Aerosols ‘shrinking’ India’s monsoon:

Aerosols ‘shrinking’ India’s monsoon:

Context:

Researchers have come to the conclusion that aerosols may be weakening the Indian monsoon even more than greenhouse gases.

Details:

• A team led by climatologist R. Krishnan of the Indian Institute of Tropical Meteorology, Pune, has come up to the conclusion that aerosols affect the strength of the Indian monsoon.
• The team had used an upgraded forecasting model that was used this year by the India Meteorological Department for forecasts.
• This model is expected to help India prepare its first home-grown forecast of climate change from global warming, and be part of the Intergovernmental Panel on Climate Change reports.

How do aerosols weaken the Indian Monsoon?

• The monsoon is mainly driven by the thermal contrast between the Indian subcontinent and the adjoining ocean.
• As the land warms up the air above the land surface is heated and rises up and is less dense than the cool air over the ocean.
• This contrast in temperatures and densities causes the cool moisture-bearing winds from the western Indian Ocean to move into the land mass and bring monsoon rains to the subcontinent.
• However, the scenario is changed due to the presence of greenhouse gases, aerosols in the atmosphere or if the land has been subjected to increased deforestation. The researchers have come to the conclusion that aerosols may be weakening the monsoon more than greenhouse gases (GHCs)
• Anthropogenic atmospheric aerosols, including sulphates, black carbon, nitrates and dust accumulate over the Indo-Gangetic plains.
• These reduce incoming solar radiation over northern India and the northern Indian Ocean and lead to cooling on both land and sea resulting in a lowered thermal contrast.
• Hence, monsoon winds and circulation are weakened as both land and sea are cooled due to aerosol accumulation.

What are aerosols?

• An aerosol is a colloid of fine solid particles or liquid droplets, in the air or another gas.
• Aerosols can occur naturally, originating from volcanoes, dust storms, forest and grassland fires, living vegetation, and sea spray.
• Human activities, such as the burning of fossil fuels and the alteration of natural surface cover, also generate aerosols

• Aerosol particles larger than about 1 micrometre in size are produced by windblown dust and sea salt from sea spray and bursting bubbles.
• Aerosols smaller than 1 micrometre are mostly formed by condensation processes such as conversion of sulphur dioxide (SO2) gas (released from volcanic eruptions) to sulphate particles and by formation of soot and smoke during burning processes
• After formation, the aerosols are mixed and transported by atmospheric motions and are primarily removed by cloud and precipitation processes.

Importance of Aerosols

Aerosols serve as media upon which chemical reactions can occur. 

• The most significant of these reactions are those that lead to the destruction of stratospheric ozone.
• During winter in the Polar Regions, aerosols grow to form polar stratospheric clouds.
• The large surface areas of these cloud particles provide sites for chemical reactions to take place.
• These reactions lead to the formation of large amounts of reactive chlorine and, ultimately, to the destruction of ozone in the stratosphere.

Cloud formation

• Aerosols are also believed to have an “indirect” effect on climate by changing the properties of clouds.
• Had there been no aerosols, there would have been no clouds.
• It is very difficult to form cloud droplets without small aerosol particles acting as “seeds” to start the formation of cloud droplets.
• As aerosol concentration increases within a cloud, the water in the cloud gets spread over many more particles, each of which is correspondingly smaller.
• Smaller particles fall more slowly in the atmosphere and decrease the amount of rainfall.

In this way, changing aerosols in the atmosphere can change the frequency of cloud occurrence, cloud thickness, and rainfall amounts.