The following transcript has been edited for clarity and length.
Imagine you’re flying through our atmosphere in a spaceship the size of an atom. As you cruise through this tiny world, you pass more than 400,000 nitrogen molecules, 100,000 oxygen molecules, 219 carbon dioxide molecules, before you find what you’re looking for: a single molecule of methane.
Methane is one of the most powerful yet misunderstood greenhouse gasses on Earth.
To understand the story behind this potent greenhouse gas, we’re going to follow 100 methane molecules throughout their entire existence, from “birth” to “death.”
The math behind our 100 methane molecules
Our goal for this story was to give a simple, clear, and engaging overview of where methane comes from and how it impacts our planet. Rather than bog the explainer down in too many percentages, we decided to follow the path of 100 imaginary molecules of methane, using data from a variety of sources.
We used data from the 2017 Global Methane Budget, which compiles top-down (starting with known emissions and using models to determine likely sources) and bottom-up (starting with known sources and extrapolating their emissions contributions) methane estimates from a number of different sources and regions. We chose to focus on the U.S., but the report includes estimates for a number of different countries or geographical regions.
We also used data and information from the National Academies of Sciences 2017 report on U.S. methane emissions. This report, written by top U.S. methane experts, includes more detailed data and commentary on the country’s methane sources from the EPA’s 2017 Greenhouse Gas Index.
In order to fit all the methane sources into one comprehensive story, we used a combination of top-down and bottom-up approaches. For our broad categories (natural sources, agriculture and waste, and fossil fuels), we used U.S. top-down estimates from the 2017 Global Methane Budget. The report presents a number of top-down estimates, so we chose the median value for the decade of 2008-2017. This report includes similar data for a number of countries and regions, so it should be easy to replicate for other countries.
The 2017 National Academies report includes detailed bottom-up estimates for different U.S. sectors, like manure, natural gas, and landfills. In order to streamline these two data sources, we took these bottom-up estimates and scaled them proportionally to fit in the broader top-down sources.
While this mixed approach allows us to include multiple types of methane information in one central visual, it does have some limitations. Bottom-up and top-down estimates don’t always perfectly fit together. Bottom-up estimates give us extremely detailed information on methane sources, but can be prone to undercounts, overcounts, or missing unknown sources.
This approach is based on reliable sources, and works as a basic primer for U.S. methane sources — but it’s by no means a definitive source for methane emissions. Methane emissions are always changing, and — hopefully — our ability to track those emissions will keep improving.
Just like CO2, methane’s been on the rise since the industrial revolution. While the rise of CO2 has been almost entirely driven by a single source — the burning of fossil fuels like oil, gas, and coal — the story is a bit more complicated for methane. Each of our 100 molecules look alike, but they come from dozens of completely different sources, from Earth’s most ancient life forms, to our food, energy, and waste.
Like the rest of the world, methane emissions in the United States generally fall into three main categories: the natural world, agriculture and waste, and fossil fuels.
The natural world
Across the world, wetlands, tideflats, and hot springs naturally release hundreds of millions of tons of methane into the atmosphere each year. Wetlands are full of plant life. But when those plants get submerged underwater, they are eaten by a type of methane-producing microorganism called archaea.
Archaea — as the name implies — are some of the most ancient forms of life on earth. They’ve likely existed for roughly 3.5 billion years! Archaea evolved on a much harsher Earth, full of extreme temperatures and conditions — and very little oxygen in its atmosphere. Today, these ancient microorganisms still thrive all around the world, in similarly inhospitable environments that are boiling hot, acidic, or devoid of oxygen.
Archaea living throughout U.S. wetlands produce nearly all of the country’s naturally-occurring methane. Only a few come from other places like mud volcanoes and hot springs, which vent methane from deep underground. For most of earth’s history, naturally occurring methane was the only source of methane – but in the last few centuries, humans have found countless ways to add more of it to the atmosphere.
Agriculture and waste
The next source of our 100 methane molecules: agriculture and waste. Chickens, pigs, and sheep all produce some methane, but no one produces methane quite like cows.
Cows love to eat grass, which is actually quite difficult to digest. To get around this challenge, cows have four stomachs filled with millions of grass-digesting microbes — including archaea, which convert a lot of that grass into methane.
This methane escapes through farts and burps, and even dissolves into the animal’s bloodstream and escapes through its breath.
So, how do scientists measure this? One method is a technique called (I swear this isn’t a fart joke) bottom-up estimation. They start by sticking a cow in a sealed room, and measuring how much methane builds up over the course of a few days. They do this for lots of types of cows, multiply that by the number of cows in the country, and viola: They get a number! While this method is not as precise as actual samples taken from the atmosphere, it gives us detailed information that would otherwise be invisible, like how much methane comes from, say, goats or bison.
So, using bottom-up estimates, we know that around 18 of our 100 methane molecules come from livestock burps — nearly all from cows.
After farm animals have digested their food and burped all their burps, another 7 methane molecules come from their poop. This is especially true in concentrated animal feeding operations, which have so many animals that farmers rely on giant tanks or pits just to store all the poop. Of course, these dark, sealed-off pits are perfect feasting grounds for methane-producing archaea. As more farms have turned to concentrated manure systems, these methane emissions have been on the rise in recent decades.
The next step of our journey brings us from animal waste to human waste. Most of our trash is stored in big underground landfills — another perfect feasting ground for, you guessed it, archaea. Landfills have also gotten bigger and more concentrated in recent decades. This isn’t great if you happen to live near one, but when it comes to curbing methane emissions, there may actually be a silver lining. That’s because these large, centralized landfills are often required to capture their methane emissions, decreasing the amount that escapes into the atmosphere by roughly a third.
The methane that does escape still accounts for 13 of our 100 molecules. The remaining three molecules come from sources like wastewater, biomass burning, and certain crops like rice.
At last, we’ve reached our final category: fossil fuels, responsible for a quarter of our methane molecules. Fossil fuels come from ancient carbon, buried underground for millions of years.
Oil and coal deposits are usually full of methane bubbles. And then there’s natural gas, which is just a marketing term for methane. When people extract any of these fossil fuels from the ground, some of it escapes into the atmosphere, sometimes in little leaks and sometimes in big bursts. And as natural gas — aka methane — moves across our country’s massive fossil fuel network, it leaks into the atmosphere at pretty much every step.
Leaks aren’t consistent from location to location, so this is one area where bottom-up estimates fall short. Instead, scientists rely on another technique, called, well, top-down estimation. Scientists take real measurements of the atmosphere — collecting air samples from planes or towers; or using sensors on satellites. And on powerful computers, they run weather simulations in reverse, to trace the emissions back to their sources
Top-down estimates are based on actual real-world measurements, but aren’t as detailed as bottom-up estimates. Neither of them is perfect, but together they can help paint a picture of what our methane emissions actually look like.
What happens to methane in the atmosphere?
Now that they’ve been released into the atmosphere, our 100 methane molecules are busy warming the planet. In these first days in the atmosphere, a ton of methane traps 80 times more heat than a ton of CO2.
The reason why methane is so much more powerful is kind of surprising. As different greenhouse gasses build up in the atmosphere, each new molecule gets a little less efficient than the last at trapping heat. For example, CO2 was once an even more powerful greenhouse gas, but now there’s so much of it that each new molecule is a little weaker than the last. Methane, on the other hand, is pretty rare! So every new molecule packs a huge warming punch.
The high atmosphere is an inhospitable place, full of super reactive chemicals, called radicals. In less than a decade, the radicals break down the vast majority of methane, and turn it into carbon dioxide — the other really bad greenhouse gas! This second life as CO2 really matters for our molecules that came from fossil fuels, and makes these emitters especially bad.
Most of our hundred molecules were plants at some point in the recent past. And plants suck CO2 out of the atmosphere. This creates a natural cycle, where CO2 grows a plant, is broken into methane, and then is broken back into CO2, where the process can repeat itself.
But that’s not the case for fossil fuels. Fossil fuels come from ancient carbon stored in the ground. When fossil fuel methane eventually breaks down, it’s adding new CO2 to our atmosphere, causing our planet to heat up even faster! Because of this, methane from fossil fuels can warm the planet as much as 38 percent more than methane from other sources!.
After methane has broken into CO2, the molecules are slowly absorbed into oceans, fields, and forests over centuries.
Until at last, roughly a thousand years since the start of our journey, our hundredth molecule is consumed by a flower, bringing our story of the life and death — and second life — of methane to an end.