3. Short-lived climate pollutants (SLCPs)

Substances that cause global warming and air pollution

Professor Ramanathan joined the national Center for Atmospheric Research (NCAR) in 1976 and began his research with meteorologist Roland Madden. In 1980, they predicted that global warming would begin to show signs by 2000. Unfortunately, their prediction came true although the observed global warming was lower than that they had predicted.

1981, Age 37

1981, Age 37 (third from the left) with NCAR Researchers

Professor Ramanathan and other scientists wondered why the warming was lower than predicted. They suspected that the warming has been partially offset by a cooling effect caused by certain particles in the air, or air pollution, also from fossil fuels.
This is where Professor Ramanathan's credo comes into play. In 1995, with Dr. Paul Crutzen (Nobel Prize laureate in Chemistry) and Dr. Mitra from India, he devised a large-scale international experiment, the Indian Ocean Experiment, to observe the cooling effect. With more than 200 scientists from six countries, this major experiment took place in 1998-1999 and involved the satellite data collection and observations by six aircraft and two ships.

The experiment observed a three-kilometer-thick cloud of air pollution, the so-called atmospheric brown clouds (ABCs), covering much of the Arabian Sea, the Bay of Bengal, and the Indian subcontinent, and found that it was blocking 10 to 15 percent of sunlight. The air pollution blocking the sunlight was largely made of tiny particles called aerosols. The experiment also identified that the aerosols blocking sunlight in the brown clouds fall into two categories. The first category of particles is sulfate and nitrate aerosols, which act like mirrors and reflect sunlight back to space. This causes global cooling. The second category consists of black carbon particles, which absorb sunlight and warm the planet.

Black carbon is a particle. It is the black soot produced by burning coal, diesel engine fumes, or from cooking food in stoves and cookers. Greenhouse gases trap heat and black carbon traps sunlight. Both warm the planet. Sulfates and nitrates (emitted by fossil fuels) reflect sunlight and cool the planet. This cooling effect of aerosols offsets part of the warming from greenhouse gases. That was why the actual global warming level was lower than that predicted by the Professor Ramanathan's research group.
There is another issue. Both black carbon particles and sulfate/nitrate particles in ABCs cause serious health problems to humans. Together these particles cause over 6 million premature deaths worldwide through diseases like asthma, lung cancer, and cardio-vascular disease. When these particles block sunlight and reduce the sunlight reaching the oceans, the amount of sea water evaporation is reduced. Less water evaporation from the sea causes a reduction in rainfall on land, which leads to drought.

Professor Ramanathan stated that these global, regional and health impacts caused by ABCs are perhaps the most compelling reasons to regulate air pollution worldwide. However, there are some political considerations that need to be taken into account when regulating and reducing air pollution. That is, it is not the responsibility of any one particular region or country. Studies have already shown that pollution from the United States crosses the Atlantic Ocean and reaches Europe and pollution from East Asia crosses the Pacific Ocean to the United States. Because air is circulating around the world, we don't know the exact origin of the pollution in question. We are never going to solve this problem by pointing fingers at each other. He also says, just like COVID-19, air pollution emissions from anywhere will have impact everywhere around the world over time.

After the Indian Ocean Experiment, Professor Ramanathan realized a common framework is needed to solve both the global warming problem and the air pollution problem. He focused on four pollutants, methane, ozone, hydrofluorocarbon, and black carbon, which are (per ton of these chemicals) more potent global warming inducers than carbon dioxide and in addition have catastrophic effects on human health. After being emitted, while carbon dioxide remains in the air for 100 to 1,000 years, methane, ozone, and hydrofluorocarbon (HFCs), which were developed to replace CFCs, remain for about 10 years, and black carbon remains only for about 10 days. What does this all mean? If all diesel vehicles in the world were fitted with filters, the filters would remove the black carbon and no new black carbon would be released into the atmosphere from diesel vehicles. Then, the black carbon in the air today would disappear in a matter of weeks, which would bend the curve of global warming quickly (in less than 10 years), and there would be no air pollution from black carbon.

Though black carbon is also released from other activities such as burning coal and wood, if we can drastically reduce emissions of short-lived elements like methane, ozone, HFCs, and black carbon, we can significantly slow down the rate of global warming.
Professor Ramanathan named these four substances short-lived climate pollutants (SLCPs) because they remain in the air for a short period of time. He wrote several scientific papers with a number of researchers to show that reducing SLCPs would be highly effective in slowing global warming and would also reduce damage to crops and human health. In 2010, he was contacted by the United Nations to lead an international committee on SLCPs. He accepted, invited an independent scientist to lead the report and he served as vice-chair. The committee published a report and founded the Climate and Clean Air Coalition (CCAC) in 2012, which aims to reduce SLCPs.

The CCAC is the first UN initiative to focus specifically on SLCPs. Initially launched with only six countries, the CCAC now includes more than 60 countries covering all regions of the world and supports a wide range of activities to reduce SLCPs. In November 2021, the reduction of SLCP emissions, which Professor Ramanathan had been advocating for years, was finally agreed upon at COP26.

4. To take a step forward


Prof. Veerabhadran Ramanathan