Keeping Score On Climate: How We Measure Greenhouse Gases : Short Wave Host Emily Kwong wants to keep an eye on her carbon footprint. Most of it consists of greenhouse gas emissions from driving her car or buying meat at the grocery store. But it's not so obvious how to measure those emissions, or how factories, cargo ships, or even whole countries measure theirs.

Enter: NPR science reporter Rebecca Hersher. Together, Rebecca and Emily break down how greenhouse gas emissions are tallied ... and why those measurements are so important in figuring out who's responsible for cleaning up.

What should we measure next? Email the show at shortwave@npr.org.

Keeping Score On Climate: How We Measure Greenhouse Gases

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EMILY KWONG, HOST:

You're listening to SHORT WAVE from NPR.

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REBECCA HERSHER, BYLINE: Hey, Emily.

KWONG: Hey, Rebecca Hersher.

HERSHER: How do you get around? Like, do you drive? Do you take the bus, walk?

KWONG: I run. I run everywhere - up and down the streets, all around...

HERSHER: (Laughter) Sure.

KWONG: ...D.C.

HERSHER: Sure. Sure. Yes. You're in amazing shape. You run everywhere.

KWONG: (Laughter) No. No, no, no. I'm really not. It's not even humanly possible to run everywhere I'd like to. So I usually Metro. I take a lot of public transit. Sometimes I drive, especially when I come to visit you and say hello. What do you do?

HERSHER: I like to bike, but if I'm being honest, I also drive a lot. And I have a gas car, so I think about climate change a lot when I do that.

KWONG: But, you know...

HERSHER: I know.

KWONG: ...That's how you get around.

HERSHER: But when you drive, because you do drive sometimes, do you ever wonder, like, how much carbon dioxide is this car releasing?

KWONG: Yeah. Actually, there should be a word for this anxiety, like your carbon-footprint anxiety. It mostly happens to me when I order something online. You know, I see a lot of stuff, and then I'm like, ooh, Emily, don't buy that. Shop local. What are you doing? I also experience this when I buy meat at the grocery store. All I can think about is the methane emissions.

HERSHER: OK.

KWONG: But I don't want to give up meat, either. I mean, there's just so many forms of greenhouse gases that are inextricably linked to how we live.

HERSHER: Yeah. Totally. And we, like, humanity - large we - need to know exactly how much greenhouse gas pollution is being released. Like...

KWONG: Yeah.

HERSHER: ...You need that information if you want to predict how quickly climate change will happen in the future.

KWONG: You make good points. It's true. But it's causing me to think about this differently. I mean, greenhouse gases, we can't see them. How do we even measure all of them accurately?

HERSHER: Dude, I was hoping you would ask me that. I'm going to tell you, today on the show, how we measure greenhouse gas emissions all over the earth. From cars and trucks and ships and forests and power plants and cows and fields of wheat, emissions are everywhere, and measuring them can be a real bear.

I'm Rebecca Hersher.

KWONG: And I'm Emily Kwong. You're listening to SHORT WAVE, the daily science podcast from NPR.

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KWONG: OK. Let's not dance around this. Some greenhouse gases are contributing to global warming more than others, right? Carbon dioxide is the big one, but others play their part, too. You've got methane, nitrous oxide, a whole bunch of industrial gases. It's kind of a soup. So, Rebecca, my first question about measuring these is if you want to figure out how much of this stuff is being released into the atmosphere, where do you even start?

HERSHER: Well, the first step is figuring out where they're coming from. You know, what is releasing them?

KWONG: Yeah. So separating out, like, cars versus trucks versus trains.

HERSHER: Yeah. Or even a little bit more broadly, like, if you're trying to get the big picture. So transportation - that would include everything you just listed - versus power plants versus agriculture.

KWONG: OK. All right.

HERSHER: And some of those things are easier to measure than others, right? Like, let's take the easier things first. These are things that are concentrated sources - stuff like tailpipes, exhaust pipes in general, smokestacks on factories. I actually talked to, arguably, one of the world experts on this. His name is Kent Johnson.

KENT JOHNSON: I'm a research faculty here at the University of California, Riverside. Most of my time is - it's been spent studying emissions from vehicles - yard tractors, trucks, cars, pretty much anything that has an emissions source, even hamburgers. You name it. If it burns and it has an emissions, we measure it.

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KWONG: OK. So, Kent, good on you. He's the guy you call when you want to know how much CO2 your hamburger grill is releasing.

HERSHER: Yep. Or your brand-new sportscar or the trucks that lug things around at your warehouse. And basically, you know, not to oversimplify a career's worth of work, but he and his team figure out how to attach a sensor to the exhaust pipe of whatever it is, and then they just measure what comes out. Voila. Easy.

KWONG: Yes, easy. But it only tells you about emissions from one hamburger grill or power plant or, at best, that one type of car. So how do you add...

HERSHER: Right.

KWONG: ...It all up to know how much is being emitted by all of the cars on the road or all of the power plants?

HERSHER: Yeah. This is where computer models come in. So people like Kent Johnson, they measure lots of tailpipes - right? - over and over and over. And other researchers are estimating how many cars and trucks are on the road and how far they're driving. And then they're plugging all of that information into a computer model to estimate total emissions from, in this case, vehicles.

KWONG: Can computer models really capture all of it, though? They can make estimates, but it can't possibly measure everything - every transportation emission - which is data we need, given that transportation, you know, is one of the single largest sources of greenhouse gases.

HERSHER: Yeah, it's one of the big two, actually. The other is power plants. So, yeah, how do we know if our computer models are correct? I had the same question.

KWONG: Yeah.

HERSHER: And the answer is actually really neat because it's really simple, at least in some cases. So the best way to explain how it works is with ships.

KWONG: OK. Cargo ships, container ships, ships that bring me the things I spontaneously buy on the internet.

HERSHER: (Laughter) They also transport a lot of other things, though, not just the things you bought.

KWONG: (Laughter).

HERSHER: For example, they transport oil and gas, so they're burning oil to move oil from A to B. But global shipping, it's a big source of greenhouse gas emissions. These ships are huge. And let's just say you want to measure exactly how much carbon dioxide is being released by these huge ships. What do you do?

KWONG: You fix them with a sensor on their ship stack.

HERSHER: Yeah. See; now you're thinking like our expert, Kent Johnson. You could do that, but it'd be really hard. You need to get permission from all the companies that own these ships. You need to physically attach sensors in a way that can withstand long ocean voyages. I talked to Bryan Comer about this. He studies shipping emissions at the International Council on Clean Transportation.

BRYAN COMER: It's a pretty difficult environment on board a ship. It's hot. They're out at sea. They're moving back and forth.

KWONG: OK. But then how do we know what ships are emitting? We got to measure it somehow.

HERSHER: Yeah. So there's an alternative. And, Emily, it is so freaking elegant. You can basically just calculate the emissions based on the fuel.

KWONG: Oh, yeah.

COMER: Carbon dioxide is really easy, as long as we know how much fuel the ship is consuming, and we know, with pretty good accuracy, what the carbon content is of different marine fuels.

KWONG: So they're basically using the fuel supply for ships to calculate the emissions, measuring greenhouse gases with math.

HERSHER: Yeah, exactly. And you don't need to measure what comes out of the ship's exhaust pipe; you can just calculate how much carbon was in the fuel that it burned.

COMER: For marine fuels, it usually ends up that for every ton of fuel that you burn, you emit about 3.1 to 3.2 tons of carbon dioxide.

HERSHER: And then this is where it gets really cool. You can scale it up, right? You take the number for each voyage, for each ship, and then for all the ships you can estimate pretty accurately how much carbon dioxide the entire industry is releasing.

COMER: So overall, the global shipping industry is emitting about a billion tons of carbon dioxide each year.

HERSHER: Elegant, right?

KWONG: Math. Saves the world again.

HERSHER: (Laughter).

KWONG: So those are some of the ways scientists measure greenhouse gas emissions from tailpipes and smokestacks, things that are directly hooked up to fossil fuels. But, Rebecca, you said those concentrated sources are the easier ones to measure. There are other sources of greenhouse gases that are - I don't know. They're, like, less concentrated. They don't...

HERSHER: Right.

KWONG: ...Spurt out of a exhaust pipe. I'm talking about methane emissions on farms - right? - or carbon dioxide from logging camps, food waste sitting in a landfill.

HERSHER: Right.

KWONG: How do we measure emissions from those places?

HERSHER: Well, I won't lie. It's a lot harder, basically, because of exactly what you said. They're more spread out. So there's...

KWONG: Yeah.

HERSHER: ...No fossil fuel shortcut, like with ships. Like, a field of wheat isn't burning a specific amount of diesel, right?

KWONG: No.

HERSHER: But the land can be a big source of greenhouse gas emissions, especially if humans are disrupting it by turning over the soil and cutting down trees and raising cows, for example.

KWONG: Right. Cow burps are a huge source of methane, in particular.

HERSHER: Yeah. Great fact. Terrifying fact. Another one - if you turn over the soil to farm it or you cut down trees, you're releasing carbon that's stored in the ground or getting rid of plants that were sucking up carbon dioxide from the air. So I talked to Ben Houlton about this. He's an expert. He works at Cornell University.

BENJAMIN HOULTON: We know that today, agriculture, whether it's application of fertilizer or management practices or livestock, coupled with the clearing of land, contribute about a quarter - that's 25% - of global greenhouse gas emissions.

KWONG: If I could let out a long wolf whistle, I would. That is a lot.

HERSHER: (Laughter) Oh, you're bad at whistling.

KWONG: A quarter - I'm very bad at whistling. But how does he even know this number? How do you measure something like agriculture when it's so spread out?

HERSHER: Yeah, there are a bunch of methods, actually. So at the smallest scale, you can just use a bucket.

KWONG: What do you mean, a bucket?

HERSHER: I mean, it has a fancier name, but it's basically a bucket.

KWONG: Oh, OK.

HOULTON: Imagine a two-gallon bucket that you turn upside down, and you put that over the soil. And what you're trying to do is capture the gases that are being emitted from the soil, from the little, microscopic creatures that live in the soil, from what's happening with the plants.

HERSHER: So you take what you've collected, and you measure how much carbon dioxide or other greenhouse gases were released. And then you extrapolate. You say, this is how much was released in one square foot, let's say. This field is 500 square feet - math, math, math. The end.

KWONG: But as accurate as this may be, it's kind of also impossible to scale that up to the entire Earth. Like, who has time to put a bucket in every field during every harvest season? There's no way.

HERSHER: Yeah. No. These buckets are not going to cut it. So the buckets are only one tool. There are also these towers that can measure gases rising from larger pieces of land using infrared sensors. And those can cover an area about 10 football fields, according to Houlton. But to really get a global picture, you have to combine those smaller measurement techniques with satellites.

KWONG: Oh, that makes sense 'cause if you really want a picture of the earth, satellites are the way to go.

HERSHER: Yeah, but to be clear, you really need both. You need satellites telling you, for example, how much land is covered in trees, and then you need an instrument on the ground that tells you how much carbon dioxide is being released or absorbed by...

KWONG: Right.

HERSHER: ...Those trees. So one measurement is really only useful with the other one.

KWONG: In combination - OK. So, Rebecca, we've covered the major sources of greenhouse gases, how those measurements are made. And I do see how that would add up to a pretty good estimate of total greenhouse gas emissions for the whole planet. But here's the thing. At the beginning, we were talking about my emissions, like how much carbon dioxide is released when I drive or take the Metro or turn on a burner for tea. Are we closer to knowing this answer on an individual level?

HERSHER: Sort of. Like, for your car, you could do the same little hack that Bryan Comer is using for ships but for your personal vehicle, right? You could add up how much gas you buy and burn every year, look up the conversion for how much carbon that releases, get a general sense. But you're right. These collective sources - they're harder. Like, you live in an apartment building, right?

KWONG: I do.

HERSHER: So you and your neighbors share the cost of heating that building. And sometimes you take the subway or a bus, and that's before we even get into thornier things, like the carbon footprint of that meat that you're eating or what you wear.

KWONG: So you're saying it's pretty difficult to measure my carbon footprint?

HERSHER: Yeah. And it's just not going to be very accurate. But we...

KWONG: Yeah.

HERSHER: ...Can say general things. Like, the United States, as a country, is responsible for the most total greenhouse gas emissions over time. And if you compare the people who live in the U.S. to the people who live in other countries that have emitted large portions of the total greenhouse gases, the U.S. actually has the highest average per-person emissions, right? So it's safe to say that your carbon footprint probably isn't great.

KWONG: Yeah. I get it. My used hybrid electric car won't save me now.

HERSHER: (Laughter) No. no.

KWONG: The point you're making is important, that we in the U.S. are big-time emitters. But how do we compare all this internationally and get a sense of that? Like, how do we measure U.S. emissions versus China versus Brazil?

HERSHER: Yeah, that's a good question. And it's definitely a measurement question, right? How do we compare countries to each other? And this is an area where measurement is a total mess.

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HERSHER: Because it depends on who you ask. Like, if I get on a plane in the U.S. and fly to China, which country counts those emissions? Or what about a company that's headquartered in the United States but has factories around the world? If that company says it's going to cut its emissions by 10%, what does that mean? Which country gets to count that reduction? How do you make sure it doesn't get counted twice? This, honestly, is one reason that international climate negotiations take so long. They're talking about measurement, which is another way of talking about fairness. And we are still a long way from fairness when it comes to greenhouse gas emissions on Earth.

KWONG: Right. Climate change, it's just such an existential threat to the entire planet, but there are big actors, and these measurements are pointing fingers towards those actors and who is ultimately responsible for that and who should, perhaps, pay for the cost of building a new, cleaner world.

HERSHER: Yes, totally.

KWONG: Becky, thank you so much for breaking down this process for all of us so we know how we arrive at the numbers we have.

HERSHER: Thank you.

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KWONG: This episode was produced by Margaret Cirino, edited by Gabriel Spitzer and fact-checked by Rachel Carlson. Our audio engineer was Stu Rushfield. Gisele Grayson is our senior supervising editor. Beth Donovan and Anya Grundmann are our senior director and our senior vice president of programming. Thanks for listening to SHORT WAVE, the daily science podcast from NPR.

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