Researchers this week released two debate-stoking new studies about methane, the powerful, climate-warming gas that scientists say promises an expeditious target for controlling global warming in the coming decades.
One study, published in the journal Nature Wednesday, found that the extraction of fossil fuels since the industrial revolution has released more climate-warming methane—as much as 40 percent more—than scientists previously thought, a startling number that could drastically upend currently agreed-upon global emissions targets in the Paris climate agreement.
The other study, published Thursday in Science, found that "minimal" methane was emitted from permafrost and geologic seepage as Earth was emerging from the last ice age, a finding that suggests that less methane than scientists thought might be emitted in coming decades as the polar regions warm.
Both reports are likely to trigger more debate about the sources of methane and what's to blame for them.
Methane is the second-most influential greenhouse gas, after carbon dioxide, but has a much greater warming effect over a shorter period of time, meaning that cutting methane emissions is an especially effective and expedient way of slowing climate change.
Though it's clear that methane concentrations in the atmosphere have doubled since the industrial revolution—and are rising—researchers have not pinned down how much methane comes from natural geologic sources, like seepage from ocean vents or mud volcanoes, and how much from burning fossil fuels.
Fossil Fuel Emissions Vastly Underestimated
In the Nature study, the researchers looked at ice core samples from pre-industrial Greenland and air samples from the Antarctic. They analyzed the samples' isotopic fingerprints to determine how much of the methane originated from biological sources, like wetlands, and how much from natural geologic—or fossil—sources. They found that the natural geologic sources emitted about 5 million tons of methane. The estimate currently used by scientists, which is based on a different measurement method, puts the figure at 50 million tons.
"That's the major new measure," said Benjamin Hmiel, a postdoctoral researcher at the University of Rochester who led the study. "We're not saying there's a change to total methane. We're saying that biogenic is a larger slice of the pie and the fossil slice is smaller. And within our data we only start to see a growth in fossil methane around the end of the 19th century, coinciding with industrialization."
That finding suggests that emissions from fossil fuel extraction are vastly underestimated—by 25 to 40 percent, the authors say—because only man-made emissions from fossil fuel extraction can explain the difference.
"The paper deepens a scientific debate, started three years ago, about the magnitude of global methane emissions from natural geologic seeps, and in extension, that of the fossil fuel industry," explained Stefan Schwietzke, a scientist and methane expert with the Environmental Defense Fund. "The heart of the debate is about the validity of emission estimates from two very different types of measurements with their unique strengths and weaknesses. To understand the emission magnitudes, both methods need to be reconciled."
At least one prominent methane expert has already challenged one of the study's primary conclusions.
Daniel Jacob, who leads the atmospheric chemistry modeling group at Harvard University, said that the study's finding that previous estimates of geologic methane were too low is "an important result" and "will greatly help me and others interpreting observations of atmospheric methane to infer global and regional methane fluxes."
But, he said, Hmiel and his co-authors go too far when they say that post-industrial methane emissions from fossil fuel production are correspondingly higher. "I totally disagree with this inference," Jacob said. "The natural geological source is irrelevant for these estimates."
Jacob explained that, in his modeling, he would instead lower overall methane estimates from all sources.
An Analog for Current Warming?
On Thursday, another study, also led by a University of Rochester postdoctoral student, was published in the journal Science. It also looked at ice core measurements, but from the last deglaciation, beginning around 20,000 years ago.
"In some ways, that period is an analog for current warming," said Michael Dyonisius, the study's lead author. "In the last deglaciation, there's an increase of greenhouse gasses that are clearly responsible for taking the Earth out of the ice age. The feedback is the same. There's more greenhouse gas emissions and the Earth's becoming warmer."
Dyonisius and his colleagues looked at isotopes of methane in ice core samples and found that methane emissions from ancient permafrost and hydrates—large deposits of methane under the ocean—were relatively small, about 19 million tons a year. They said that means that future methane emissions from these sources are likely to be lower than many researchers have projected.
"We show that if there's low methane emissions from old permafrost during that period, then likely the permafrost in the coming decades would also not emit a lot of methane," Dyonisius said.
But he noted that permafrost thawing and releasing methane will continue to be a problem in the overall greenhouse gas budget of the atmosphere.
"We're not saying permafrost melt is not a big deal," Dyonisius said.
He added, "The main difference is that in the last deglaciation we went from a cold period to a warm period—the Holocene. But now we're going from a warm state into an even warmer state."
One prominent methane researcher disagreed with the paper's conclusion that methane emissions will be minimal in coming decades, noting that most projections say it will rise significantly.
"The warming that's going to happen this century should induce permafrost thaw. It's already happening," said Katey Walter Anthony, a methane specialist with the University of Alaska-Fairbanks. "I don't agree with their statement. There's uncertainty, but there's going to be methane released in the future."
While the papers may stir conversation about past and future methane sources, researchers—both involved in the studies and not—warn that the goal of reducing methane remains crucial.
"The contribution of fossil fuel methane emissions to the increase in methane concentrations over the past century is large and reducing emissions will reduce warming," Schwietzke stressed. "This debate does not change that reality."
A subhead in an earlier version of this story mischaracterized the findings of one study. The study found that fossil fuel-related methane emissions have been underestimated, not overestimated. In addition, the article mischaracterized the status of the lead author of the Science study and, in one of several times he was mentioned, misspelled his last name. Michael Dyonisius is a doctoral student, not a postdoctral student. Also, the figure 19 million tons was incorrectly used to describe an estimate of future methane emissions from ancient permafrost; the figure refers to emissions in Earth's last deglaciation.