiple factors: “the surrounding ecological context… the size of the city, [considering] both the area and population size of the city… and then will be the shape of the city, and [its] development patterns.”

Much of this heat retained by urban materials stems from the waste heat released by the cities’ own energy consumption. Last week’s report considered the global energy consumption of 2006, which reached 16 terawatts (one terawatt is 1 trillion watts). Most notably, close to 7 terawatts were consumed in major cities throughout the Northern Hemisphere.

The study’s design and results

At UC-San Diego, scientists Guang Zhang, Ming Cai and Aixue Hu ran five 100-year global climate models (GCMs) based on historical climatological forcings between 1981-2000 to determine the potential climate impacts from non-renewable energy consumption of major cities, as stated in their journal publication. The results from this experiment provided an explanation for the difference between the actual observed warming and the expected warming produced by computer models during the winter months.

When you consider major cities consuming a minimum 0.4 W m² of energy, the subsequent surface air temperature can rise by 1° C. The global effect, then, stems from the fact that many of these major, coastal cities lay beneath atmospheric circulative cells that transfer warm and cool air polewards across the earth’s surface.

“What we found is that energy use from multiple urban areas collectively can warm the atmosphere remotely, thousands of miles away from the energy consumption regions… This is accomplished through atmospheric circulation change,” states Guang Zhang, lead researcher at the Scripps Institution of Oceanography (UC-San Diego).

The energy consumed in major cities “represents a direct external energy source for the climate system because fossil-fuel burning releases energy sequestered millions of years ago,” the report explains. And while this energy source contributes only a small percentage of the energy transported by natural mechanisms, it is highly concentrated in major cities. Clearly, this phenomenon poses global implications as society continues to urbanize the planet.

Increased urbanization and UHI effects

The United Nations’ 2011 World Urbanization Prospects report cited 3.6 billion people living in urban environments, translating to 52% of the world’s population. This number, they predict, will double by 2050. With more people living in urban environments, resulting in an increase in localized energy consumption, it is plausible that UHI effects will become more noticeable in the foreseeable future.

Cynthia Rosenzweig, a NASA Goddard Institute for Space Studies scientist, believes that increased climate change will inevitably increase the UHI effect. She writes, “right now, we average about 14 days each summer above 90 degrees (Fahrenheit) in New York. In a couple of decades, we could be experiencing 30 days or more.” In order for future generations to elude an interminably warming planet, the scientific community pursues new and innovative practices and technology to combat the issue.

A potential solution?

During a summer heat wave in 2011, all New York City buildings held 170° F surface temperatures, except one. With a recorded surface temperature of 128° F, one lone building in the middle of the city used a white roof cover to dissipate the sun’s rays. As as initiative of Mayor Michael Bloomberg’s plan to reduce greenhouse gas emissions, NYC° CoolRoofs works to convert dark, heat-absorbing rooftops to a simple white membrane, shedding light to help cut energy costs and greenhouse gas emissions simultaneously.

Since its launch in 2012, NYC° CoolRoofs has covered 2.5 million square feet of rooftop for a total of 288 New York City buildings, while engaging almost 3,000 volunteers. According to the program, the materials cost about $0.30 per square foot on average; however, NYC° CoolRoofs sees that investment paying for itself in three years.

At Columbia University, research scientist Stuart Gaffin published a paper detailing white roofs as “an ambitious effort with real potential to lower city temperatures and energy bills… from a climate and urban heat island standpoint, it makes a lot of sense to install bright, white roofs. That’s why we say, ‘Bright is the new black.’”

And while science agrees on the capabilities of white roofs to reduce energy costs and greenhouse gas emissions, some cite decreased precipitation as cause for concern.

A research team at Arizona State University warn that applying white surfaces to city roofs could affect hydroclimatology. Matei Georgescu, who works as an assistant professor in ASU’s School of Geographical Sciences and Urban Planning, says that while "raising the reflectivity of buildings by painting their roofs white is an effective way of reducing higher average temperatures caused by urban expansion… increased reflectivity also modifies hydroclimatic processes and… can lead to a significant reduction of rainfall.”

With increased climate change and urbanization, the UHI effect will continue to challenge human development, making it critical for society to develop powerful adaptation and mitigation strategies to prepare for an unforeseeable future.