Aerosols, like smoke or dust suspended in the lower layers of the atmosphere, can either heat up the planet by trapping solar radiation or cool it by reflecting sunlight into space. Previous research has shown, for example, that absorption of radiation by aerosols can significantly heat the atmosphere over the Bay of Bengal region. This region dramatically influences the Indian summer monsoon; any change in the effect of aerosols can have a critical impact on the local climate.
Little is known, however, about how much aerosols that are present much higher up—above the clouds—contribute to this warming. Their contribution could depend on the amount of cloud cover below them, studies have shown. Some scientists have proposed a tipping point called “critical cloud fraction” (CCF)—the amount of cloud cover at which above-cloud aerosols switch from having a cooling to a warming effect.
Now, scientists at the Indian Institute of Science, Bengaluru, have estimated this CCF value over the Bay of Bengal region, and demonstrated, for the first time, that it changes with the nature and type of aerosols present above the clouds.
“CCF indicates when aerosols above the clouds will change from cooling the earth to heating the earth,” explained Prof. S. K. Satheesh, senior author of the study published in the journal Atmospheric Science Letters published by the Royal Meteorological Society. “Previous investigators had argued that CCF is the same all over the globe. We have shown that this is not the case and it varies on account of changes in chemical composition of aerosols.”
The researchers found that CCF shifted from a higher value during the post-monsoon period (September-November) to less than half of that value during winter (December-February).
Upon investigating further, they found that this shift corresponded to a change in the nature of the aerosols over the region during those periods. In the post-monsoon season, for instance, the aerosols present were those that absorb less sunlight, such as sea salt aerosols. In their presence, a larger cloud coverage will be needed for the aerosols to have a warming effect.
However, later, in winter, the aerosol composition shifted to those that absorb more solar radiation, such as black carbon and sulfate emissions likely originating from India, China and Myanmar, leading to a lower warming threshold, or CCF value.
The analysis, using data collected for over four years from multiple satellites, was carried out by researchers at the Centre for Atmospheric and Oceanic Sciences and the Divecha Centre for Climate Change at IISc. This study is the first to analyse the nature and influence of above-cloud aerosols over the Bay of Bengal region. The study also demonstrates that it may be possible to estimate aerosol composition in the atmosphere above the clouds using satellite data.
Efforts are on to investigate the potential regional impacts of the shift in CCF, says Prof. Satheesh.
Editor's Note: The above story was originally published on 30 March 2015 by the Science Media Center at IISc and is now republished.