Urban heat
Cities sizzle as more people move in
By Sid Perkins
In life, as in boxing, the combined effects of a one-two punch are often more devastating than either blow alone. Imagine, then, the devastation from a triple whammy that city dwellers might suffer this century as three unfavorable trends converge to afflict an already warming world.
First, there’s temperature. According to the Intergovernmental Panel on Climate Change, Earth’s average global temperature has risen about 0.74 degrees Celsius in the past century (SN: 2/10/07, p. 83), an increase almost certainly linked to the rising concentrations of carbon dioxide and other heat-trapping greenhouse gases that human activities have released into Earth’s atmosphere. IPCC scientists suggest this warming trend will continue, and indeed accelerate: In the next 20 years, average global temperature will rise another 0.4 degrees C or so, they estimate.
Then, there’s population growth. Now home to more than 6.6 billion people, Earth will see its population increase by about 25 percent, or 1.65 billion, by the year 2030 (SN: 10/13/07, p. 235). Even with substantial increases in energy efficiency, this dramatically larger population will likely trigger a rise in total energy consumption, further boosting emissions of greenhouse gases.
Third, there’s urbanization. Much of the world’s population growth in coming decades will occur within cities, where residents are exposed to warmer-than-average conditions due to the “urban heat island” effect. People living in large, well-developed areas typically experience temperatures several degrees warmer than do residents of the rural areas nearby.
The confluence of these trends could create a “perfect storm” that places urban dwellers at increased environmental risk, says Walt Dabberdt, president of the American Meteorological Society in 2008. Besides the risk from rising sea levels — much of the projected population increases will be in coastal cities — urban dwellers could be exposed to more-frequent heat waves, higher levels of pollution and a myriad of health concerns (SN: 7/3/04, p. 10).
Possibly of more importance, much of the population growth will result in urban sprawl in what are now smaller metropolitan areas — a trend that will lead to dramatically larger numbers of people living within urban heat islands.
Future changes in climate, as well as the effects of those changes on regional and local weather, must be an integral part of urban planning, Dabberdt notes. The way urban heat islands affect climate overall is also important: If more people move to places that require increasing amounts of air conditioning, even more greenhouse gases will be emitted.
“Cities are major contributors to anthropogenic climate change,” says Dabberdt, who is also the Boulder, Colo.–based chief science officer for Vaisala Corp., an international manufacturer of weather-monitoring systems and instruments. Overall, he says, urban areas are directly or indirectly responsible for about 80 percent of the emissions of planet-warming greenhouse gases such as carbon dioxide. While many of those emissions are produced in the cities by industrial activity or vehicles, others are generated in coal-fired power plants that are far from the cities but are still driven by urban demand for power.
The topic of urban heat occupied many researchers who gathered in January for the 2009 annual meeting of the American Meteorological Society, held appropriately enough in Phoenix, the site of many recent studies of the urban heat island effect. While some scientists are studying how to minimize the temperature-boosting effects of urban heat islands, others are studying how urban dwellers can substantially reduce the amount of greenhouse gases that are emitted in the first place.
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From farm to suburb