The focus of climate change researchers has been on carbon dioxide emissions over the last few decades. But, it is only recently that the role of carbon aerosols in climate change has been understood. The work on carbon aerosols was pioneered by Professor S. K. Satheesh.

Satheesh has studied aerosols in detail, spending almost six years in Lakshadweep and the Maldives islands. His initial work on sea salt aerosols and dust aerosols explained how natural aerosols affect the monsoon. But it was his seminal work on carbon aerosols that catapulted him to the fore. He established, for the first time, the impact of these particles on climate change, and on the Indian monsoon in particular.

He is the recipient of several awards, including the Shanti Swarup Bhatnagar, J.C. Bose Fellowship, TWAS Prize from the World Academy of Science, and the Infosys Prize. He also chairs the Divecha Centre for Climate Change at the Indian Institute of Science (IISc).

Edited excerpts from an interview:


You started your career in engineering, but you ended up studying climate change. How did that transition happen?


I did a BSc in physics, then a Masters with a specialisation in Applied Electronics. Originally, I wanted to become an electronics engineer. Most of my classmates are engineers in other institutions now. But I did a project with high frequency Doppler radar in the University of Kerala. It was originally installed by the Indian Space Research Organisation (ISRO) at the Vikram Sarabhai Space Centre. That got me interested in atmospheric sciences.

That radar was mainly for studying the earth’s ionosphere. At that time communication was affected because of ionospheric interference. But the ionosphere is at very high levels—at 80 km level— and it had some effect on communication.

I realised that lower atmospheric processes, where we are all living, has greater importance and a bigger societal application. So I changed from electronics to atmospheric or climate science. I joined the Vikram Sarabhai Space Centre for studying atmospheric aerosol in 1993. I’ve been working on atmospheric aerosol for 27 years now. 


So what are aerosols and why is it so important to study them?


Atmospheric aerosols are anything suspended in the atmosphere in solid or liquid phase. These are microscopic particles suspended in the atmosphere. It has to be for at least a few days to have an effect on climate. These particles can be natural origin, for example, sea salt droplets, or dust aerosol coming out of deserts or soil from any region. They can also originate from anthropogenic activity, like, small black particles emitted from vehicles, or industrial sulfate.

It’s important to study them because they affect climate, health, agriculture, satellite, remote sensing, etc. The applications are innumerable. It affects agriculture because it affects solar radiation. Particles suspended in the atmosphere are like a haze and can affect solar radiation reaching the surface and, in turn, crop yield.

Aerosols can affect satellite remote sensing by forming a blanket between the satellite and the surface of the earth as well as solar energy potential. There is an increase in particle concentrations at around 2 per cent per annum. By 2050, if it goes on like this, concentrations will double or triple over the present level. And there will be a drastic decrease in solar radiation reaching earth.

Aerosols can affect climate in different ways. One is direct—due to scattering and absorption. This affects surface atmospheric temperature, because particles like black carbon from biomass burning, industrial burning, or vehicle emission, affect our ability to absorb radiation. And that will in turn lead to heating of the surface. So it will complement global warming by carbon dioxide.

The other way of interaction is indirect. These particles could act as cloud condensation nuclei in the atmosphere. Water vapour cannot condense unless you have a surface for condensation. It’s a well-known fact. So unless aerosols are present, water vapour cannot condense to form cloud droplets and it will not rain. So aerosols are very important. But the surface properties are also important—whether water vapour can condense or not depends on the type of particle.

The overall effect, whether direct or indirect, depends on whether it is natural or anthropogenic. The latter have the potential to affect climate substantially. That is why we are interested in anthropogenic or man-made aerosols. Also, we can control their release. That is the biggest advantage. Natural aerosols we cannot control. Fortunately, they don’t have a huge climate impact.


Your earliest research was in marine aerosols. You performed some interesting experiments then with long periods spent at sea.


I wanted to study the impact of marine aerosols on the monsoon. So we established an observatory in Minicoy Island of the Lakshadweep group. Because Minicoy is far from the mainland—it is the southernmost—I spent almost two years there. We set up an observatory in 1995.

The purpose was to study two things—one was marine aerosols because of wind speed.  How much wind speed is required to produce a given amount of sea salt aerosol? Natural aerosols like sea salt aerosol are cooling agents.

If we have a large quantity of sea salt aerosols they will have a cooling effect and offset global warming. That was my interest—whether aerosols can offset global warming.

Another issue was transport of mineral dust aerosol from the Sahara to the Arabian Sea and whether it would affect the monsoon. It comes from the Horn of Africa and part of it reaches the Indian Ocean. Both these aerosols are in general cooling. Dust aerosols absorb a little heat, but the scattering effects are more important. Scattering disperses part of the energy back to space. Energy lost from the system leads to cooling.

We believed both sea salt and dust aerosols would have an effect on the monsoon. We worked out how much wind was needed to produce how much sea salt aerosol and its climatic impact—the same thing with dust aerosols. That was the main object of our study. It was one of the early studies in India. Nobody had studied marine aerosols in India at that time until then.


Why was it difficult to study marine aerosols?


It was difficult because reaching Minicoy is difficult. The vessel stops 10 km short and we make the rest of the journey by fishing boat. We needed advanced instrumentation to measure the output. Each trip was three to four months. And I managed that for two to three years. Then I completed my PhD and went to the US for post-doctorate studies at the University of California.


How did you get to studying anthropogenic aerosols?


Anthropogenic aerosols have much more impact on climate. After the Lakshadweep experiment, when I went for post-doctoral studies, they wanted to study anthropogenic aerosols. We established a station in the Republic of Maldives on a small island—Kaashidhoo.

I spent two and a half to three years there during post-doctoral studies, but not continuously. I depended on the local people. I still remember Ashraf and Nasim from the Maldives Meteorological Service. Kaashidhoo is not a tourist island. We had access with special permission from the government. So I was given permission to stay there. Outsiders cannot go to Kaashidhoo.

Two things—one is transported from South Asia: India, China, East Asia. Anthropogenic aerosols are transported to Indian Ocean region and there they affect the monsoon. Our main interest was transport of aerosol. In the Maldives or Lakshadweep, there are no anthropogenic aerosols.

It was called the Indian Ocean experiment. The monsoon circulation changes from about November to March, from the East Asian region to the Indian Ocean region. During the monsoon season, June to around October, this wind is from the land to the ocean. In that time, we collected a lot of data on anthropogenic aerosols.

Until then the general belief—not only in India but the whole world—was that aerosols cooled the planet. In the experiment was conducted from 1996 to 1999 we showed that aerosols have light-absorbing properties and can complement global warming. They can heat the atmosphere.

Until then most studies focused on light scattering aerosols, like sea salt, dust, sulfates. Even with industrial aerosols, anthropogenic aerosols, the thrust was sulfate. Mostly they were working on sulfate because it is a cooling agent. From that time onwards, black carbon aerosols got a lot of attention. That is about 1999 to 2000. It attracted a lot of media attention. I published in Nature. In The Hindu a whole page featured my work.


Why was it neglected all this while?


The measurements were very limited. Even aerosols were an evolving topic at that time. From 1950 aerosols were being studied for their indirect effect—on clouds, their ability to act as cloud condensation nuclei was the main topic. But by the ’70s and ’80s, dust transport became a topic.

But again, it was mostly scattering. So nobody bothered. But once black carbon got attention, a lot of studies began to appear. Then the United Nations environmental programme announced it as a short-lived climate force.

Methane and black carbon particles are short-lived climate forces. Now in the latest IPCC report, Chapter 6 is on short-lived climate forces only for this purpose—aerosols. Short-lived because in the case of carbon dioxide the lifetime is hundreds of years. If you stop emitting carbon dioxide now, its effects will linger for several hundred years, unless some high technology carbon sequestration process is found economically viable. Everybody knows how to take it out but it is not economically viable.

For black carbon it takes only a few months to disperse. During the Beijing Olympics, one main issue was pollution. So they removed all the old cars, shut down thermal power plants and in a matter of months, they cleaned the air completely. The idea is that if you reduce black carbon aerosol particles or sources, you can achieve an effect that is quick.


You are also one of the lead authors in the IPCC report on aerosols. Can you give an overview?


The chapter 7 assessment is on clouds and aerosols and I was one of the lead authors. That report considered the black carbon effect, and we came up with some numbers which are larger than the previous estimate. We found that the effect of black carbon is greater than was estimated, more than we thought originally. That was the main conclusion.


In terms of measuring the impact, where does black carbon rank?


Second to CO2 is black carbon. The highest impact is from carbon dioxide. And the effects increase in extreme weather events. The World Health Organization in 2014 declared black carbon a carcinogen. Look at PM 2.5 particles—half of that is black carbon. If inhaled, it can cause cancer.

Other than rainfall onset and intensity there is distribution of rainfall. If you take the all-India mean for the last several years, you don’t see much variation—some fluctuations but no clear trend. It will be all within the uncertainty limits. But if you see the regional distribution, how much in Kerala, how much in Karnataka or in south India or north or northeast, that shows lots of differences

What has changed is regional distribution. One year we have huge quantities of rain in Kerala, another year it is a drought situation. That is not understood well. It remains a mystery how black carbon affects the Indian monsoon. One thing is clear, it does not affect total rainfall but clearly does affect regional distribution.

We have carried out a lot of black carbon measurements using ISRO aircraft. It has an instrument-filled aircraft in Hyderabad. We submit a proposal and take it all over India and measure black carbon, put the data in climate models and make an assessment of the impact.

What we have seen is that if black carbon amounts increase in the coming years, there will be a reduction of rainfall in south India, and an increase in northern and central India to some extent. And it will migrate it to the Northeast.

The large quantities of black carbon in the higher levels of northern India create huge absorption and warming at altitude—three to four km up. And that creates convection and the air moves up. That vacuum drags moisture from the south. All the moisture goes there due to pressure. This is a preliminary assessment.


So it’s already  happening?


It’s already happening.


So to what extent do you see regional variation in future?


If we have a huge amount of black carbon in northern India, we will have less rain over southern India. But we have lots of problems with data availability. In a climate model we need large numbers of variables. We don’t have that information. Whatever we measure is only a limited number of things. To predict the next 10 years or 15 years, we need much more data.


Specifically, what are the big sources of anthropogenic aerosols in India?


Coal-fired thermal power plants are a major reason, but the government has taken some steps to reduce the number. In a phased manner they will shut down plants. Then, clearly widespread and distributed all over the country, there is vehicular emission. There is no control on the number of vehicles registered. It was only two million in 1970. It is now 200 million. The curve is a steep one. We have rules and regulations on vehicular emission, but I don’t think they are implemented properly. If all 200 million vehicles follow emission standards, things will improve substantially.


What about recent decisions by car companies to stop manufacturing diesel cars?


Yeah, diesel smoke contains a lot of black carbon. Petrol is better, but we are advocating for renewable energy like solar vehicles. If you see the campus you can see a lot of autos. In electric also, our main objection is that charging is done with electricity produced by conventional methods.

If the charging stations can be changed to solar, that will be good. I wanted to buy an electric vehicle, but I have no option for charging. I am using a petrol vehicle. Charging stations should be everywhere or people will not buy electric vehicles. In Kerala, they started electric buses. It didn’t work because of lack of charging stations.


The US has withdrawn from the climate change agreement. So what do we do now?


More than mitigation, adaptation is required. As the US has withdrawn, many other countries may not take climate change mitigation seriously. Because it means you have to spend a lot of money. Government will have to spend a lot of money in the budget. If that is not done adaptation is the next step. You can see the Kyoto Protocol, the Montreal Protocol, global cooperation was not there. So when one country or other is not cooperating, it will not work. India contributed just six per cent of the CO2 emissions globally. So even if we reduce a little bit, it is not going to make much difference. The major players will have to reduce their share.

 That is why all these IPCC 1.5 reports are out now. Theoretically it is possible to contain global warming to 1 or 1.5C. But global cooperation is needed. By 2030, 45 per cent of CO2 emissions should be reduced. That is what the 1.5C report says.


What adaptation strategies do we need to follow?


As in farming, when we cannot say how much rain will fall, we have to change strategies accordingly. Adaptation is more important than mitigation. Mitigation needs a lot of policy decisions. Adaptation means, change or perish.


Could Kuttanad farming in Kerala be a model for climate adaptation, where they work with saline water?


Yes, yes. They have already adapted to the changes. That is a model you can adopt. But different places have different environments. Here in Karnataka, if somebody wants to use it, there may be scope to use it as a model. Future Earth is a new initiative by the United Nations for sustainable development. I am the director of South Asia for that, and we are looking to implement these solutions.

The objective is to make sure that we follow sustainable pathways; the ultimate aim is a sustainable development pathway for South Asia. That is why I advocate for mitigation and adaptation.


I was wondering if the pollution crisis in Delhi and the rest of north India is related to aerosols. 


Oh, yes, it is all aerosols. The Himalayas block it on one side and the high hills in Bihar on the other. So it is like a canal, a valley between two hills. It would circulate and get diluted otherwise by atmospheric circulation. But the Gangetic plains’ population densities are high and there are many thermal power plants. So emissions are very high. These two factors ensure that aerosols are confined in that valley. In winter, the atmospheric boundary layer is shallow. So, all the pollution is confined to a band of 500 metres or one kilometre. In summer, the height goes up to 3-4  km so pollution gets diluted. In winter, it’s all like a gas chamber inside. In Delhi specifically, agricultural burning in Punjab and Haryana is causing it. Deepavali also comes in that season. They both add to a lot of pollution.


So, what can be done? 


Farming practices can be changed. Instead of burning crop stubble they can follow advanced practices. I know of The Nature Conservancy working on it in Punjab. They are coming up with solutions. Farming practices can be modified. Awareness programmes are needed. A subsidy from the government is also needed as farmers will not be able to or want to suddenly change to a new way.


Will this solve the problem?


Because of the two hills—the geography that confines the air, and how the weather is, nobody can do anything. But, if we control emissions from thermal power plants and vehicles, it can solve the problem to some extent. Diesel should be strictly prohibited and electric vehicles used as much as possible. Heavy vehicles contribute only 1.2 per cent in terms of total registered vehicles but they contribute 35 per cent of the emission. That is one target area if we want to reduce emissions. Two wheelers make for 60 per cent of the vehicles but contribute only 9 per cent to total vehicular emissions.


What new areas should climate research look into?


A new area is air quality and health. It’s because of the co-benefits. We are seeing an increase in the number of respiratory illnesses, especially among school children. If you count the number of days we lose—it’s one of the parameters we look at—because of respiratory problems, the air quality is not good. If you reduce aerosol particles, it’s not only good for the climate but also advantageous to human health.

Water, food and energy—these are the three main objectives of the future of the programme. Agriculture and crop yield and biodiversity changes are another area we’re looking at. The Center for Ecological Sciences, IISc, maintains a large number of biodiversity plots in Mudumalai (in Tamil Nadu). We use the data to study biodiversity. In September 2019, we are organising an international conference with over 1,000 participants. It’s about a sustainable water future. In the field of energy, we are working on transparent solar cells, which can be pasted on glass windows. It will not affect the outside view. It will be like a thin film but it will produce energy.

In water quality, we have a group studying arsenic and fluoride contamination in groundwater. And another group is studying natural uranium contamination. There are several places where the incidence of cancer is very high. That group is studying soil contamination. For example, in some parts of Kerala, banana contains a lot of radioactive material and that was the cause for a rise in the number of cancer cases in Alapuzha, Kollam and surrounding areas. The group is studying it with the Bhabha Atomic Research Centre in Mumbai. In total, we have 91 staff members, providing a lot of diversity.

We also have outreach programmes. We work with schools. We install air quality monitors in schools and ask students to do projects with that. We have identified 15-20 schools but we want 100 schools. It’s a school project, students will get credit. And they will get to understand the serious consequences of air pollution. Doctors will talk to them about the numbers they are measuring and what effects those numbers have on their health. It is to create interest among them. In future they may consider this as a topic. We also do climate change awareness programmes for school children.


What is the next major work that you are looking at?


Our next big step on black light absorbing particles is aircraft emissions. I wrote in a journal that there is no control on aircraft emission as we have with Bharat II and Euro II for vehicles on the road. There is no control, no regulations on aircraft emissions anywhere in the world. The United Nations is considering it. We have reported very recently on it—nobody else has reported it so far. High altitude aircraft, especially international flights, emit these particles at an altitude of 12-14 km, and it’s easily injected at that level. Surface emissions have to go through several mechanisms to reach the top of the atmosphere. Clouds can remove it, rain can wash it down. There are other circulation processes involved. Surface emissions cannot easily reach high level.

But the particles emitted by an aircraft at high altitude are already injected at that level. They can easily go to stratospheric altitudes. There are several laboratory experiments showing they can destroy the ozone layer. If that happens, we will have a global catastrophe on our hands. It will be exactly like what happened with CFCs once upon a time.  So, some  regulations are required for aircraft emissions.


Have you been able to quantify the emissions or the extent of the damage?


We need to conduct measurements at that level. No aircraft available in India can fly at 25 km. What we do is we send balloons and make measurements. From whatever limited data we have, we feel it can affect the ozone layer, and delay the expected recovery of the ozone hole by several decades.


What percentage of total emissions would be from aircraft? Is it comparable to vehicular emission?


It is small. But while vehicular emission can affect humans they cannot go so far up. Our preliminary estimates show that 80 per cent of the particles at that level are from aircraft and the rest from the surface. But more studies are required on this; it is only an emerging thing. This will take time.


You grew up in a rural area that wasn’t very developed and where life was difficult. Development usually involves spending more energy. How do you balance development objectives with climate change effects?


Yes, it may be difficult, but I believe it should be controlled. Uncontrolled development is not good. My village has changed very much. There is a TV in every house, water supply and electricity. But in rural areas, development is in a controlled manner. A Current Science article a few months ago showed Bangalore in 1970 and now. They showed construction activity or built-up area in red. In 1970, it was all green, almost everywhere. Now, there is green only at some spots. The whole of Bangalore is red. If we develop in a controlled manner, it will be good for the climate.


So while bringing all our people out of poverty, can we still maintain our carbon emissions within limits?


It is possible if we follow the guidelines. Development might be slow but it will be sustainable. It’s possible.