Possible causes of Global Dimming[10]
There are many possible causes which have been identified in accordance with the following simplified model, after Darnell et al. (1992)
E0 : Extra-terrestrial radiation
τr : Rayleigh scattering
τg : permanent gas absorption
τw : absorption by water vapor
τa : absorption and scattering by aerosols
τc : absorption and scattering by cloud components
Extra-terrestrial radiation
The intensity of solar irradiation directly outside the earth’s atmosphere on a horizontal surface is almost constant at around 1,350 W/m2, the so-called “Solar Constant”.
Rayleigh scattering and gas absorption
The increase in Rayleigh scattering and gas absorption reduce Eg↓. It implies a much greater change in the known gaseous composition of the atmosphere compare to have been reported. Of course the effects of increases in concentrations of new gases of anthropogenic origin, which are extremely radiatively active in the solar spectrum, remains a remote possibility.
Water vapor
The influence of atmospheric water vapor content, measured as column water vapor, CWV, on atmospheric absorption has been quantified by models and regression studies. These approaches give an increase in short-wave absorption for CWV increases. Changes in the water vapor content of the atmosphere indicate that a small and variable increase has occurred during the last 20 years. Inorganic particles such as sulfate tend to take up more water vapor, grow larger, and therefore scatter more radiation than organic particles. However, the magnitudes of the increases reported are insufficient to significantly affect τw.
Aerosols & Clouds
Changes in aerosol loading and cloud cover and their interactions are not only the most probable causes of the contemporary global dimming phenomenon but will also be the most complex and difficult to quantify.
When fossil fuels are burned than not only green housegases but also by-products are produced. These by-products are comprised of a variety of pollutants like ash, soot and sulfur dioxide. These impurities are responsible for changing the properties of the clouds.
These polluted particles result in increasing the number of droplets per cloud. The reflective property of these clouds thus increase. These clouds sent the rays of sun back into the space instead of letting it reach the surface of the earth. Also, some climate scientists have theorized that aircraft contrails (also called vapor trails) are implicated in global dimming. Actually during September 11, 2001 attacks, an increase in diurnal temperature variation of over 1 °C (1.8 °F) was observed in some parts of the U.S.
