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Should we be worried about Earth getting warmer if we cut coal power?

Coal plants, despite some cooling effects from their complex mix of pollutants, are a major contributor to global warming. If we shut plants down, there might be some extra warming at first, but overall, temperatures will cool.

 

December 14, 2020

Wait a second—how could the Earth get warmer if we cut coal power? Coal plants, like all fossil fuel plants, emit carbon dioxide (CO2), which is a greenhouse gas that traps heat in our atmosphere. So cutting coal power should help stop global warming… right?

It will—but when we get into the details of how coal power changes temperatures, it gets a little more complicated. Coal power doesn’t just emit carbon dioxide. It also emits fine particles called aerosols. And aerosols’ effects on climate change are not as straightforward as CO2’s.

“Generally, coal power produces two kinds of aerosol emissions,” says Dr. Chien Wang, an atmospheric chemist at MIT’s Joint Program on the Science and Policy of Global Change. “One is called black carbon, or soot.” Soot helps warm the Earth: its black color absorbs heat and can contribute to melting ice.

The second type of aerosol emission, sulfates, has a cooling effect. Water droplets attach to the sulfates, easily condensing into clouds. Those clouds then reflect the sun’s heat into space, lowering temperatures on Earth.1 Sulfates usually form clouds about a mile above Earth’s surface, where they can cool the Earth by a lot.

And this means that shutting down coal plants cuts not only CO2, but also soot and sulfur particle emissions. “If you cut coal burning, indeed you reduce CO2 emissions, which reduces heating,” says Wang. “But at the same time you cut [sulfate] aerosols, which otherwise would have cooled the climate system.”

So which changes temperatures the most—CO2 or sulfate? It depends on the timeframe you look at. 

As Wang notes, CO2 has a long lifetime: it can last in the atmosphere for hundreds of years. Sulfate aerosols leave the atmosphere much more quickly, after just a few days. Why does this matter?

Here’s one way to think about it: imagine you’re cooking a bowl of soup over a campfire. The CO2 emitted by coal plants is like the fire’s fuel. Fuel burns for a long time, and makes the soup hotter and hotter over time. Similarly, coal plants are emitting more and more CO2, causing temperatures to climb.

The sulfates are like ice cubes thrown into the soup. The ice cubes lower the temperature of the soup, but they don’t last very long. By the time you’ve added new ice cubes, the old ones have already melted. In the same way, sulfates don’t last long in the atmosphere. Even though coal plants add new sulfates to the air constantly, the old ones are leaving the atmosphere just as fast.

Now imagine you stop tending the soup altogether. With no more ice being added, the soup gets warmer quickly. It takes a lot longer for the fire to die down, since it still has a lot of fuel. But since you’ve stopped adding fuel, over time, the soup will eventually cool.

So if we shut down coal plants, the “ice cubes” of the atmosphere—the sulfate aerosols—will disappear quickly, since the coal plants aren’t emitting them anymore. But even though the “fuel”—CO2—is no longer being produced, we will have to live with the heating effects of past emissions for much longer. Only as the existing CO2 slowly leaves the atmosphere will temperatures fall again.

There’s another twist in this story. Wang says that we are unsure exactly how powerful the cooling aerosols from coal plants are. This is because there are many other aerosols, including natural ones like sand and dust, that also help form clouds and reflect sunlight. Whether human-made aerosols do a lot more cooling on top of natural aerosols, or just a little, is a question researchers like Wang are still trying to answer. What’s not up for debate is the fact that greenhouse gases like CO2 heat the Earth—definitely more than sulfates are cooling it.

Aerosol emissions from coal plants don’t only affect global temperatures. They are also harmful to our health; they can travel to our lungs and cause diseases. This gives us two good reasons to cut emissions from coal power: because it heats the Earth, and because of air pollution.

So, while the effect of burning coal on the climate is complex, Wang is clear that sulfates are not a good reason to keep using coal power, with its mix of greenhouse gases and other dangerous pollutants. “In particular in places where air pollution causes haze, I think [countries] have no choice but to cut coal power,” he says. “But it will serve the best purpose for the community if policymakers realize these additional climate effects.” In other words, it’s useful to know how sulfate aerosols affect the climate, but it doesn’t change the basic fact that coal power is warming the Earth.

 

Thank you to Bruce Parker of Alexandria, Virginia, for the question.

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Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International license (CC BY-NC-SA 4.0).
Footnotes

1 All clouds affect the Earth's temperature by reflecting light, but exactly how they affect temperature can vary depending on height, time of day, and other factors. MIT's Today I Learned: Climate podcast has an episode on clouds that explores this phenomenon in more detail.

Blisse Kong
by Blisse Kong, MIT Climate Portal Writing Team
Headshot of Dr. Chien Wang
featuring guest expert Dr. Chien Wang, an atmospheric chemist at MIT’s Joint Program on the Science and Policy of Global Change
Related MIT Groups
More Resources for Learning
MIT Climate Portal: "How much global warming has been hidden by the cooling effect from sulfur produced by burning coal and oil?"
MIT Joint Program on the Science and Policy of Global Change: "How might a steep decline in aerosols impact the climate?"
NASA: "Just 5 questions—aerosols"
Intergovernmental Panel on Climate Change: "Clouds and Aerosols" (Report)
International Energy Agency: "Coal 2023" (Report)
Topics
Atmosphere
Climate Modeling
Fossil Fuels

Want to learn more?

Listen to this episode of MIT's "Today I Learned: Climate" podcast on clouds and climate change.

Dan Cziczo: [00:00:00] The clouds that are in our atmosphere now are not the same ones as hundreds of years ago before industries really started to develop. As we, as humans, put more and more particles into the atmosphere, we're making more and more sites on which clouds form. It's changing the precipitation of the planet.

Laur Hesse Fisher: [00:00:22] Thanks for joining us on Today I Learned Climate, where you learn about climate change from real scientists. I'm Laur Hesse Fisher the MIT Environmental Solutions Initiative. In our last episode, we spoke about condensation trails, better known as contrails, those lines that look like clouds that trail behind airplanes and how they actually warm the planet. But, what about natural clouds? Do they warm the planet, too? What do they have to do with climate change? To help us better understand this, I spoke with a clouds expert at MIT's Earth, Atmospheric and Planetary Sciences Department.

Dan Cziczo: [00:00:59] My name is Dan Cziczo. I am an Atmospheric Chemist. I have a joint appointment in civil and environment engineering and I've been at MIT for eight years now.

Laur Hesse Fisher: [00:01:09] Professor Cziczo studies how humans have changed the composition of clouds and what that means for our planet. Yeah, you heard me correctly. Human activity has changed clouds. That is a crazy concept. Professor Cziczo has been studying this since the 1990s.

Dan Cziczo: [00:01:27] As part of my PhD, we started looking at the impact that humans could have on particulate matter, on clouds in the atmosphere and how that might participate in our understanding of climate.

Laur Hesse Fisher: [00:01:38] Particulate matter is going to be really important in this episode. It's important to understanding how clouds form and how they work. So, basically, particulate matter is the collection of extremely small particles in our air and our atmosphere.

Dan Cziczo: [00:01:52] You might think of that dirty truck that's in front of you on your way into work and it's belching smoke out. So, that would be a type of human particulate matter, what we would call anthropogenic particulate matter. And, of course, there's also natural sources for the particles that are always around us.

Laur Hesse Fisher: [00:02:08] That's like sea salt, dust from dust storms, ash from fires. These are all examples of particulate matter that occurs naturally.

Dan Cziczo: [00:02:16] And so, our environment, the environment around us is always this complex mixture of natural particles and human-made particles. So, we think about dust storms as being something natural that's always happened and that's true, but we've also holed up ecosystems, grasslands, and turned them into graze lands or into farmland. So, when the wind comes by, dirt and soil that would have otherwise been held by those root systems of those plants aren't there anymore because of human activities. So, as humans, what we've actually done is we've unintentionally increased the amount of these natural particles in the world.

Laur Hesse Fisher: [00:02:53] So, there's this natural particulate matter that's just a normal part of the earth's cycle and there's more of it because of human activity. There are other unnatural particles, like from car exhaust, that we've added that's never been there before. Okay, so what does this have to do with clouds?

Dan Cziczo: [00:03:12] They don't just sort of form out of water vapor. They actually condense in a little particle. And so, as we as humans put more and more particles into the atmosphere, we're making more and more sites on which clouds form. Well, what does that mean? It means that each one of the droplets that forms on all of those new particles is only going to be about half as large, so they're not going to weigh as much. That means that they're not going to start raining as quickly.

Laur Hesse Fisher: [00:03:37] What makes this even more interesting and even more complicated to study is that some particles are better at forming clouds than others are. Some particles are better at making the ice crystals that become snow and some create clouds that reflect more light than others. These particles all interact with each other and their environment. So, they behave differently depending on what other kinds of particles are around and also what the climate is like where you are.

Dan Cziczo: [00:04:06] So, you might put that one particle that we're talking about in the air around Boston and it might have one effect, but if you put it around, we were talking about Los Angeles, it might have a completely different effect because the temperature is different. The relative humidity is different. The environment around it is different. Then, we could take that specific particle that we're talking about and put it in another location around the planet, over the poles or over the equator and it's going to act completely differently.

Laur Hesse Fisher: [00:04:33] The reason why this is so important is because this is what Professor Cziczo and other scientists and engineers need to know to understand how these particles impact rainfall.

Dan Cziczo: [00:04:44] One of those complicated factors that we're sort of trying to tease out right now is where are we going to see less rainfall? Where are we going to see more rainfall? Where are we going to see more snowfall? Where are we going to see less snowfall and so on.

Laur Hesse Fisher: [00:04:58] So, how do you even study that? Are you actually looking at the composition of the atmosphere or you have, I don't know, labs where you're recreating artificial environments?

Dan Cziczo: [00:05:10] Yeah, both of those things actually. We do two types of studies. One is we make clouds and the other one is that we find clouds. So, we have a couple of strategies of doing that. Probably the most fun is that we get on research aircraft and they belong to folks like NASA or NOAA or the Department of Energy. We put our instruments on them. We try to go out, we try to sweep up those droplets and ice crystals and then figure out what the particles are that are inside. Once we've done that, then we go back to the laboratory and we have a series of cloud chambers in our lab where we can manipulate temperature and relative humidity in a very controlled way. And then, be able to figure out like if we tweak particles in the future, if human industry acts differently, how is that going to affect climate?

Laur Hesse Fisher: [00:05:51] But, there's something else at play here. In our last episode, we talked about how contrails warm the planet because they absorb more heat than they reflect. For those who missed that episode, here's a 10 second summary. Contrails absorb heat, radiating off of the earth, like how a jacket keeps in your body heat. And, at the same time, it also acts like a shade, reflecting the sunlight that would have normally warmed the earth. So, the question is, do real clouds act the same way?

Dan Cziczo: [00:06:20] Oh, that's a great question. Contrails are a fascinating topic and they're very unique in the sense that they actually warm climate. They're one of the few clouds that net warms climate as opposed to cooling it. What's the reason for that? It's because they tend to be very high in the atmosphere. They tend to be so high up that the amount of sunlight that they reflect back into space, the cooling effect that they have is actually overwhelmed by the amount of heat that's trying to make it off the surface of the planet that they trap.

Laur Hesse Fisher: [00:06:53] Okay, so because contrails are so high, they act more like a jacket than like a shade. But, the real clouds floating below cool the planet because they're able to reflect much more light and provide more shade. Okay, but wait a minute. If clouds help cool the planet, then couldn't that help with reducing global warming?

Dan Cziczo: [00:07:15] Oh, boy. I think we might need two or three more hours of podcasts to talk about this. When I say that particles and clouds cause some amount of cooling that's less than the warming we've experienced, it won't be a huge leap for people to say, "Well, if particles and clouds create a little bit of cooling, why don't we put more particles in or make more clouds and then we can offset all of that warming." Unfortunately, for those people that are thinking about that, I don't want to burst your bubble, but somebody has already thought of that. It's a topic that is often called geoengineering.

So, this is the idea of manipulating the climate of the planet around us for an end result, ultimately trying to keep the temperature close to what it is now or what it has been in the past. There's a whole number of side effects that go along with those. Some of those side effects are as bad or worse than the temperature changes that we're seeing in the planet around us. If we were to actually try to augment or increase the amount of particles, we might have adverse effects on health of people.

Laur Hesse Fisher: [00:08:22] And on coral reefs and marine life, which are hurt by increased CO2, no matter how much sunlight is coming in. That's another episode we'll have to do.

Dan Cziczo: [00:08:33] The "solution" might not be as bad as even the problem. And, I'm not sure that folks would want to make that trade. The truth of the matter is that we, as scientists, have for over a hundred years now, understood the impact the greenhouses gases are going to have around the planet. This was not a surprise. This was nothing new. As scientists, we knew that this was a real situation and something that was coming for a long period of time. There's really no questioning that or what's going to happen moving into the future.

But, the one thing that I would like to get across to the folks is that there is some uncertainty in that. So, is the planet going to be one degree warmer or three degrees warmer. Those are very different planets moving into the future. So, bear with us while we're trying to figure that out. We're not going to give you false information, but what we're going to try to do is be as up front as possible about what the range of possibilities of the future are and how we might adapt to that, moving into that future.

Laur Hesse Fisher: [00:09:32] So, while scientists are clear that human-caused climate change is happening, they're still trying to figure out how fast this is going to happen and what the impacts are going to be. The question about what we do now, which future we prepare for, given the range of possible futures that we might experience because of climate change, well, that's not up to science. It's up to us.

Dan Cziczo: [00:09:58] We, as scientists, are highly certain that the temperature of the planet has risen and will continue to rise. The amount that is rises is very secondary to the fact that it is rising. Uncertainty is not a call for inaction. The less CO2, the less greenhouses gases that we all can produce on an individual level, as well as a societal level, is really important right now. So, the choices that we're going to make now are going to have ramifications for the next generation, the generation after that, really, hundreds and even a thousand years into the future.

So, it's something that we have to start having frank discussion about. Not just sort of reading the catastrophic news articles and things like that, but really thinking about what we're going to do about energy usage, moving into the future; you know, what world we want to leave the generation to inherit.

Laur Hesse Fisher: [00:10:54] So, what can you do as an individual? Well, there's a lot. But one of the reasons that we created this podcast series is because we wanted to help you better understand what's going on with climate science and climate change and how it all works so that you can better share and open up conversations with other people. So, share this podcast with someone in your life. Share with them something that you learned about climate change because of this. We hope that will help open up a conversation, so that we can have these frank discussions about what we can collectively do. I want to thank Professor Dan Cziczo for speaking with me and to you, for tuning into Today I Learned Climate, brought to you by the MIT Environmental Solutions Initiative.

You can find other climate change 101s on our website, tilclimate.mit.edu. That's tilclimate.mit.edu. What do you want to know about climate change and how it will affect us? Do you still have a lingering question from one of our episodes? Let us know by tweeting #tilclimate. That's tilclimate or sending an email to climate@mit.edu. Thanks so much and see you next time.

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