The urbanization following China's rapid economic growth has impacted 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, causing the so-called Urban Heat Island effect.
The direct consequence of the urban heat island is an increase in the surface temperature in the local area, which has already had palpable influence over people's lives, according to an international research team.
The team, led by Zhou Liming from the School of Earth and Atmospheric Sciences, Georgia Institute of Technology in the United States, found urbanization in Southeast China has caused a temperature increase of 0.05 C per decade on average.
"Our estimated warming that is attributable to urbanization is much larger than previous estimates for other periods and locations," says Zhou.
The team's findings were made public recently in a paper published in the Proceedings of National Academy of Sciences.
He noted that China has experienced rapid urbanization and dramatic economic growth since its reform process started in late 1978, which many atmospheric scientists assume may have given rise to the urban heat island effect.
Zhou and his collaborators 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, where rapid urbanization has occurred over the past two decades.
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 says.
From 1978 to 2000, China's gross domestic product grew at an average annual rate of 9.5 per cent, compared with 2.5 per cent for developed countries and 5 percent for other developing countries.
The number of small towns soared from 2,176 to 20,312, nearly double that of the world average during this period; the number of cities increased from 190 to 663; and the urban population rose from 18 to 39 percent of the total, their paper revealed.
One of the major indicators they used to measure the impact of urbanization on climate is called Diurnal Temperature Range(DTR), that is, the daily maximum temperature minus the daily minimum. It has been generally believed that a decline in the DTR is the fingerprint of steady increase in temperature near the Earth surface, either due to urbanization or global warming.
To measure the variations of the DTR, Zhou and his collaborators 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 that consist of 194 well distributed observation stations, in the area where most of China's urbanization has occurred.
According to them, this region has the highest meteorological station density; the most uniform station distribution, the minimal non-climatic effects; and most consistent observation data in China.
They found a decline in the DTR at most stations, with the largest decrease in the eastern and southern coastal areas where rapid urbanization has occurred, suggesting a rise in near surface temperature.
The decrease of DTR is greatest in the Yangtze and Pearl River deltas and is generally larger 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, urban population, and so on.
"If urbanization is responsible for the reduction in the DTR, changes should be correlated with factors known to affect urbanization." Zhou says in the paper.
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 surface, which differs between urban and rural areas.
They found the greenness substantially decreased in eastern and southern provinces but increased over the important agricultural areas of northern and western provinces.
Researchers, however, also admitted some uncertainties remain in their estimates as the urban heat island may involve many non-urban factors such as clouds and changes in solar radiation.
Zhou said the work should be seen 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)