Warming in the Arctic is intensifying methane emissions, contributing to a vicious feedback loop that could accelerate climate change even more, according to a new study published May 7 in Nature.
"Methane is a very potent greenhouse gas that we need to address urgently," said co-author Xin (Lindsay) Lan , a climate scientist at CU Boulder's Cooperative Institute for Research in Environmental Sciences (CIRES) . "Our study suggests that a significant portion of the recent rise in atmospheric methane originates from natural sources driven by climate change. Our emission reduction efforts need to be more aggressive."
Methane is the second most abundant human-produced greenhouse gas after carbon dioxide. But an equal amount of methane traps about 30 times more heat than CO₂ over a 100-year time frame. Methane has been responsible for roughly a quarter of the planet's warming since the Industrial Revolution.
Lan has spent the past decade tracking methane concentrations in the atmosphere at Boulder's Global Monitoring Laboratory at the National Oceanic and Atmospheric Administration (NOAA).
Lan and her colleagues at NOAA have observed a rapid increase in atmospheric methane levels in recent years. While previous studies have shown fossil fuel production accounts for 30% of global methane emissions, Lan and colleagues have noticed a steady increase in emissions from microbial sources since 2007.
These microbes, specifically a group known as archaea, produce methane as a byproduct of their metabolism in environments like wetlands, landfills and livestock's digestive systems.
Together, microbial emissions contribute to nearly half of global methane emissions, but it remains unclear which specific sources are driving this increase.
"While long-term methane trends are important to investigate, we also need to look at seasonal variations to understand how individual sources are changing and how the natural mechanisms that remove methane from the atmosphere are evolving," Lan said.
A vicious cycle
To get a clearer picture, Lan and her team analyzed seasonal fluctuations in atmospheric methane levels over the past four decades.
They found that methane's seasonal amplitude — the difference between peak and lowest methane levels within a year— has been decreasing in northern high-latitude regions, including the Arctic.
Using computer models, the team showed that this trend since the 1980s is largely a result of increased methane emissions from wetlands. Increased precipitation in the Arctic has expanded the region's wetlands by 25% during the warmer months. Rising temperatures have also been melting some of the perpetually frozen soil layer deep underground, known as permafrost, in summer.
The melted, waterlogged soils have provided ideal conditions for archaea to thrive, leading to higher methane emissions which in turn could accelerate warming further.
Scientists have long warned about such climate feedback loops, but the precise scale and speed of these effects remain uncertain. Lan said this new study added another piece of evidence that natural methane emissions have already been responding to a warming climate.
"This study, along with a few previous studies, has provided indirect evidence on potential climate feedback on methane emissions, which would be beyond our ability to control directly," Lan said.
The sharp increase in atmospheric methane and its climate feedback effects since 2007 resemble the planet's most dramatic warming events that brought past ice ages to an end, according to Lan's previous research .
"Our hope is that by rapidly reducing emissions, we can avoid triggering more severe and abrupt climate feedback that could lead to catastrophic events," she said.
Methane sponges
The team's simulations also found a 10% increase in the levels of hydroxyl (OH) radical since 1984. These radicals are highly reactive molecules that can soak up and remove methane and other air pollutants.
Because these molecules stay in the air for less than a second before they react with other compounds, scientists cannot directly measure them globally. In the past, researchers had assumed the OH levels remained constant over the years when calculating atmospheric methane emissions, but this study suggested that assumption might be wrong.
"Our result showed that we've been underestimating how much methane the atmosphere has been removing, which means that there's actually more methane being emitted than we previously estimated," Lan said.
Understanding the specific source of emission is vital in designing climate mitigation policies. While microbial emissions are responsible for most of the methane growth, human-produced methane from burning fossil fuels remains an important contributor.
"We need to aggressively cut all greenhouse gas emissions from the sources we can control," Lan said. She added that the world's permafrost currently holds at least twice as much carbon as is currently in the atmosphere. If future warming causes widespread permafrost thaw and releases that carbon, it could trigger irreversible changes to the planet's climate. "We need to address the feedback loop before reaching that tipping point."
