Rising levels of aerosol in the atmosphere and rampant deforestation have ensured a steady decrease in the volume of rainfall every year in South Asian countries, especially in India.
Nirad Mudur l Bangalore
Rising levels of aerosol in the atmosphere and rampant deforestation have ensured a steady decrease in the volume of rainfall every year in South Asian countries, especially in India, which is first in line for the South-west monsoons.
A review of more than 100 recent research articles appearing in the online edition of Nature Climate Change on June 24, has observed that increasing levels of aerosol (fine solid particles or liquid droplets in a gas used in refrigerators, air fresheners and deodorants) in the atmosphere and land-use changes have affected ideal conditions for good monsoon rain.
The reviewers, Hariharasubramanian Annamalai, senior researcher and graduate faculty of meteorology, International Pacific Research Centre at the University of Hawaii, and Andrew Turner from the department of meteorology, University of Reading, have observed a rise in carbon dioxide concentration of up to 70 parts per million by volume and in global warming by about 0.50°C in the past six decades.
This increased the atmospheric moisture during the monsoon season. Though this ought to have led to more rainfall, it did not happen.
One of the main causes for this strange phenomenon is the increase in atmospheric aerosol and rampant land-use change, they said.
“South Asia’s increasing industrialization… and widespread biomass burning and use of cooking fires mean there are large and increasing local emissions of scattering and (sunlight) absorbing aerosols. This limits solar radiation reaching Earth’s surface, (in turn) reducing monsoon rainfall,” Annamalai told DNA in an email.
An earlier study led by S Ramachandran, scientist at the space and atmospheric sciences division of Physical Research Laboratory (PRL), Ahmedabad, conducted from 2000 to 2010 found that the ability of aerosols to scatter/absorb sunlight in the atmosphere in India had risen by 40%. The trend was noticed to be higher in western and southern India. The PRL study, conducted over 30 locations in various parts of the country, also attributed this disturbing trend to biomass burning and rapid urbanisation and industrialisation.
Annamalai explained how land-use changes affects monsoon rainfall. “The speed of low-level moisture-laden winds reduces over land because of “roughness” (caused by forests and a range of flora) and results in moisture convergence,” he said. “But change in land-use (from forests to cropland in India) has led to low level moisture-laden winds for the past several years resulting in less rainfall.”
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