GLOBAL DIMMING: ITS CAUSES, IMPACTS AND SOME POSSIBLE CONTROL MEASURES
Sun is the source of all energy available on earth. The amount of solar radiation reaching the earth's surface gradually declines, termed as "Global dimming". Changes in pan evaporation, diurnal temperature range (DTR) and transmissivity of the earth's atmosphere support the incidence of global dimming. Global dimming is mainly caused by increased amount of particles (aerosols) in the air which can be determined by the aerosol optical depth (AOD), increased cloud thickness and cloud cover due to sea ice melting and changes in the land use. The albedo (short wave reflectivity) of the earth is increased by non-BC (black carbon) aerosols such as sulfate, nitrates and organics in atmospheric brown clouds (ABCs) which result in an increased amount of reflected radiation to the space. Albedo of the cloud is also increased as the non-BC aerosols nucleate cloud drops which is known as "cloud-albedo-effect". Volcanic eruptions and desertification also add large amount of aerosols and dusts respectively in the earth's atmosphere. Global dimming results in some ecological problems viz. changes in hydrological cycle, rainfall pattern and rate of photosynthesis. It sometimes causes drought, failure of monsoon and decrease or absence of seasonal rainfall in northern latitudes. Solar radiation controls photosynthesis by controlling the opening and closure of stomata and by maintaining CO 2 concentration. Nevertheless, global dimming also harms human health indirectly. Thus actions could be taken to control this phenomenon due to its direct and indirect impacts on global ecosystem. By cultivating photo insensitive crop varieties we can minimize the adverse effects of global dimming on photosynthesis. Air pollution can be controlled by using some alternative fuels and strictly regulating the emissions from industries thereby lowering the aerosols content in the atmosphere and by these we can escape from the ill effects of global dimming.
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