For Immediate Release
Researchers led by the Georgia Institute of Technology report that the mean surface temperature in the region has risen 0.09 degrees Fahrenheit (0.05 degrees Celsius) per decade since 1979. Also, nighttime low temperatures have risen much faster than the daytime high temperatures. The average reduction of the day-to-night temperature range was 0.24 degrees F (0.132 degrees C) per decade. Their findings will appear in the June 29 print edition of the journal Proceedings of the National Academy of Sciences.
To estimate the temperature changes due to urbanization, researchers
used a new approach that integrated meteorological station observations,
model-assimilated temperature predictions, satellite-measured greenness
and China's census data. The modeling data - provided by the National
Oceanic and Atmospheric Administration's Centers for Environmental
Prediction and the U.S. Department of Energy
- is considered more accurate than previous information because of its
improvements in accounting for temperature range differences affected
by cloud cover and soil moisture, the researchers note.
"These results are further evidence of the human impact on climate,"
says lead author Liming
Zhou, a Georgia Tech researcher working with Professor Robert
Dickinson, a global climate modeler in the Georgia Tech School
of Earth and Atmospheric Sciences.
Carbon dioxide from industrial and automobile emissions has been suspected
to be the primary force in global warming. Scientists have attributed
a 0.9 degrees F (0.5 degrees C) increase in global temperature in the
20th century to a significant atmospheric increase of greenhouse gases,
including carbon dioxide. They predict this increase will continue through
the 21st century and cause continued increases in extreme weather, rising
sea levels, and the retreat of glaciers and polar ice caps.
"Human-induced changes in land use - such as urbanization, deforestation, and agricultural and irrigation practices - can affect local and regional climate and even large-scale atmospheric circulations," Zhou explains. "They may have changed climate as much as greenhouse gases over some particular regions of land."
It is not yet possible to establish the extent to which these temperature
changes affect climate on a larger scale, Zhou adds. More research must
be done to make this determination because it's a challenge to differentiate
the impact of land use changes on climate from that of industrial emissions
because both tend to warm the earth and decrease the day-to-night temperature
Most scientists agree that land use changes from urbanization create
an urban heat island (UHI) that is partially responsible for the observed
warming over land during the past few decades. The cities' predominance
of buildings, roads and paved surfaces with little vegetation largely
explains the UHI effect.
"In this study, we focused on the climate effect of urbanization
in China because it is a good case study at the maximum end of the UHI
spectrum," says Zhou, who grew up in western China and worked as
a weather forecaster at China's National Meteorological Center for four
years before earning a doctoral degree at Boston University in 2002.
China has experienced rapid urbanization and dramatic economic growth
since its reform process started in late 1978. Its temperature change
attributed to UHI is larger than the estimated 0.11 to 0.49 degrees F
(0.06 to 0.27 degrees C) in the United States during the 20th century.
Because this study's analysis is from a country with a much higher population
density and focused on a period of rapid urbanization, the researchers
expected their results to give higher values than those estimated in other
locations and over longer periods, the authors note in their journal article.
In addition to Zhou and Dickinson, the authors include Yuhong Tian of
Georgia Tech, Jingyun Fang of Peking University, Qingxiang Li of the China
Meteorological Administration, Robert Kaufmann and Ranga Myneni of Boston
University, and Compton Tucker of NASA.
Also, the researchers caution that their estimates do not represent the
urbanization effect globally, nor should they be interpreted as a denial
of global warming, Zhou says.
"The UHI effect is responsible for real climatic change in urban
areas, but it may not be representative of large areas," he explains.
"Although significant in magnitude, our estimated UHI is still relatively
small compared to the background temperature trends documented in the
Chinese long-term climate record.
"However, considering its intensity and spatial extent, combined
with other urban-related land use changes and increased urban pollutants,
urbanization in China may have affected climate far beyond urban areas,"
Scientists are already giving more attention to the UHI effect and other
land use changes, such as deforestation. Global climate modelers, such
as Georgia Tech's Dickinson, are working to accurately account for these
effects, as well as the impact of aerosols and soot, in their predictive
Dickinson, who is president of the American Geophysical Union, says: "The identification of global warming in the observational records of global temperature patterns is statistically well established through numerous detailed studies. However, to assess the total, current risks of climate change for human welfare, studies such as this are suggesting the necessity to add to this global warming signal, that resulting from urbanization and other land use changes."
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