Rapid urbanization in southeastern China in the past 25 years is responsible for an estimated warming rate much larger than previous estimates for other periods and locations, according to a new study funded by NASA.
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 range.
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," Zhou adds.
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 models.
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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