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Aerosol, clouds blocking sunlight over India

Biplab Das

doi:10.1038/nindia.2010.58 Published online 4 May 2010

Clouds with aerosols conspire to block and reflect away sunlight over the Indian subcontinent, a new research claims. This results in what is called 'solar dimming' lessening the amount of sunlight that reaches Indian soil.

Interestingly, solar dimming continues unabated over India, in contrast to most of the world where dimming has given way to brightening since the late eighties.

"Dimming affects climate change by reducing surface temperature and weakening the hydrological cycle, which in turn could have serious effects on vegetation and agriculture," according to the lead researcher B. N. Goswami, director of Indian Institute of Meteorology, Pune.

Understanding the cause of this significant dimming is very important in the backdrop of continuously increasing surface temperature of the region, he notes.

Researchers B. N. Goswami (left) & B. Padma Kumari.

Traditionally, solar dimming has been attributed entirely to aerosols, micron-sized (or smaller) particles of sulfates, black and organic carbon, dust and even sea salt. "The rate of dimming is twice as large during cloudy conditions as compared to that during clear skies. This is a new insight and establishes that clouds play a more important role in continued dimming over Indian monsoon region," Goswami says.

Some aerosols scatter and others absorb the incoming solar radiation. Aerosol particulates act as nuclei points for cloud condensation. They can lead to more cloudiness — a phenomenon called the indirect aerosol effect — which reflects sunlight away.

To understand the interaction between aerosol, cloud and radiation, the researchers analysed the 'daily mean surface reaching solar radiation' data recorded at different locations in India between 1981 and 2006. They compared this data from urban stations with that at a rural, high-altitude location least affected by industrialisation and urbanisation.

Solar dimming under clear sky conditions were likely due to increase in aerosol emissions from human activities. The trends in clear sky represented direct effects of aerosols by scattering and absorption of incoming solar radiation. The rural hill station of Kodaikanal showed lesser aerosol loading as compared to urban stations.

The researchers found that the rate of decrease of solar radiation during cloudy conditions was double than that during clear skies. This shows that clouds play a larger role in the observed solar dimming than aerosols. Clouds contribute more to dimming during summer since 'increasingly deeper clouds cover increasingly larger area'. "This increasing trend of deeper clouds is largely due to increasing instability of the atmosphere and could be partly due to indirect effect of aerosols," Goswami adds.

Reduced solar dimming has far-reaching implications for the Indian subcontinent. According to Goswami, the evaporation over continental India shows a significant decreasing trend. Further, the monsoon rainfall during the period shows a weak decreasing trend. The monsoon hydrological cycle is weakening over the past three to four decades.

Solar dimming also affects farming. Goswami says agricultural productivity of light-loving plants such as peppers and tomatoes may decline as predicted by some researchers. However, some plants do better in more limited, diffused light.

Experts are calling it a timely research to establish the important role of clouds in solar dimming. Sachchida Nand Tripathi of Indian Institute of Technology, Kanpur told Nature India that there's consensus among peers that anthropogenic aerosols are rising and will contribute to more solar dimming over the Indian subcontinent.


References

  1. Kumari, P. B. et al. Seminal role of clouds on solar dimming over the Indian monsoon region. Geophys. Res. Lett. 37, L06703 (2010) | Article