Super Pollutants: Carbon Dioxide’s Evil Cousins

Published Apr 23, 2019

Dr. Geeta Persad outlines four types of super pollutants that play a significant role in the climate change equation.

In this episode
  • Colleen and Geeta discuss short lived climate pollutants
  • Geeta explains what black carbon is and how it's different than carbon dioxide
  • Geeta talks about cow farts
  • Colleen asks whether kids can still expect to see coal in their Christmas stockings
Timing and cues

Opener (0:00-0:33)
Intro (0:33-1:58)
Interview Part 1 (1:58-11:06)
Break (11:06-11:51)
Interview Part 2 (11:51-24:29)
Sidelining Science throw (24:29-24:43)
Sidelining Science (24:43-28:03)
Outro (28:03-29:00)

Related content
Full transcript

Colleen: Geeta, welcome to the "Got Science? Podcast".

Geeta: Thank you. I'm excited to be here.

Colleen: So when we talk about how to curb the worst effects of climate change, the conversation is focused on carbon dioxide, but you study another type of pollutant that plays a significant role in the climate equation called short-lived climate pollutants, or SLCPs as they're known. So, what are short-lived climate pollutants?

Geeta: Yes. So, short-lived climate pollutants, or SLCPs, or super pollutants as they're also sometimes referred to, basically just refers to a group of climate pollutants that lasts in the atmosphere for a much shorter amount of time than carbon dioxide does, but have an outsized impact on climate warming while they're up there. So, this includes types of pollutants like methane, which people are maybe familiar with on its own as a climate pollutant, but also black carbon, which is the dark light-absorbing part of soot, tropospheric ozone, which is the component of smog and then also HFCs or hydrofluorocarbons, which are an industrial chemical that's used in refrigeration and air conditioning. So, it really...the term SLCPs or super pollutants, is really just sort of a catch-all for all of these other types of climate pollutants that we might need to think about differently in a policy context than we think about carbon dioxide.

Colleen: So, if we go through each one of them, so soot that comes from emissions from cars? Or where else does that come from?

Geeta: It's basically comes from, anytime we burn something without enough oxygen, we produce soot, and black carbon is the climate warming component of that soot. So, it can come from things like agricultural burning, so when farmers burn agricultural waste on their fields, which is something that's done in a lot of developing countries, that produces black carbon. A lot of industrial processes produce black carbon, so coal-fired power plants are a source, also anything that's diesel burning will produce a lot of black carbon, so, for example, diesel vehicles, and also especially freight vehicles, and off-road vehicles that tend to use diesel and have relatively less filtering, those will be a source of black carbon. Wildfires are a source of black carbon, cookstoves. So, basically anywhere where we're burning stuff and you see smoke, you see black carbon.

Colleen: So, black carbon is a little different in the way it contributes to climate change and so, how is it different?

Geeta: Yeah. Pollutants like carbon dioxide and methane and HFCs, all warm climate by trapping heat radiation coming off of the earth's surface, but black carbon actually warms the climate by trapping sunlight, so because it's dark, it's absorbing sunlight in the same way that a dark jacket would absorb sunlight. And, that actually creates a lot of differences for how those climate impacts actually manifests for people and for plants, and also for how we designed policy to mitigate it. So, because black carbon is trapping sunlight, that actually means that where it's emitted matters for what its overall climate impact ends up being. So, black carbon emitted from China might not have the same climate impact as a black carbon particle emitted from the U.S., and that's very different from something like carbon dioxide or methane where a molecule of that anywhere has the same climate impact.

Colleen: So, if I understand that, so the difference is in the sun? What is the difference between...

Geeta: Yeah. So, the difference comes from the fact that the...because the black carbon is interacting with sunlight and that's how it warms the climate system, how much it warms the climate system is going to depend on how much sunlight is present, wherever it's being emitted, but also how much sunlight the earth would be reflecting if that black carbon wasn't there, which also depends on where that black carbon is emitted. So, for example, if you have a bunch of black carbon sitting over a glacier, that's going to have a much bigger effect on climate warming because the glacier would otherwise be reflecting a bunch of sunlight back to space. Whereas, if you're putting that black carbon out over the ocean, which is already dark, you're going to have a much smaller impact on climate, because you're going to be absorbing less sunlight that would otherwise have been reflected back to space.

Colleen: So, what does this mean in terms of how we make policy?

Geeta: So, a lot of the policy that we've designed so far is based on carbon dioxide, and that allows us to assume that if someone mitigates carbon dioxide in California, it's going to have the same benefits for it's climate as if someone mitigates carbon dioxide in New Jersey, but with black carbon, that's actually not the case. The climate benefits or the climate penalties that you get from mitigating or emitting this is going to be different depending on who is emitting this, what time of year they're emitting it, what the source of the black carbon is. So, black carbon that's coming out of the tailpipe of a car is going to have a different lifetime in the atmosphere and a different impact on climate than black carbon coming out of the top of a smokestack. So, it requires you to have a lot more granularity in how you design your policy to make sure that you're actually counting your climate impacts correctly. Whereas, for carbon dioxide, you really don't have to care about where these pollutants are coming from, and so, that means when you're setting a target for reducing your black carbon emissions, you have to have some understanding within that of where those reductions are coming from, who is going to be reducing them, and it also really crucially means that you can't trade-off black carbon reductions against carbon dioxide reductions because they're just impacting the climate system in a totally different way.

Colleen: So, it's apples and oranges...

Geeta: Exactly.

Colleen: ...and you can't...Is it doable to regulate black carbon?

Geeta: Yeah, it's definitely doable to put a cap on how much emissions we're going to produce, and to target all of the different sources that we know are producing a lot of these emissions, because at the end of the day, we know that once this stuff gets up into the atmosphere, it has a strong warming effect on climate, and that's something that we know we want to prevent. And so, even though it may be really difficult to create an exact formula for what the overall climate impact of emissions from each of these different sources is going to be, we do know that we need to reduce emissions. And so, I think designing a policy that incentivizes reducing those emissions is pretty easy, we just don't want to try to do it in the same way that we have designed our carbon dioxide mitigation policy.

Colleen: So, as long as we don't just set carbon dioxide aside, this is a positive change, that we could see results really quickly?

Geeta: Yeah, basically, as soon as you stop emitting something like black carbon, you get it out of the atmosphere and you start seeing the benefits in terms of reduced climate warming, and you also start seeing benefits with air quality, and with asthma, and with human health. So, there are really a lot of ways for this to be win-win, as long as we can make sure we're increasing our ambition across the board.

Colleen: And then, what about methane?

Geeta: Yeah. So, methane is one component of natural gas, so when we think about some of the industrial sources, a lot of them come from natural gas infrastructure, so leakage or emissions in natural gas pipelines or storage sites, but there are also natural sources of methane, so a lot of swamps produce methane. It's a byproduct of certain biological digestion processes, so you'll see this in swamps where you have a lot of decomposing matter, but you also see it in animals, so people may have heard about cow farts and methane.

Colleen: Yes. Yes, I was thinking of that.

Geeta: Yes. So, ruminants, things like cows and sheep are also a source of methane in their digestive system, so the agricultural sector is one of the big sources of human-caused methane emissions. Mostly through raising of livestock, but then also through things like rice production where you also have you created, sort of, a swamp-like environment where some of those biological methane emissions can also be produced.

Colleen: So, Geeta, are you telling me that my dog might be a source?

Geeta: You know, I don't know that anyone has assessed to the net methane emissions from people's pets.

Colleen: Okay.

Geeta: That's an interesting question. Although, dogs are not ruminants, so I'm not sure if their digestive system produces methane in the same way, or at the same rates...

Colleen: I think...

Geeta: ...I'll have to look into that more and let you know.

Colleen: I think this needs more research.

Geeta: Yes. Yes. This could definitely be a winner of an Ig Nobel award.

Colleen: Yes, that would be fun.

Geeta: Wouldn't it?

Colleen: That would be a lot of fun.


Colleen: Moving on to hydrofluorocarbons…

Geeta: Yes, they're a mouthful. HFCs are entirely manmade, they don't have natural sources, but they're used in a lot of our refrigeration, in air conditioning and insulation processes. And so, where human-induced emissions of those come from is, if you have leakage, say, in your refrigerator while it's in use, then you'll have HFC emissions into the atmosphere or another big, sort of, place in the life cycle where that happens is at end of life of something like a refrigerator when it's sitting in a landfill or something. If it's not disposed of properly, then you can get HFC leaking out of those systems and going into the atmosphere where it traps heat.

Colleen: Okay. And then, tropospheric ozone.

Geeta: Yes. Ozone is basically just O3 or 3 Oxygen. And, most people might be familiar with stratospheric ozone, which is just ozone molecules in the stratosphere, but tropospheric ozone can also form and that's when it's these ozone molecules happening in the surface layer of the atmosphere, closer to the air that we're breathing. And, it's formed from air pollutants that are produced by plants, but also by cars, and by industrial activities, and these things, sort of, all go into the atmosphere and create smog, which includes tropospheric ozone.

Colleen: Oh, okay. I see. It's smog...

Geeta: Yes.

Colleen: ...more or less.

Geeta: It's more or less smog minus a couple of things.

Colleen: So, where are the highest concentrations of these different short-lived climate pollutants?

Geeta: Yeah. So, in terms of emissions, for things like black carbon, you tend to get lots of emissions of black carbon wherever you have pretty unregulated combustion happening, so in a lot of industrial...really industrialized areas that are also developing, and so, that don't yet have very strict air-quality regulations, that's where you'll tend to get really high emissions. So, for example, places like India and China, they're really big sources of black-carbon emissions from cookstoves, from brick kilns, from cement production. And, because black carbon only lasts in the atmosphere for a pretty short amount of time, it doesn't have enough time in the atmosphere to get very far from where it's emitted, so you tend to see these concentrations of atmospheric black carbon near where it's being emitted from the surface. So, if you were to look at a snapshot of the atmosphere and see where the black carbon is located, it tends to be concentrated over these places like India and China where you have a lot of these processes, these combustion processes that are putting that into the atmosphere.

Colleen: So, what areas in the United States do we find most concentrations?

Geeta: Yeah, so historically, a lot of U.S. industrial activity has been on the coast, so in the Eastern seaboard, and then also on the West Coast. So if you look at a distribution of where emissions of these particles are coming from in the U.S., it's mostly coming from the Eastern U.S. and also from, sort of, industrial areas along the West Coast of the U.S. Agricultural emissions, you're going to get wherever you have a lot of agricultural activity happening, wherever you have a lot of livestock, so there's lots of livestock, for example, in the Central Valley of California, also in the U.S. Midwest you have a lot of sources of the, sort of, agricultural methane emissions. And then, wherever you have natural gas infrastructure, which is basically all over the U.S., you're going to get a lot of these methane emissions.

Colleen: And that, the natural gas, I know when we were talking a couple of weeks ago, you were saying that there are many places in that structure where you can get leaks.

Geeta: Yeah.

Colleen: Which is harder to, sort of, contain it, right? It's just leaking potentially anywhere from tank to...

Geeta: To use point.

Colleen: ...Yeah.

Geeta: Yeah. I think one of the big challenges that colleagues of mine that are more focused on methane-leak detection and methane observations, one of the big challenges they've found is that, we really don't have many systems in place right now to detect where methane is leaking out of our natural gas infrastructure. So, for example, there was a study done in Boston a few years ago that used infrared cameras to see where methane was leaking out of the city's really old natural gas pipeline infrastructure that was bringing natural gas into people's homes for heating and cooking, and they found all sorts of hotspots all over the city where methane was leaking out of that infrastructure. But, that kind of really detailed analysis hasn't really been done yet for the whole U.S., so most people estimate that the amount of methane that's leaking out of our infrastructure is much, much higher than estimates that we currently are thinking about when we think about our methane emissions.

Colleen: So how long does each of them stay in the atmosphere?

Geeta: This is one of the challenges of the fact that short-lived climate pollutants is a phrase that's used to dump a bunch of different types of pollutants together, so each of these types of pollutants has a pretty different lifetime in the atmosphere. So, black carbon lasts in the atmosphere for weeks. HFCs and methane will last for something like a decade, and tropospheric ozone is somewhere, sort of, in the middle on the, sort of, weeks to months length.

Colleen: So, let's compare that to carbon dioxide. How long does carbon dioxide stay, stick around?

Geeta: Yeah. So, carbon dioxide stays in the atmosphere for hundreds of years. So, you're thinking about something like carbon dioxide that sticks around for hundreds of years, versus something that sticks around for decades, versus something like black carbon that only sticks around for days. You can imagine that the climate effects of something that lasts for a hundred years versus a couple of days is going to be pretty different, but also how we might think about that in a policy context is going to be pretty different as well.

Colleen: How do we get rid of them, do they dissipate naturally or do we have to take them out by some method?

Geeta: For black carbon, it gets removed from the atmosphere naturally by either settling out, so just, sort of, gravitationally floating down to the ground or it'll get rained out, so it'll get washed out, so that's often why the air seems clearer after a rainstorm is, a lot of those particulates, those particle air pollution is getting washed out. For some of these gas-based, short-lived climate pollutants, they're basically decaying into other types of chemicals that are not as climate impactful, or they're being taken up by the sort of natural carbon cycle that we have on our planet.

Colleen: So, there's an advantage to focusing on short-lived climate pollutants because we could have a big impact?

Geeta: Yeah.

Colleen: So, tell me a little bit about...California is the first state to put short-lived climate pollutant targets into policy, so tell me a little bit about that.

Geeta: Yeah. So, short-lived climate pollutants can really help us achieve some of our climate goals in the near term, because once we stop emitting them, then they get removed from the atmosphere much more quickly, so we start seeing the climate benefits of stopping those emissions much more quickly than we would for carbon dioxide. So, states like California now have said, "Okay, we're going to set targets to reduce short-lived climate pollutants specifically by, say, in the next 20 years or so." I think the biggest thing that California has been trying to do is create alternatives, so in the agricultural sector, they've been trying to encourage dairy farmers and livestock owners to manage the manure that comes from the animals more strategically, and to do it, there are technologies now that allow you to capture the methane that's released from the manure that the animals produce, and then you can put that back into a natural gas pipeline or otherwise, try to remove it...prevent it from getting into the atmosphere. So, that's one strategy, there are also more, sort of, far-out-there strategies to try to change animal's digestive processes, so by modifying their diet to try to get them to produce less methane to begin with. That's a little bit more far out than some of these technologies that just allow you to capture the manure...or the methane from the manure.

In terms of methane coming from the natural gas sector, I mean, a big piece of reducing methane leakage from the natural gas sector is to reduce our dependence on natural gas. So, that's one way of reducing those emissions. Another is to try to prevent more of the methane leakage from the natural gas infrastructure, so there's a lot that owners of natural gas infrastructure can do to monitor their infrastructure more carefully, to catch leaks early, and to do more of the maintenance that prevents those leaks from happening. So, that's in terms of methane. For things like black carbon, the U.S. has already made a lot of progress on reducing its black carbon emissions by moving to cleaner forms of combustion. So, whether it's moving away from diesel vehicles, or putting particulate filters on diesel vehicles so that less of that black carbon goes into the atmosphere, in California, reducing diesel vehicle miles traveled is going to be a big focus, and also reducing off-road diesel vehicles that currently are pretty unregulated. And then, also trying to create incentives for people to move away from burning of agricultural waste or other, sort of, combustion like that that can produce black carbon.

Colleen: So, is there a danger that if we're focusing on the short-term pollutants that we're, sort of, kicking the can down the road and we'll ignore the bigger carbon dioxide that's going to stick around for a really, really long time?

Geeta: Yeah. There are a lot of benefits to focusing on short-lived climate pollutants now. We could prevent ourselves from crossing certain thresholds in the climate system that could have really damaging impacts on vulnerable communities. We can also get a lot of immediate air quality benefits from reducing emissions of these pollutants, so they're definitely a win-win in that respect, but I think, one thing that's become really clear over the last several years is that we really need to be mitigating everything. We can't use the fact that the short-lived climate pollutants are really attractive to mitigate and are, in some cases, easier to mitigate than carbon dioxide. We can't use that as an excuse to do less on carbon dioxide mitigation. We absolutely have to be increasing our ambition on carbon dioxide emission reductions at the same time as we're increasing our ambition on reducing short-lived climate pollutants. And so, I think one of the challenges with designing policy that incentivizes short-lived climate pollutant reductions is that, you have to design it in such a way that you don't create trade-offs for people that provides openings for them to reduce ambition on their carbon dioxide mitigation, because the short-lived climate pollutant mitigation looks easier.

Colleen: So, California is often the test bed for forward-thinking climate policies. Do you know if any other states are thinking about implementing a similar policy?

Geeta: So, hydrofluorocarbons are one place where there's been a lot of state coordination on putting regulations in place to reduce how much they're being used in refrigeration and also how carefully they're managed once they're in these products to make sure there isn't leakage. So, a few states including New York, and I think maybe Michigan, and one or two others, around the time of the Global Climate Action Summit that happened here in California last September, all announced that they were going to achieve similar regulations on reducing HFC emissions that California had recently passed and that were also passed at the international level as part of the Montreal Protocol. So, there's been more work happening at the state level to all band together and make sure that all of the states are doing their part in achieving these international goals to keep our HFC emissions from growing.

Colleen: My final question, it's specifically about black carbon. And so, will black carbon replace coal in our kids' Christmas stockings?

Geeta: Well, they're actually made up of, sort of, the same stuff. The black carbon is just what happens when the coal gets into the atmosphere after you burn it.

Colleen: The lump is much neater.

Geeta: It is, it is. It's true, and I don't...but I don't think the kids will enjoy the black carbon any more than they enjoyed the coal, in fact, it might be much worse.

Colleen: Well, thanks, Geeta, for joining me.

Geeta: Thank you very much, this was a great conversation.

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Sidelining Science: Shreya Durvasula
Editing and music: Brian Middleton
Research and writing: Pamela Worth
Executive producer: Rich Hayes
Host: Colleen MacDonald

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