Urban heat islands – city areas that are warmer than surrounding countryside – could adversely influence climate in certain regions of the world, especially in countries like China where the economy is growing fast and cities are getting bigger. This critical but often ignored aspect of regional climate change should be taken into account when developing new models, say researchers in Beijing.
The amount of green vegetation cover in a region directly influences local climate by affecting how water and heat are exchanged between the land and atmosphere. However, understanding such interactions – especially in humid, subtropical areas – remains a challenge.
Now, Gensuo Jia of the Institute of Atmospheric Physics at the Chinese Academy of Sciences and colleagues have analysed how changing vegetation cover in urban and rural areas in the Pearl River Delta region of south-east China affects local climate. The region has seen rapid economic development in recent years, which has dramatically altered the landscape; formerly green, lush areas are now covered over with concrete. These changes can be clearly identified with the Terra moderate resolution imaging spectrometer (MODIS).
MODIS, like other such weather satellites, measures the greenness of a landscape using light-reflection data. Green plants reflect light in the near infrared wavelengths of the electromagnetic spectrum but they absorb red wavelengths for photosynthesis. Using near-infrared and red-wavelength data, scientists can calculate a "normalized difference vegetation index" (NDVI) – the bigger the index, the greener the landscape.
Jia's team examined the change in area of urban, wild land and urban–rural fringes from 2000–2008. The results showed that more photosynthesis occurs in the wild lands and stable urban landscapes than the urban–rural fringe areas, where cities are now expanding and there is less and less greenery.
The researchers also found that increased temperatures from climate warming speed up vegetation growth more than increased rainfall does. This is to be expected in tropical areas, such as the region studied, where conditions are already humid.
"Vegetation is an important driving factor in local climate dynamics," says team member Yonghong Hu of the Institute of Atmospheric Physics. "It influences how much solar radiation is absorbed, surface and air temperatures, and the amount of water evaporating from land."
Replacing green areas with concrete leads to less water evaporating from the surface, and so less water in the air. Concrete also absorbs more radiation from the Sun, heating up the land surface and near-surface air. This hot air rises, which, in turn makes it harder for precipitation to form. The overall result is dryer air in the entire region.
Climate models need to take such processes into account, says Jia, who believes that maintaining green areas in cities is crucial for preventing large-scale urban heat islands.
The researchers will now look at how local effects can spread over entire regions and test how land-use change affects regional climate models.
The scientists reported their work in Environmental Research Letters.
Now, Gensuo Jia of the Institute of Atmospheric Physics at the Chinese Academy of Sciences and colleagues have analysed how changing vegetation cover in urban and rural areas in the Pearl River Delta region of south-east China affects local climate. The region has seen rapid economic development in recent years, which has dramatically altered the landscape; formerly green, lush areas are now covered over with concrete. These changes can be clearly identified with the Terra moderate resolution imaging spectrometer (MODIS).
MODIS, like other such weather satellites, measures the greenness of a landscape using light-reflection data. Green plants reflect light in the near infrared wavelengths of the electromagnetic spectrum but they absorb red wavelengths for photosynthesis. Using near-infrared and red-wavelength data, scientists can calculate a "normalized difference vegetation index" (NDVI) – the bigger the index, the greener the landscape.
Jia's team examined the change in area of urban, wild land and urban–rural fringes from 2000–2008. The results showed that more photosynthesis occurs in the wild lands and stable urban landscapes than the urban–rural fringe areas, where cities are now expanding and there is less and less greenery.
The researchers also found that increased temperatures from climate warming speed up vegetation growth more than increased rainfall does. This is to be expected in tropical areas, such as the region studied, where conditions are already humid.
"Vegetation is an important driving factor in local climate dynamics," says team member Yonghong Hu of the Institute of Atmospheric Physics. "It influences how much solar radiation is absorbed, surface and air temperatures, and the amount of water evaporating from land."
Replacing green areas with concrete leads to less water evaporating from the surface, and so less water in the air. Concrete also absorbs more radiation from the Sun, heating up the land surface and near-surface air. This hot air rises, which, in turn makes it harder for precipitation to form. The overall result is dryer air in the entire region.
Climate models need to take such processes into account, says Jia, who believes that maintaining green areas in cities is crucial for preventing large-scale urban heat islands.
The researchers will now look at how local effects can spread over entire regions and test how land-use change affects regional climate models.
The scientists reported their work in Environmental Research Letters.
About the author
Belle Dumé is a contributing editor to environmentalresearchweb.
