Every cow and one out of three humans breathes out methane.
The gut microbiome is the key to understanding why and how to change it.
During San Francisco Climate Week we took a cow burp methane sensor to the Gigaton Salon to find out: how much methane do humans breathe out? We measured 57 people and one out of three (33%) of them breathe out methane! Which as it turns out the medical literature agrees - 33% of humans breathe out methane1! My favorite moment, was when my friend Sarah Golden, the Executive Director of the Verge Sustainability Conference got the “high score” breathing out of 50ppm. That’s as high a value as we measure inside of dairy barns! So what’s with all the hot air!?
(p.s. If you’re looking for why livestock methane is important, check out my first post.)
Before you get too concerned about the greenhouse gas impact of your digestive tract - humans exhale much less methane than cattle. Researchers have done the math, and the global impact is at least 100x smaller.
The climate impact of human breath breath is tiny, less than a percentage of livestock methane burps. This makes sense as cows regularly burp out 100x Sarah’s high score.2 However, the variability is enormous - >25x between the lowest and highest humans we detected. If everyone exhaled methane like Sarah, we’d be seriously talking about mitigation strategies. And while all cows exhale a lot of methane, it’s also very variable between cows. If you measured two different cows on planet earth, you might find one is exhaling 5x the methane as the other. So what explains all that variability? And what can we do about it?
First off - we know methane exhalation can be much lower. Anything that eats grass has a special digestive system to break down the tough biopolymer cellulose. Unfortunately, the chemical environment needed to break down cellulose is the perfect low oxygen environment for methane production. But both kangaroos and cattle eat grass, but kangaroos exhale less than one-tenth the emissions of cattle per kilogram of food they eat. By the way, when we talk about methane emissions from cattle, to give a fair comparison we always divide it by how much they eat or how much milk or meat they produce. That’s because methane comes from digesting food, so the more food digested, the more methane produced.
In lay terms, what causes methane production is a combination of cattle (or human) genetics, gut microbiome genetics, and the food eaten.
For nerds like me:3
On the methane per milk measurement, we see a factor of 10 between the worst and best cows, as shown in the y-axis of the figure above.
Within a single farm, we see a 2x difference between the heavy exhalers of methane and the light exhalers.
We see a factor of 1.5 to 2 difference between high forage (grass) and low forage diets.
We see substantial differences in the gut microbiomes of low and high methane emitting cows.
Let’s take a human lens on that first. Methane exhalation is used by medicine as a test for digestive issues. For example, people who are intolerant to the lactose exhale methane if they drink milk. And people with the gut disorder Small Intestine Bacterial Overgrowth (SIBO) exhale it as well. But there’s quite a lot of mystery to it - some people just breathe out methane. Inside of your gut is a garden of microbes, an ecological community known as the gut microbiome. These microbes do all the heavy lifting of digestion and have enormous and mysterious impacts on health, and intelligence. For example, a near-diabetic friend of mine got a gastric bypass surgery to reset his insulin. It worked - and unexpectedly after a lifetime of asexuality he felt sexual attraction for the first time.
Cattle that eat tough-to-digest grass release much more methane - 2x to 4x as much - than those eating grain like corn. This leads to a learning counterintuitive to many people. Most cattle methane comes not from animals in feedlots or dairies eating a diet high in easily-digestible corn, but from cattle eating grass out in pasture or rangeland. The mitigation solution is to tune what the cattle eat - plant better lower-methane diets for them in pastures and feed lower methane diets for them in dairies.
Genetics is the next lever for mitigating emissions, which can be done in livestock through selective breeding. Researchers in New Zealand have shown selective breeding can reduce methane emissions in sheep by at least 13% and work is progressing on getting similar results in cattle. It’s a complex lever - animal genetics influence the gut microbiome, with these sheep showing markedly different microbiome properties. The immune system regulates the populations of the microbes in it, like a gardener pulling out weeds. And human genetics influences that immune system.
More on the immune system - it develops a template of which microbial species it prefers in infancy. For that reason, livestock researchers are showing that early interventions in baby cattle (calves) can lead to long-term reductions in methane long past infancy. These interventions would be incredibly cost-effective and scalable - because they’d only need to be done once. And human medical researchers are working towards ensuring that every infant has the best microbiome for its life-long health and happiness. The technology to make these edits is advancing rapidly. The Innovation Genomics Institute, whose leader Dr. Jennifer Doudna won the Nobel Prize in 2020 for the development of CRISPR genome editing, is working to re-engineer the gut microbiomes of cattle to reduce their methane. In humans, they’re working to cure childhood asthma.
We understand enough of this methane variability to work on solutions for them, which groups like the Global Methane Hub and the Bezos Earth Fund are actively funding today. Yet always, we need more creative ideas for funders to fund based on the science of methane variability. We need to understand that variability better, which means science funding to tease apart the fundamentals of methane and the microbiome. Much of the current work in livestock methane mitigation came out of a paper by Dr. Sinead Leahy mapping out the biochemical steps of methane production. Each step in that process is an opportunity for a targeted intervention. Finally, we need the tools that make science and solution development as easy and inexpensive as possible, so that good ideas and knowledge are the limiters. That means lower-cost and complexity measurement tools for methane as well as those that tease apart the underlying biology that drives it.
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Doc Brown
Gratitudes:
I am grateful to the Gigaton Salon organizers - Harvest Thermal, Impulse Labs, and StarShot Capital, especially Zoe Samuel, the master planner of the most entertaining event at SF Climate Week.
I am also grateful to: Emma Kulow and Sarah Golden for letting me use photos of them; Emma Kulow & Daniel Goodwin for providing feedback on drafts; Frankie Myers & Evan Abel for working on this hardware with me; Ladan Amouzergar, Stephanie Geerlings, Joe Rollin, & Kaizo Naka for helping with the booth.
The medical threshold is 2ppm. All people exhale an almost undetectable 100ppb. Our sensor (Axteris Compact LGD CH4) rated to detect 1ppm methane signals, seemed to detect 200 ppb methane signals, though this may be interference from water vapor or carbon dioxide in the breath.
An average of 0.5% is very high when you consider that at 4.5% methane, air is explosive. Fortunately, evolution abhors an explosive cow.
Professor Gerald Sussman said in class at a MIT about a particularly nerdy point, “For those of you into this sort of thing, this is the sort of thing you’re into.” This is the Tao of nerdiness.