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
are, on average, 1.5 degrees Celsius warmer than those in undeveloped areas
nearby. Likewise, a weather station in Vienna,
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
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
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.
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.”