UPSC IAS Interview 2017-18

Vegetation-An effective tool to change climate

Scientific experts agrees that global climate change is accelerating. Human activities are responsible for the vast majority of climate change. If it is left unchecked, climate change threatens to harm current and future generations. The emission of heat trapping gases by vehicles, industrial processes, power plants and deforestations cause climate change. Rising temperature and changing patterns of precipitation drastically shift vegetation. We forget that this vegetation plays an important role in changing the climate of a city. It is also effective in controlling the microclimate. Plants, shrubs and trees cool the environment when they absorb radiation for photosynthesis. They are useful in shading a particular part of the structure and ground for reducing the heat gain and reflected radiation. By releasing moisture, they help raise the humidity level. Vegetation also creates different air flow patterns by causing minor pressure differences, and thus can be used to direct or divert the prevailing wing advantages.
Based on the requirement of a climate, appropriate type of trees can be selected to preserve good climate of cities. Planning deciduous trees such as mulberry to shade east and west walls would prove beneficial in hot and dry zones. In summer, they provide shade from intense morning and evening sun, reduce glare, as well as cut off hot breezes. On the other hand, deciduous trees shed their leaves in winter and allow solar radiation to heat the building. The cooling effect of vegetation in hot and dry climates comes predominantly from evaporation, while in hot humid climates the shading effect is more significant.
Trees can be used as windbreaks to protect both buildings and outer areas such as lawns and patios from both hot and cold winds. The velocity reduction behind the windbreak depends on their height, density, cross-sectional shape, width, and length, the first two being the most important factors. When the wind does not blow perpendicular to the windbreak, the sheltered area is decreased. The rate of infiltration in buildings is proportional to the wind speed reduction in extreme climates, than to attempt to maximize the distance over which the windbreak is effective.
In cold climates, windbreak can reduce the heat loss in buildings by reducing wind flow over the buildings, thereby reducing convection and infiltration losses. A single-row of high density trees in the form of a windbreak can reduce infiltration in a residence by about 60% when planted about four tree heights from the building. This corresponds to about 15% reduction in energy costs.
Thus, trees can be effectively used to control the microclimate.

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