Many man-made objects — buildings, parking lots and cars, to name a few — absorb the sun’s radiation much more effectively than plants, soil and rocks do, a trend that transforms cities into “urban heat islands.” This phenomenon has led some researchers to claim that urban sprawl has skewed weather data and is the true cause of global warming in recent decades. But a new study indicates that this idea is just a lot of hot air.

During the past 100 years, the global average temperature has risen about 0.74 degrees Celsius (SN: 2/10/07, p. 83). Some scientists have blamed some, if not all, of that warming on urban heat islands, says David Lister, a climate scientist at the University of East Anglia in Norwich, England.

For instance, previous studies indicate that conditions in central London are, on average, 1.5 degrees Celsius warmer than those in undeveloped areas nearby. Likewise, a weather station in Vienna, Austria, registers about 0.3 degrees C higher than the surrounding countryside. However, Lister and his colleagues report in an upcoming Journal of Geophysical Research (Atmospheres) that those cities have been well-developed for so long that temperatures in urban and rural areas fluctuate at roughly the same rate.

From 1981 to 2006, the temperature measured at a rooftop weather station in central London rose, on average, at a rate of 0.61 degrees C per decade. During the same period, the average temperature recorded at Heathrow Airport, a semi-urban site about 25 kilometers west of central London, rose just a bit more, about 0.65 degrees C per decade. Similarly, scientists at a rural station in Rothamsted, about 39 kilometers northwest of central London, tallied a temperature increase in the same range, about 0.67 degrees C per decade during that 26-year interval.

Meteorological data from Austria depict the same trend, the researchers note. At an urban weather station about 5 kilometers north of central Vienna, from 1961 to 2006 the average temperature rose at a rate of 0.36 degrees C per decade. Meanwhile, the temperature increase at a rural site 15 kilometers east of downtown Vienna wasn’t much different, about 0.33 degrees C per decade during the same period.

“Urban sites may well be warmer than rural sites, but the overall trends are the same,” says Thomas C. Peterson, a climatologist at the National Climatic Data Center in Asheville, N.C. “The new study “is excellent work … that doesn’t just look at trends or at average temperatures, it does both,” he says.

However, the new research indicates that the urban heat islands in rapidly developing regions in China aren’t fully mature. So, the long-term temperature trends measured there in recent years show both the effects of global warming and an influence of ongoing sprawl, says Lister.

To wit: The average temperature measured at Chinese weather stations that include the most rapidly developing portions of the country jumped about 0.25 degrees C per decade between 1951 and 2004. During the same period, sea surface temperatures just east of China — which the team used as a baseline, because eastern China has so few rural weather stations — rose only 0.1 degrees C per decade.

In other words, says Lister, about 60 percent of the warming trend observed at the Chinese stations is real, and about 40 percent of the warming can be attributed to the still-growing urban heat island. At some time in the future, when the nation’s development has stabilized, warming trends at the urban and rural weather stations should be approximately the same.

China has only recently begun to release meteorological data gathered at many of its weather stations, says David Parker, a climate scientist at the Met Office Hadley Centre in Exeter, England. That information is crucial for scientists, he adds: “The more data that we analyze, the more we can understand global trends in temperature.”

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