Air Pollution and SRM

Pete Irvine: [00:00:00] Welcome to Climate Reflections, the SRM360 podcast where we discuss sunlight reflection methods, or SRM: ideas to reduce the impacts of climate change by reflecting sunlight away from the earth. I’m your host, Dr. Pete Irvine, and I’m a climate scientist who has been studying SRM since 2009.

Did you know that some forms of air pollution have actually helped reduce global warming?

This raises an uncomfortable question. Has clean air legislation – so important to protecting human health and ecosystems – inadvertently accelerated global warming? In today’s episode, we discuss the links between air pollution, climate change, and SRM. To be specific, we focus on stratospheric aerosol injection, or SAI, the SRM proposal to reflect sunlight by adding tiny reflective particles high in the atmosphere, in an effort to cool the planet without the same health impacts as air pollution.

A side note to our listeners, if you’re not familiar with SAI, you can learn much more about [00:01:00] how SAI would work and some of the geopolitical concerns it raises by checking out our previous podcast episodes or by visiting SRM360.org. Today we’re focusing on air pollution. How would SAI differ from air pollution and what could happen when the sulfate particles used for SAI eventually fall back to earth?

We spoke with two experts about air pollution, its impact on the climate, and how SAI would differ from it. They were Dr. Daniele Visioni, an assistant professor of Atmospheric Sciences at Cornell University, who is an expert on SRM and its impacts, and a frequent contributor to SRM360, as well as Oliver Morton, science writer and a senior and briefings editor for The Economist.

First, a very quick history of air pollution. More than 2000 years ago, writers in the ancient world had already acknowledged air pollution and its harmful impacts. Even before the Industrial Revolution in Europe, there were treaties on air pollution as well as evidence of air pollution preserved in the geologic record.

Modern efforts to clean the air started in the 1950s following the London Smog event of December, 1952, which killed [00:02:00] about 12,000 people. In the same year, scientists began characterizing smog in Los Angeles. Here’s Oliver Morton on the health impacts of air pollution.

Oliver Morton: I think anyone who’s ever been in a smoggy developing country city, or in older days in a smoggy developed country city, knows why it matters.

There are bits of nastiness that will float in the atmosphere and they get into your lungs and it can feel uncomfortable, and it can also lead to various forms of disease.

Pete Irvine: The deadly 1952 London Smog event, which inspired clean air legislation in the UK was specifically caused by sulfate particulate pollution.

Sulfate particles, a byproduct of the burning of dirty fossil fuels, have substantial impacts on human health. Sulfate pollution also has impacts on the environment, specifically acid rain. A hot topic in the 1970s and eighties when dead lakes and dying forests near and downwind of industrial centers began to emerge.

Daniele Visioni has more.

Daniele Visioni: Anybody who is [00:03:00] sort of in their forties and fifties, so not me, uh, but have been told a lot of time, well, what about acid rain? Nobody in their thirties think about acid rain because it’s a problem that in the Global North, we sort of solved in the eighties and nineties, there was a strong effort to reduce sulfate emissions and then reduce sulfate deposition that mostly comes out in terms of acid rain.

Pete Irvine: The health and environmental impacts of sulfate pollution were well understood by the 2000s, but another impact was becoming increasingly clear as a scientific understanding of climate change evolved. We turn back to Daniele Visioni for more.

Daniele Visioni: These aerosols of which we emit so much from the surface and that are so harmful to people and environments, they have reflected part of the incoming solar radiation, and they have prevented warming from being much, much higher than it could have been.

Pete Irvine: In 2006, Dr. Paul Crutzen, a Nobel Prize winning atmospheric chemist made the unorthodox suggestion to add sulfate particles high in the atmosphere to reflect [00:04:00] sunlight and slow global warming. When Crutzen suggested this, he faced immediate backlash from his colleagues for potentially undermining efforts to curb CO2 emissions, but he was addressing what he perceived as a conundrum for policymakers.

Here’s Oliver Morton again.

Oliver Morton: The basic argument in Crutzen’s 2006 paper is that efforts to reduce air pollution are reducing the effect specifically of sulfate particles on the climate because there are fewer sulfate particles. And so by removing these sulfate particles, the various regulatory authorities involved are saving lots of lives, which is good, but they are also increasing the amount of warming that is actually felt, because some of the warming from greenhouse gases has historically been offset by cooling, by air pollution. And so given this, Paul saw this as a dilemma for policy makers, that’s what the paper’s actually called. It’s called ‘Albedo enhancement [00:05:00] by stratospheric sulfur injections: a contribution to resolve a policy dilemma?’ Note the uptone at the end, there’s a question mark.

Pete Irvine: At the time in 2006, the World Health Organization estimated that there were about 500,000 deaths attributable to air pollution each year, but the real number was likely about 10 times higher. Crutzen saw clear benefits to cleaning up the air people breathe and offered an option he estimated would retain the cooling effects of sulfate pollution without compromising people’s health.

Here’s Oliver Morton again to explain Crutzen’s proposal.

Oliver Morton: He says that if you put the sulfur in the stratosphere rather than in the lower atmosphere, the troposphere, which is where all the sort of like mucky mixing around of everyday matter occurs, then the sulfate particles will last in the atmosphere much longer because the stratosphere doesn’t have these turbulent flows and it’s, it’s very layered.

That’s why it’s called the stratosphere, because it has this layered [00:06:00] stratographic vibe to it. And so if you put the sulfur in the stratosphere, you can use a lot less sulfur, but because it lasts a lot longer, you can actually get the same amount of cooling as the much larger amount of sulfur that you are seeing in the troposphere.

And so Paul’s suggestion was that one way to think about a possible way forward was to offset the removal of lots of tropospheric sulfur with the injection of a small amount of stratospheric sulfur that would have nothing like the same deleterious effect on ecosystems or human health, but would have more or less the same effect on masking some global warming.

Pete Irvine: Sulfate particles are just one component of air pollution.

Other pollutants of major concern for human health include soot, ground level ozone, carbon monoxide, and nitrogen dioxide. While many of these pollutants trap heat and warm the earth, sulfate particles reflect sunlight and [00:07:00] have a cooling effect. Crutzen and others hypothesize that reducing sulfate pollution would reveal significantly warmer temperatures due to climate change, and subsequent studies have shown this to be the case. In 2023, the IPCC estimated that air pollution’s net cooling effect masked about 0.4 Celsius of warming. In 2020, the International Maritime Organization or IMO put into effect new fuel rules for the shipping industry. These rules called for reduction in the amount of sulfur and fuel used in shipping. What impact did this have on climate?

Here’s Daniele Visioni again.

Daniele Visioni: So in 2020 there has been a pretty significant drop in sulfate emissions from international shipping. We can observe that. There’s now a lot of research also observing the fact that this did create a reduction in the amount of clouds, especially over the North Atlantic.

And so there is now a growing body of literature that is trying to actually measure and quantify how much this 2020 IMO [00:08:00] regulation has affected global temperature. It’s kind of hard to not see the connection and say, oh, we’ve had this rule, and then after a few years when the cloud, these clouds have gone and the ocean has been able to take up more of the heat from the sun, and we see more warming, are the two things connected? Especially considering that already in 2012 there were estimates of the fact that this was gonna have a warming effect. To how much temperature effect this translate is pretty complicated. And to me, aside from debating the actual number, it is interesting what this tells us about a lot of other things about how, what this tell tells us about how do we get to a scientific consensus, how would we monitor something like this in the case of SAI, how little this thing was monitored in 2020, this the most fascinating thing to me. So, two things. The first one was that there was a very famous, well to me, very famous paper in 2013 that analyze this for the first [00:09:00] time and said, this is what would happen if you cut shipping emissions.

And they said there would be a massive air quality impact. Port cities would be very happy. We would see a bit of warming, and then towards the end they said you could achieve the same air quality impact without losing the cooling potential. By only turning on this, these scrubbers, these filters that remove the sulfate from the emissions only when you’re close to port and let the sulfate continue when you are in the open ocean where we don’t care about our quality.

They say, but we’re not gonna talk about this too much because these, these voluntary, voluntarily keeping emission is akin to geoengineering. And this is unpopular to talk about. This was 2013, it was another time. So I think that this is interesting in what it tells us about how we conceive of, uh, deliberate or inadvertent modifications of the climate.

And the other part is the fact that clearly. [00:10:00] If we had been way more concerned with this than as we are now, there would’ve been better plans to observe clouds over the North Atlantic on January 1st, 2020. We would’ve discussed this a lot more. We would’ve had modeling simulations in place to quantify this effect in much better ways.

It’s just true that people try to ignore aerosols a little bit too much because they  lead to uncomfortable questions. Uh, and the main question is, does it make sense to remove them if they contribute to warming? If warming is the, on the main thing we should be scared about for the 21st century.

And so I think that this shipping, uh, case is a clear example of all of these contradictions finally coming into full view of the public. And actually the scientific community didn’t have really great answers in terms of how to define a consensus. Why haven’t [00:11:00] we talked about this a lot more and so on.

Pete Irvine: Temperatures are continuing to rise and 2024 was the first year on record where average global temperatures were 1.5 degrees Celsius above the pre-industrial average. A recent study led by James Hansen attributes some of 2024’s leap in global temperatures to the new shipping fuel standard that went into effect in 2020 that Daniele described.

In their study, Hansen and his co-authors found that previous work had underestimated the cooling effect of aerosols and that we have yet to see the full impacts of unmasking the warming from cleaning up our shipping fuels. This is precisely the conundrum Paul Crutzen described in 2006. So how would SAI differ from air pollution?

We hear again from Daniele Visioni and then Oliver Morton.

Daniele Visioni: Cooling potential of the sulfate aerosols in the stratosphere is a hundred times higher than the cooling potential of the sulfate at the surface. So you would need actually a pretty tiny fraction to achieve a substantial cooling [00:12:00] compared to how much we’ve been emitting as humans. To give numbers, we as humanity produce every year around a hundred million tons of sulfate from human activities. And with 1 million ton, one to two, maybe three, you would cool the planet by a fraction of a degree.

Oliver Morton: Paul’s insight was that air pollution cools the world, but it does so very inefficiently.

You have to have a lot of sulfate emissions to get the amount of cooling that you see, and that amount of sulfate emissions down at the surface of the earth, combined with other things, leads to a lot of disease and mortality. If you put those same aerosol particles, those same sulfates into the stratosphere, they will have a much longer life than they do in the lower atmosphere.

They will last up there for a year or more, rather than for around a week. And so they will have be about 50 times more efficient [00:13:00] in terms of the cooling that you get per kilogram of sulfur. And if you get that sort of higher efficiency, you can think about cooling using relatively little sulfur that gets deposited all over the world rather than close to places where you are burning sulfur.

And so you could imagine massively reducing mortality without reducing the cooling effect.

Pete Irvine: The amount of sulfate needed to cool the planet is much lower if it’s added to the stratosphere, and so it should have a much smaller health and other environmental impacts on the surface. However, it would have other impacts on the stratosphere itself.

In the late 20th century, it became clear that human activities were creating ozone depleting substances, damaging the stratospheric ozone layer, the part of the atmosphere that protects the earth’s surface from the sun’s most damaging ultraviolet radiation. Starting in the 1970s, the world came together to reverse that damage by phasing out ozone depleting substances through the Montreal Protocol. Could injecting sulfate particles into the stratosphere have an unwanted impact on stratospheric ozone?

We hear [00:14:00] again from Daniele Visioni.

Daniele Visioni: Aerosols would definitely affect stratospheric ozone close to the pole. It would definitely favor more ozone depletion, which would mean that we set us back a couple of decades, at least, in terms of recovery after the, the ozone hole. At low latitudes and mid latitudes, the effect based on our climate modeling seems pretty small to be honest, and mostly positive inside, uh, in, in sign. As in it looks like there would be a little bit more ozone due to dynamical reasons. Uh, but these are probably small. But of course, at high and mid to high latitude, there would be some ozone depletion, uh, that would need to be monitored and understood.

Now, the ozone depletion is also driven by what we call ozone depleting substances, so CFCs and HCFCs. Uh, and so as the decades move forward and these CFCs go away [00:15:00] because we’re not producing them anymore, these risk of depletion would go down for sure.

Pete Irvine: Besides the risk of ozone depletion, sulfate pollution also causes acid rain.

How much could SAI add to acid rain? Here’s Daniele again.

Daniele Visioni: Of course, if you’re putting the sulfate up in the stratosphere, eventually it will come down. The big difference in this case is that first of all, you’re putting it sort of away from where people live. So once it mixes in the stratosphere, most of that would come out over the oceans because they cover most of the surface of the planet.

Uh, and also that you would need actually a pretty tiny fraction to achieve a substantial cooling compared to how much we’ve been emitting as humans.

Pete Irvine: To recap sulfate pollution reflects sunlight lowering temperatures. As this pollution is cleaned up, saving millions of lives each year, more warming is unmasked.

SAI using sulfate aerosols would require less material to produce the same amount of cooling and would likely contribute little to acid rain. However, it could delay [00:16:00] the recovery of the ozone hole by decades. So where does Paul Crutzen’s suggestion to study SAI stand 19 years later? Do the sentiments or motivations behind the subject being taboo in 2006 still shape the discussion of SRM in 2025?

Paul Crutzen believed that emissions reductions were in fact, the only way to solve the problem of climate change, but he also believed that hoping for rapid emissions cuts to resolve the issue quickly was a pious wish. How does that ‘pious’ wish look today? Here’s Oliver Morton with some final thoughts on the subject.

Oliver Morton: The people studying this at that time, and still to some extent now, were overwhelmingly natural scientists and they thought that the proper thing was for the world to be returned in some way to its natural state. So, one of the longer lasting impacts of Paul’s idea of the Anthropocene, is that, that’s not necessarily something that’s still in your hands, but people said we can’t be trying to engineer a system we don’t understand. [00:17:00] They said that to do this would be like giving methadone to a heroin addict. It just treats the symptoms. And they weren’t, I think taking as seriously as they might have done the fact that the symptoms are actually the harm being done to people. There are still quite a lot of people who don’t want to see SRM discussed at all as a policy option and have severe doubts about the wisdom of doing any research on the topic at all.

So yeah, I think there is still some distortion. Of the argument, but I think the situation is better than it was when people were saying to Paul and Steve Schneider, you just can’t publish this. Because among other things, they weren’t disagreeing that this idea was a plausible idea. They may have thought it was very ill-advised, but they were kind of saying, let’s not even think about it.

And I think that that’s in the end, an indefensible position. I [00:18:00] would argue that the dynamic is changing now largely because of the far reduced costs of solar power of batteries, and to a slightly lesser extent of wind. So you could say that we are getting towards some point of genuine inflection, but emissions are still going up.

And also the other trend that Paul was aware of is also continuing, which is the amount of sulfate pollution is going down. In general, the earth has been getting a little less bright, and part of that is because of less air pollution. I think that’s undoubted and a world that’s getting a little less bright, a little less reflective is other things being equal, absorbing more heat.

That has something to do with the accelerated warming.

Pete Irvine: In 2006, when Paul Crutzen first suggested lowering temperatures with sulfates, the world was cooler than it is now. The global average temperature was less than 0.7 Celsius above the pre-industrial average, but every year in the past [00:19:00] decade has been hotter than this.

The understanding of air pollution impacts on human health has also grown since Crutzen made his first proposal. Crutzen himself acknowledged in 2015 that he had underestimated deaths due to air pollution by a factor of about 10. We are still facing the same dilemma. Crutzen laid out 19 years ago, CO2 emissions have continued to rise, air pollution’s cooling effects continue to be lost, and global temperatures are surging. While we know much more about SAI and other SRM proposals than we did when Crutzen made this suggestion, there are still many uncertainties and risks that need to be explored.

That’s it for today’s episode of Climate Reflections. Thanks for listening. This is a new podcast and we’re hoping to build our audience, so if you enjoyed it, please do share it on social media or recommend it to a friend. If you have a question about SRM or just want to find out more, go to our website, SRM360.org. We answer questions from the audience in our monthly news roundup, so you might hear your question answered there. You can find a transcript of today’s episode with links to sources on our website, so [00:20:00] please check it out.