NEW DELHI: In the just concluded winter session of Parliament, Union Minister of Earth Sciences Kiren Rijiju shared his concerns about the rise of aerosol levels over the Indo-Gangetic Plain and the Himalayan foothills. Solar light absorbing carbonaceous aerosols or black carbon (BC) are altering rainfall patterns, accelerating melting of glaciers and snow and impacting cloud formation at the regional level. India represents a unique case for aerosol loading, properties and their effects. Varying aerosol sources get activated at different spatial and temporal scales.
This changing nature of aerosols temporally and spatially when coupled with different land use nature av cross India, produces a very complex aerosol radiation-cloud-precipitation-climate interaction, Rijiju said. Studies show that at least 75% of the aerosol absorption over the Indo-Gangetic Plain and the Himalayas throughout the year comes from BC.
Besides, aerosols alone account for over 50% of the total warming of the lower atmosphere. In an interview with the New Indian Express, Dr Mrutyunjay Mohapatra, Director General of Indian Meteorological Department, said eastern India’s long range period rainfall has reduced significantly in the past few decades due to high BC levels. Uttar Pradesh, Madhya Pradesh, Chhattisgarh, Jharkhand and some parts of the Northeast have witnessed significant reduction in seasonal as well as annual rainfall in the past few decades.
What is aerosol and its role in regulating the atmosphere?
An aerosol — natural or anthropogenic — is a suspension of fine solid particles or liquid droplets in the air. It plays an important role in the formation of fog, mist and brown clouds. Some aerosols absorb solar radiation resulting in warming the atmosphere while others scatter and reflect the heat, resulting in the cooling of the atmosphere. BC aerosols contribute significantly (20-70%) to the total atmospheric load of particulate matter and account for more than 50% of total warming of the lower atmosphere. BC absorbs solar radiation, causing a warming effect as it settles on glaciers and accelerates melting.
What is the source of BC aerosols and why is it a concern?
BC is the main anthropogenic light-absorbing constituent present in aerosols. Its main sources are the combustion of fossil fuels (such as gasoline, oil, and coal), wood and other biomass. It is mostly emitted by vehicles (largely diesel ones), industrial production, household use of solid fuels, agricultural biomass and waste burning.
According to a study, the highest BC contributor was biomass burning (37%), followed by transport (both on-road and non-road transport) at 25%, residential biofuels at 18% and industry at 11%. The rest are residential biomass and coal. Besides, large aerosol concentration does not allow cloud droplets to grow, thus inhibiting rainfall and decreasing visibility in the lower atmosphere.
The Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR5), while quantifying the warming potential of each pollutant, stated that BC can be 900 times more warming than CO2 in a 20-year time horizon. Also, BC is the largest contributor to fine particles i.e. smaller than PM2.5, causing serious respiratory and cardiovascular diseases.
Why do BC and CO2 need aligned mitigation?
There is a complex chemistry between cooling and warming components of aerosols. For example, the IPCC AR6 report says that removal of sulphur dioxide emissions that form cooling sulphate can increase the global mean surface temperature by 0.69°C.
But reduction of BC aerosols would reduce the temperature by just 0.022°C during 2041-50. “But this trade-off can be small if significant CO2 reductions are also achieved simultaneously,” the Delhi-based environment think tank Centre for Science and Environment says in its report.
Double burden of the Global South
While the Global North or the developed economies have been producing more heat-trapping gases and require accelerated energy transition to curb long lived CO2, the Global South finds itself in the polluting pathways of socio-economic development that produce more short-lived climate pollutants (SLCP) - BC, greenhouse gases like methane, CO2 and hydrofluorocarbons.
It has twin impacts: (1) Global warming further increases the reaction rate of atmospheric gases, thus increasing harmful particulate matter concentration. (2) Increasing global warming affects dust concentration, increases forest fires and impacts aerosols. And changing rainfall patterns affect the wet removal process of particulate matter. This is the double burden.