Microbes, not fossil fuels, biggest source of rising atmospheric methane levels

Scot Miller is an assistant professor in Johns Hopkins University's Department of Environmental Health and Engineering whose research focuses on greenhouse gas emissions and air pollution. His Greenhouse Gas Research Group uses observations collected from airplanes, towers, and satellites to estimate emissions across individual states and continents.

A recent study blames rising levels of climate-changing methane on microbes in wetlands, cow stomachs, and select types of agriculture. Microbes are tiny organisms that produce methane in low-oxygen environments, like animals' stomachs and wetlands. Scientists say that warming temperatures may be causing wetlands and fields to release more methane, raising concerns about a feedback loop that could drive further climate change. Here, Johns Hopkins environmental engineer Scot Miller discusses the implications of a new study on the source of climate-changing methane.

Image caption: Scot Miller

Image credit: Will Kirk / Johns Hopkins University

What is methane and why is it important to understand its sources?

Methane is the second-most important long-lived greenhouse gas in the atmosphere, and its impact on the climate is second only to that of carbon dioxide (CO₂). Human-caused CO₂ emissions are primarily caused by the combustion of fossil fuels. Methane has numerous human-caused sources, including natural gas leaks, coal mining, wastewater treatment plants, landfills, rice production, and livestock. Cows' special digestive systems cause them to burp methane into the atmosphere. Wetlands also release methane when leaves or branches from plants fall to the ground and decay anaerobically.

There are numerous state, federal, and global efforts to reduce human-caused methane emissions. However, methane emissions are challenging to quantify due to the complexity of sources. For example, leaks from natural gas systems are largely caused by a small number of problem facilities, making it difficult to pinpoint or measure emissions. Wetland emissions are equally hard to track, as scientists disagree on exactly where wetlands are located across the globe and the amount of methane they release.

Were you surprised by these findings? Why or why not?

I'm not surprised by these findings. The history of methane in the atmosphere is complex, with levels increasing in the global atmosphere through the '80s and '90s, leveling out in the early and mid-2000s, resuming growth in the late 2000s, and accelerating in recent years. Over the years, there have been many different hypotheses to explain these trends, such as the U.S. hydraulic fracturing boom in the late 2000s.

Isotope measurements, which track heavier and lighter carbon atoms, have been key to unlocking answers to this question and tell a somewhat different story. These measurements generally show that recent methane increases are being driven by microbes—either by microbes in the stomachs of cows and sheep, or microbes that break down plant material in wetlands.

However, some additional factors complicate this story. The chemistry of the global atmosphere changed in 2020 as people stopped driving cars and trucks during the COVID-19 pandemic. As a result, less methane was chemically destroyed in the atmosphere in 2020 relative to previous years, as cars release nitrogen oxide (NO_x). NO_x has an impact on the amount of hydroxide radicals in the atmosphere, and hydroxide radicals destroy methane. Some existing studies argue that these chemical changes contributed to the surge in global methane levels in 2020.

What are the implications of this new understanding of where most methane is coming from?

I think recent studies on methane emissions highlight both how far the scientific community has come and how difficult methane emissions are to track. Human-caused CO₂ emissions are arguably much easier to track, in part because many countries have good records of coal, oil, and natural gas production and use. Methane emissions are arguably more difficult to pin down, both because there are a myriad of different emissions sources and because the factors driving those emissions sources can be complex.

Overall, recent studies drive at a really important question—whether recent emissions increases are caused by natural sources or by human-caused sources. Natural sources are arguably beyond our control and may change in the future due to climate change, while there are clearer pathways to reduce human-caused emissions sources.

For example, methane emissions from the natural gas industry are often the result of malfunctioning equipment, and one way to reduce these emissions is through more proactive leak detection and repair programs. As another example, there are technologies available to capture methane from landfills instead of emitting that methane into the atmosphere.