How Air Pollution Has Put a Brake on Global Warming

In an interview with Yale Environment 360, Norwegian climate scientist Bjørn H. Samset talks about the results of his team’s recent research showing that aerosols linked to human activities cool the planet far more than previously believed.

Pollution particles emitted by diesel cars and trucks, coal-fired power plants, factories, rudimentary cook stoves, and the burning of forests are major contributors to the unhealthy pall of smog that blankets many cities and regions, particularly in the developing world. Scientists have long known that these aerosols serve to block incoming solar radiation and temporarily cool the planet, but now an international team of scientists has quantified that cooling effect, saying the earth would be 0.5 to 1.1 degree C (0.9 to 2 degrees F) warmer if that pollution were to suddenly disappear.

In an interview with Yale Environment 360, lead author Bjørn H. Samset of Norway’s Center for International Climate Research discusses the implications of this research. As countries like China make progress in reducing air pollution, regional planners should be prepared for the cleaner air to cause a jump in temperatures even above those expected under global warming scenarios. At the same time, Samset says, rising temperatures will likely lead to an increase in precipitation as more water evaporates from oceans, lakes, and rivers.

Bjørn H. Samset

Bjørn H. Samset

In Samset’s view, the recent findings should not be taken as a green light to ramp up controversial geoengineering efforts to spray aerosols into the atmosphere, a prospect he likens to Russian roulette. “In Russian roulette, you know there’s a bullet in there,” Samset told Yale 360. In the case of geoengineering, “there might not be a bullet, you might be lucky. But would you count on it? The precautionary principle argues against it.”

Yale Environment 360: With these aerosols, is particle size important?

Bjørn Samset: Yes, it is. The thing that connects all aerosols is that they are all of a size that is relevant for interaction with sunlight. The reactions with sunlight — the scattering of sunlight which leads to a cooling effect — become stronger as the aerosols grow, at least up to a certain size. For the sulfate aerosols, for instance, they tend to grow in humid air as water molecules and droplets tack onto them. The longer they are in the atmosphere, the stronger their effect becomes. There is a time element with aerosols after they are emitted, and that is where some of the detailed science is going at the moment — into tracking the evolution of these particles in the air over time.

e360: How does the presence of these particles impact climate?

Samset: They act as mirrors or as miniature clouds, and they reflect the sunlight back into space. So if the earth was surrounded with these aerosols, a lot of the sunlight would reflect back out and you would get cooling. That is exactly what we see. We believe that the volume of human-created aerosols is so great that they have counteracted the effect of global warming to a certain extent. There is a kind of tug of war taking place between the warming greenhouse gases and the mainly cooling aerosols.

“It turns out we have actually been helping ourselves - we’ve been polluting ourselves toward a slightly cooler climate.”

e360: That is ironic — pollution is actually slowing down global warming.

Samset: Yes, it turns out we have actually been helping ourselves — we’ve been polluting ourselves toward a slightly cooler climate, we’ve been mitigating climate change through pollution.

e360: How big a thermal effect do anthropogenic aerosols produce?

Samset: We think that sulfate is cooling by half a degree or maybe a bit more. So this is the cause of some concern if we clean up air pollution, as we will do, then this impact on temperature will come very rapidly as opposed to greenhouse gases, where the impact of reducing them is felt much more slowly.

e360: When you say that aerosols have led to at least half a degree Celsius of cooling, that is over how many years?

Samset: That is the interesting thing about aerosols. We are used to thinking in terms of greenhouse gases, where you emit CO2, for example, and it just stays up there for a long, long time. But that’s not true of aerosols. If we were to stop emitting them today, then in a week there would be no aerosols in the sky, it would all rain out. It’s a continual process, which in a sense makes it a bit easier to treat than greenhouse gases.

e360: If we were to end all pollution today, how much more of an effect would that have?

Samset: That’s what we tried to find out in the paper that came out earlier this year — we turned off all anthropogenic aerosol emissions from all over the world. So if you removed all our emissions today, then the world would rapidly — within a year or two — warm between a half of a degree and 1 degree Celsius additionally.

“Aerosols have gone from being a perturbation to being very relevant because of our more ambitious climate goals.”

e360: We can see the cooling effect of aerosols in the atmosphere perhaps most dramatically in massive volcanic eruptions, which can alter the earth’s weather for years. What is the difference between the impact of a volcanic eruption and that of anthropogenic aerosols?

Samset: Very little, except that a powerful volcano like Mount Pinatubo will emit mostly sulfate at very high altitudes — they will go 20 or 30 kilometers straight up all the way to the stratosphere, where they stay for a long time. Many of the particles remain above the layers of the atmosphere where rain is created, so it stays there for several years.

e360: We’ve certainly known about the impact of aerosols on climate for a long time. But my perception is that there was not a lot of attention paid to them until recently.

Samset: That’s true. Some years ago we thought that aerosols were interesting for people like me who like to study them, but not so important on the global scale, because it is really the greenhouse gases that matter. And that may be true. But then the Paris Agreement came around and it looked like there was momentum to keep the world below 2 degrees C of warming. So suddenly this half to 1 degree of cooling from aerosols — that actually begins to matter a lot more in the context of what we’re aiming for. So the aerosols have gone from being a perturbation to being actually very relevant because of our more ambitious climate goals. 

e360: That puts nations in something of a bind, doesn’t it? In places like India and China, pollution is leading to hundreds of thousands of additional deaths per year. So they have a huge incentive to cut down on their pollution. Yet in cutting pollution, they are simultaneously speeding up global warming?

Samset: It’s one of those Catch-22s. They should certainly clean up their air pollution. That’s obvious, it’s an immediate concern. Not only do they have a huge incentive to cut pollution, but they are actually doing it. There was a paper that another group did before Christmas that said that sulfur dioxide emissions in China have gone down by 75 percent since 2007. You really can see it even in the satellite images. So they really are cleaning up.

e360: What impact does that have on the climate?

A thick blanket of smog over northeastern China in December 2016, the result of industrial emissions, energy generation, and crop burning.

A thick blanket of smog over northeastern China in December 2016, the result of industrial emissions, energy generation, and crop burning. NASA

Samset: That’s what we’re trying to figure out. It’s clearly been mitigating climate change in China and India, for example. The really interesting question, I think, is what the continual reduction in aerosols will mean for rainfall, for the monsoon, for extreme weather events. And there, to be honest, we really don’t know, because the climate models that we have been using thus far have been just too coarse.

e360: What do we know about the impact of aerosols more generally on rainfall?

Samset: Several studies have shown that there has been a drying trend in the monsoon since the 1950s. And that seems to link to this cooling from aerosols. This hasn’t been published yet, but there is a study that shows that Asia would have gotten wetter over the past 100 years if it weren’t for the aerosols. We see from ground observation that large parts of China have gotten drier, and we’ve seen a slowdown of the [South Asian] monsoon.

e360: So now that aerosols are being reduced, can we expect things to get wetter?

Samset: That’s right, the drying trend has come as a result of the aerosol emissions that have increased. So when you remove the aerosols, you heat up the earth’s surface, increase evaporation, and get more rainfall. So much of Asia will get more rainfall, which to a certain extent may actually be good news, because these are regions that often have problems with accessibility to fresh water.

The other side of that is there will be greater likelihood of extreme events. There are regions that are susceptible to extreme rainfall already, there are landslides and mudslides and many people killed every year. We need to step up the effort to protect against that kind of damage.

e360: You speak of the regional impacts of aerosols. Aren’t the effects global?

Samset: For CO2, we see global effects. For aerosols, the regional effects are more profound because of the fact that they get removed from the atmosphere very efficiently. Think of a handful of sand that you pick up on the beach — gravity draws it down very quickly. This happens to aerosols as well. They are heavier than air so they fall down. As soon as there is a rain shower, it washes the air of aerosols. So there are all these processes that rapidly remove them.

“At some point you are going to want to stop this geoengineering and then all the problems are going to come back more or less immediately.”

e360: Some people have argued that your latest findings are an argument for geoengineering. Do you see it that way?

Samset: I don’t. Geoengineering is one of those things that looks nice on paper, it looks nice in climate models because it encourages us to think outside the box by simulating climate engineering experiments. But the one takeaway from all those studies is that unexpected things happen when we do this. My favorite example was a paper that tried to see if we could save the polar ice cap by emitting sulfur aerosols over the North Pole, and they found large impacts of that on the South Asia monsoon system because it changed the whole circulation system of the atmosphere.

And there is always the termination problem — at some point you are going to want to stop this geoengineering and then all the problems are going to come back more or less immediately. So my feeling after researching this for some time is that the uncertainties are so large that I just can’t advocate it.

However, it is certainly something that we do need to look into and research. Because the question is being raised by more and more planners and politicians — is there a Plan B? Probably the answer is “No, there is no Plan B.” But we need to be able to give that answer in a proper way, not just say, “No, I don’t think so.”

e360: At the moment would it be fair to say that doing geoengineering with aerosols would be something like playing Russian roulette with the climate system?

Samset: Somewhat, yes. It is a rather tough way of putting it, but you introduce the possibility of totally unexpected climate impacts. In Russian roulette you know there’s a bullet in there. In this case, there might not be a bullet. You might be lucky. But would you count on it? The precautionary principle argues against it.