A Methane Mystery
Caption: One of the methane-emitting Davis cows on a sweltering summer afternoon, 2016. (Photo credit: James Yoon)
Davis has a special place in my heart. It’s a quaint town in California nestled between Sacramento and the Bay Area, and it’s well-known for the intensely flat and bike-friendly UC Davis campus. But the real attraction in Davis is the cows. Bike from the Veterinary Medicine buildings to the rest of the UC Davis campus, and you’ll see the cows in a rectangular pen, munching hay all day. Many afternoons, I’d gaze through the fence and watch the cows vacantly stare back.
Inside each Davis cow is a factory that ferments their food into methane, a greenhouse gas thirty times more efficient at trapping heat than carbon dioxide. As methane builds up inside of their digestive tracts, they have to expel it out, one way or another. Most of it is through cow burps.
Cow burps are just one of the many processes that can change the amount of methane in the atmosphere and thus how quickly Earth warms. Atmospheric methane has generally increased over time, which we often attribute to more methane emissions—like more cows. However, the amount of methane doesn’t always grow at the same rate. For instance, methane growth slowed down in 1992, which coincided with the fall of the Soviet Union. More surprisingly, methane concentrations stopped growing in 2000 before shooting up again in 2007. Did we stop emitting methane during those seven years, or did something else cause this methane plateau?
The amount of methane in the atmosphere is like the water level in a cow’s drinking trough. At any moment, there is a certain amount of water in the trough, but the water level can change over time. The water level increases when we pour more water into the trough, but it decreases when the cows drink the water, or when the water evaporates during those sunny California afternoons. Analogously, methane concentrations are controlled by both how much methane we emit and how quickly methane breaks down. It’s difficult enough to understand methane emissions: for instance, wetlands are a large natural methane source, but we can’t force bogs to keep logs of their emissions as we do with human sources. In addition to that formidable challenge, we also need to know how quickly methane breaks down in the atmosphere, which is easier said than done.
Methane only lasts around a decade in the atmosphere, since it is broken down by the intensely reactive hydroxyl radical (OH), also known as the detergent of the atmosphere. OH “cleans” the atmosphere of methane and eventually converts methane into carbon dioxide, which thankfully transforms our favorite greenhouse gas to a less potent one. Just like how the amount of detergent you use determines how many clothes you can clean in one cycle, the amount of OH determines how quickly methane breaks down. If there’s more water vapor in the atmosphere, methane degrades faster because water vapor creates more OH. If we drive fewer cars, like during the COVID-19 lockdowns, methane degrades slower because car emissions also create OH. The atmosphere is a complex system: you change one part, and the whole system shifts.
This complexity is why we need detectives to solve this methane mystery. We have some ideas on what’s causing methane to vary over time: the COVID-19 lockdowns and increased wetland emissions may have caused 2020’s rapid methane growth, and the 2000-2007 methane plateau was possibly due to changes in OH. But as we collect more data and fit puzzle pieces together, what we think we know about methane may change. Who knows: maybe it was the cows and their methane burps all along.
James Yoon is an atmospheric chemist who is fascinated by how plants can affect the atmosphere and the air we breathe through the gases they emit. To study this question, he uses computer simulations and satellites to better represent these gases and the chemistry they undergo in the atmosphere. Through this work, he hopes to better describe how biology and air quality interact.