Scientists Puzzled by Soaring Global Methane Levels

Methane concentrations in the Earth's atmosphere are soaring—and the exact causes of the "frightening" increase are puzzling scientists.

In April 2022, the National Oceanic and Atmospheric Administration (NOAA) reported that concentrations of the gas averaged 1,895.7 parts per billion (ppb) over the past year, a new record.

In fact, the NOAA report showed that 2021 saw a rise of 17 ppb: the largest annual increase in atmospheric methane levels since systematic measurements began in 1983.

"Methane concentrations are increasing at a frightening and totally unexpected rate," NASA atmospheric scientist Benjamin Poulter told Newsweek.

"In 2020 and again in 2021, methane concentrations grew at a rate that was more than double the average over the previous decade."

Scientists say the rapid rise in atmospheric methane has significant implications because it is a potent greenhouse gas and can contribute to global warming.

While methane lingers in the atmosphere for only around 10 years or so—far shorter than carbon dioxide—it has a warming potential roughly 30 times that of CO2.

As a result, soaring atmospheric methane concentrations is "serious and a major global problem," that could put at risk the goal of limiting global warming to to 1.5–2 degrees Celsius above pre-industrial levels, Euan Nisbet, an Earth scientist at Royal Holloway, University of London, in the United Kingdom, told Newsweek.

"Methane is a major anthropogenic greenhouse gas, and its unexpected recent rise is arguably the largest deviation from the hopes of the Paris Agreement (note the recent CO2 rise was also bad). If we don't get methane under control, the Paris Agreement will fail."

Since the era of the Industrial Revolution, methane concentrations in the atmosphere had been steadily increasing in a trend "clearly" driven by fossil fuel emissions—i.e. the burning of coal, gas and oil—according to Nisbet.

But in the 1990s, this increase began to slow and levelled off entirely between around 1999 and 2006. Then, in 2007, atmospheric methane levels began to spike mysteriously again and have been increasing ever since in a trend that scientists are struggling to properly understand.

This has led researchers to ponder whether human activities or climate-change feedbacks—or indeed a combination of the factors—are responsible for the rise.

The cause of the change has been studied in recent years using various methods, ranging from taking measurements from aircraft and satellites, to running computer models.

But these studies have come to different, and sometimes conflicting, conclusions citing factors ranging from changes in tropical wetlands to increases in fossil fuel or agricultural emissions, among other factors—such as declining methane sinks in the atmosphere—as the main drivers.

According to a study published in the journal PNAS, the contradictions in these different hypotheses have arisen because our current monitoring capacities cannot definitively link recent methane variations to specific sources.

"Our monitoring systems are totally inadequate for the challenge we face," Poulter said.

Methane comes from both natural sources and human activity, with current estimates indicating that approximately 600 million tonnes are emitted into the air every year.

Natural sources account for around 40 percent of total methane emissions, with the primary one being wetland ecosystems.

Meanwhile, human-driven (or anthropogenic) sources, which account for the remaining 60 percent, including fossil fuel emissions, livestock, landfills, agriculture, and the burning of biomass.

Methane sources can also further be classified by emitting process into three categories, each of which have both anthropogenic and natural components.

These categories are biogenic (methane from the decomposition of organic matter in zero oxygen conditions, with sources such as the breath of ruminant animals, manure, rice agriculture, landfills, wetlands and sewage); thermogenic (methane originating from the breakdown of organic matter over long periods of time due to heat and pressure in the Earth's crust, with sources including geological gas seeps and fossil fuel exploitation); and pyrogenic (methane produced by the incomplete combustion of biomass with sources such as wildfires, crop waste fires and the burning of biofuels).

While scientists are still unsure about what exactly is driving the post-2007 methane spike, one line of evidence is helping to shed light on the question.

Sources of methane—a compound which consists of one carbon atom bonded to four hydrogen atoms—can be identified by their characteristic geochemical "signatures".

Carbon itself comes in two stable forms, or isotopes. The vast majority is carbon-12 (12C) while around one percent is carbon-13 (13C).

Methane emitted from fossil fuels and fires tends to have slightly more 13C, in contrast to biogenic methane, which usually contains slightly more 12C.

The term "biogenic methane" refers to methane emissions from living or biological organisms, which are produced by certain microbes in a process called methano-genesis. These microbes can be found in the guts of ruminant animals, like cows and sheep, and the soils of wetlands, among other locations.

Studies looking into bubbles trapped in ice cores show that the proportion of carbon-13 in atmospheric methane increased in the two centuries prior to 2007—in a trend driven by fossil fuel emissions, according to a report of which Nisbet is an author.

But some recent research has shown that as methane concentrations started growing again in 2007, the proportion of 13C began to fall as 12C increased. This trend, according to Nisbet and other experts, provides strong evidence that biogenic methane emissions from wetlands, cows, sewage, or landfill—or several of these factors combined—have been primarily fueling the post-2007 increase rather than fossil fuel or fire emissions.

"We don't fully know what is driving the recent increases and there are likely multiple causes," Nisbet said. "But since 2007 the carbon-12 percentage content of methane in the air has been increasing. That suggests much of the new growth is from biological sources such as cattle, natural wetlands and landfills."

Moreover, measurements from a collaborative global network run by the NOAA indicate that much of the growth has been in the tropics and sub-tropics, suggesting that biological emissions from wetlands and/or farming in these regions are rising.

"Much of the growth is in the tropics but we don't now if it is cattle or wetlands or very likely both," he said.

"There have been some recent studies on wetlands in Africa and Bolivia with huge emissions. And the cattle populations of India and Africa are huge, with extremely rapid human population growth, which gets a very large amount of its food from ruminants on pastureland. Landfill emissions are also biological and likely growing very fast in the tropics though declining in Europe and the United States."

"The fossil fuel emissions remain very big too, but are not necessarily growing fast as leak control gets better, but fires and landfills are increasing in the tropics," he said.

However, Nisbet said it is also a possibility that the capacity of the atmosphere to degrade and destroy methane through certain chemical reactions is also declining. "Maybe all factors are acting," he said.

Xin Lan, an atmospheric scientist at NOAA's Global Monitoring Laboratory in Boulder, Colorado, and colleagues have estimated that around 85 percent of the increase in methane emissions since 2007 has come from microbial, i.e. biological, sources. The remaining 15 percent of the growth came from fossil fuel sources, according to Lan and her team.

(It is important to note that biogenic methane emissions can still come from anthropogenic sources, such as cattle farming, for instance. In fact, one study published in the National Science Review found that the main factor driving the post-2007 methane growth is human activity—primarily, increased emissions from agriculture, landfills and waste, as well as from fossil fuels.)

As methane levels rise, scientists are now questioning whether or not a climate change-related feedback loop has been triggered where global warming leads to higher methane emissions, which feeds further warming. Some already believe there is a chance that this process is underway, although the answer is not yet clear.

"Is warming feeding the warming? It's an incredibly important question," Nisbet told Nature magazine. "As yet, no answer, but it very much looks that way."

Lan previously told Newsweek that natural methane production is accelerated by rain and varying temperatures, which will become more common as climate change results in many areas of the world becoming warmer and wetter.

"From natural processes, we know that wetland methane emissions are sensitive to change in precipitation and temperature," she said. "Methane production from microbes increases with increases in global temperature which is driven by long-term greenhouse gas emissions. More atmospheric methane, in turn, can further warm up the earth. That's the feedback loop we are referring to."

As the planet warms, the tropics are predicted to expand, meaning tropical agriculture will likely become more productive.

"An increase in human agriculture will likely occur with more ruminants, more crops, more crop waste fires, more use of fertilizer running off into wetlands, all leading to methane emission," Nisbet wrote in an article published in the journal Philosophical Transactions A.