Jun 01, 2023
Jun 01, 2023
The brown cloud of particulate air pollutants that now cover all of Asia and the seas around is responsible for the weak and erratic monsoon in India, about half of the global warming and a major rise in respiratory ailments like tuberculosis.
These are the findings of a series of studies carried out over 10 years by a group of around 200 scientists, a third of them from India.
The particulates - also known as aerosols - consist mainly of sulphates, nitrates and soot. Half are spewed into the air by burning coal and usage of diesel, the other half while cooking with firewood and animal dung.
The aerosols are too small to be seen by the naked eye, but they sit all over South Asia and the surrounding seas from ground level to a height of three kilometres, hitting against the flanks of the Himalayas.
Veerabhadran Ramanathan, distinguished professor of atmospheric sciences and the director of the Centre for Atmospheric Sciences at the Scripps Institution of Oceanography at the University of California, San Diego, has been leading and coordinating most of these studies since 1996, when scientists first realised that the actual warming of the earth's surface was only half of that predicted by greenhouse gas global warming models.
Are we missing something, Ramanathan wondered? Is there a cooling effect on the surface due to something else? That was when he started the 1996-98 Indian Ocean experiment to study the role of particulates on climate.
Making their observations from the Maldives, the scientists found that the north-east trade winds of the Asian winter pushes a huge brown cloud of particulate pollutants into the Indian Ocean region.
"The entire Arabian Sea and Bay of Bengal were covered by it," Ramanathan told IANS during a visit here.
"It led to a reduction of five to 10 percent in the amount of solar radiation (sun's heat and light) reaching the surface between October and March. The nitrates and sulphates were reflecting the radiation back into space, while the soot particulates were absorbing the radiation."
The effect was that the atmosphere was getting hotter than ever before, while less solar radiation reached the surface. As a consequence, the amount of water that evaporated from the Indian Ocean region was less than before.
On top of that, a hotter atmosphere meant that the water vapour that did go up was not condensing into clouds as easily.
The brown cloud was thicker in the northern parts of the Arabian Sea and the Bay of Bengal, a fact that added a third factor. The scientists found that on an average these northern seas were a half-degree Celsius cooler than the southern parts of the Indian Ocean region, a major difference that can lead to significant climate change. This meant that the 'pull' factor for the southwest monsoon winds that bring rain was being reduced, since winds move from a cooler to a warmer region.
Ramanathan found proof of this when he checked rainfall data in India from 1950. It showed a five to 10 percent decreasing trend in overall rainfall. A NASA satellite launched in 2001 also showed the brown cloud clearly.
The satellite showed the dirty cloud lapping up against the shoulders of the Himalayas from both north and south, a huge plume going from North America to the Atlantic Ocean, and another huge plume moving from Europe and over northern Asia to the Pacific Ocean.
Apart from the effect on the monsoon, the particulates had a direct effect on the retreat of the Himalayan glaciers, the scientists found. Since they sat relatively low over the earth's surface, they were directly heating the ground at high altitudes, where the glaciers are. Ramanathan said NASA satellites had found major glacier retreat on both sides of the Himalayas.
This means major Asian rivers such as the Ganges, the Indus, the Brahmaputra and the Yangtze stand to lose their major source of perennial water. Ramanathan realised that this was such a major issue that "I've to spend the rest of my life" on this.
In 2006, Ramanathan put together another group to start a new project where the scientists would measure the exact effect of the aerosols on the atmosphere.
For days together, three unmanned planes (UAVs) carried a host of instruments to gather meteorological data, flying one above the other from their base in the Maldives, all at the same time so that the researchers could compare what was happening at different levels of the atmosphere.
"The UAV on top was flying above the brown cloud to measure the amount of radiation being reflected back into space by the aerosols," Ramanathan explained. "The UAV at the bottom was measuring the dimming effect of the brown cloud. And the one in the middle kept going up and down between the other two, measuring the temperature at each layer."
The scientists found that the brown cloud was adding almost 50 percent to the normal heating of the atmosphere all over the Indian Ocean, the Arabian Sea and the Bay of Bengal.
At the same time, NASA used its new satellite CALIPSO to throw laser beams at the brown cloud, scatter the particulates, and get the exact altitude at which different kinds of particulates were to be found at various concentrations.
Once again, the CALIPSO data confirmed the presence of the brown cloud from the ground up to a height of three km in the atmosphere, over all of Asia, the Indian Ocean and the western Pacific Ocean.
This study also answered a major question about global warming. Glaciologists working in the Himalayas had found that there had been an average temperature rise of 1.2 degrees Celsius since 1950.
But the global warming model based on greenhouse gases had predicted a warming of 0.6 degrees Celsius. Now Ramanathan and his colleagues found that the aerosols contributed the other 0.6 degrees.
So, when it comes to global warming, the brown cloud is as guilty as the greenhouses gases, a finding that has major implications for the control of global warming all over the world.
The presence of the brown cloud, especially at the ground level, has another serious implication, Ramanathan pointed out. The aerosols are so small that they get into our lungs, and aggravate respiratory ailments, especially tuberculosis.
This becomes particularly bad for people who already suffer from indoor pollution because they use wood and animal dung for cooking.
Ramanathan is now in India as he is working with the Delhi-based The Energy and Resources Institute (TERI) on this indoor pollution issue - trying to encourage the use of solar cookers and cooking fuel obtained from biogas plants.
This will have two major benefits - respiratory ailments will come down, and the brown cloud will thin out, because half the particulates in the air come from cooking with wood and animal dung.
Asked if he had extrapolated his findings to make any prediction about the monsoon in India, Ramanathan (who did his engineering course at Annamalai University and got his master's degree from the Indian Institute of Science, Bangalore, before going to the State University of New York for his Ph.D) hesitated and said: "If it is going to be business as usual, we could see a doubling in the frequency of droughts in India in the next 20-30 years. But this is one prediction where I really hope I'm proved completely wrong."
More by : Joydeep Gupta