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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.

 

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. You can submit your own question to Ask MIT Climate here.

 

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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.