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)