The urbanization accompanying China's rapid economic growth has affected climate on a scale much larger than people used to think, scientists say.
The rapid increase in the urban population, particularly in southeast China, has resulted in dramatic changes in land use and created the "urban heat island" effect. The direct consequence is an increase in surface temperatures in the local area, which has already had a palpable impact on people's lives.
An international research team led by Zhou Liming from the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology in the US, found urbanization in southeast China has caused an average temperature increase of 0.05 C per decade.
"Our estimate for warming that is attributable to urbanization is much larger than previous estimates for other periods and locations," said Zhou.
The team's findings were made public recently in a paper published in the Proceedings of the National Academy of Sciences.
Many atmospheric scientists believe that China's rapid urbanization and dramatic economic growth since late 1978 may have given rise to the urban heat island effect.
Zhou and his colleagues in China for the first time presented evidence of the effect on climate, based on analysis of the impact of land-use changes on surface temperature in southeast China.
Their estimate of the temperature variations is consistent with the changes in the percentage of the urban population and in satellite-measured greenness, both characteristic of the urbanization process, Zhou said.
From 1978 to 2000, China's gross domestic product grew at an average annual rate of 9.5 percent, compared with 2.5 percent for developed countries and 5.0 percent for other developing countries.
The number of small towns soared from 2,176 to 20,312, nearly double the world average increase during this period; the number of cities jumped from 190 to 663 and the urban population rose from 18 to 39 percent of the total.
One of the major indicators the team used to measure the impact of urbanization on climate was the Diurnal Temperature Range (DTR), calculated by subtracting the daily minimum temperature from the daily maximum. It is generally believed that a decline in the DTR is the fingerprint of steady increase in temperature near the earth's surface, resulting either from urbanization or global warming.
To measure the variations in DTR, Zhou and his team used observational data of monthly daily maximum and minimum land surface air temperatures at 671 meteorological stations across China from January 1979 to December 1998, collected and processed by the National Meteorological Center of the China Meteorological Administration.
They then focused their study on 13 provinces and cities in southeast China, the area where most urbanization has occurred.
The selected area contains 194 observation stations. The team believes the region has the highest meteorological station density, the most uniform station distribution, the fewest non-climatic effects and most consistent observation data in China.
They found declines in the DTR at most stations, with the largest appearing in the eastern and southern coastal areas where rapid urbanization has occurred.
The decrease of DTR was greatest in the Yangtze and Pearl River deltas and generally more pronounced at coastal stations.
They also found the DTR change is generally consistent with several indicators of urbanization, such as the number of towns and cities and urban population.
"If urbanization is responsible for the reduction in the DTR, changes should be correlated with factors known to affect urbanization." Zhou wrote.
They used data from China's fourth census in 1990 and the fifth in 2000 to measure the changes, confirming a significant correlation between the DTR change and the urban population distribution.
Another indicator of the impact of urbanization is the satellite-measured greenness of the earth's surface, which differs between urban and rural areas.
The team found that greenness substantially decreased in the eastern and southern provinces but increased over the important agricultural areas in the north and west.
The researchers warn that some uncertainties remain in their findings, as the urban heat island may involve many non-urban factors such as clouds and changes in solar radiation.
Zhou said their work should be viewed as the first step toward a more accurate assessment.
"Our results should be interpreted as illustrative rather than definitive," he says, "we need to better characterize the system with observations and better describe and model the complex processes involved."
(China Daily July 2, 2004)
