Published recently in Nature Geoscience, the findings link widespread methane emissions from beneath the Greenland Ice Sheet to a warming period dating back just a few thousand years.
These methane data indicate that during that period, the ice sheet had retreated well beyond its present extent.
The Greenland Ice Sheet emits methane everywhere—sometimes in big amounts.
The total amount of methane coming from the Greenland Ice Sheet is relatively small when considered on a global scale.
That means the ice sheet is a highly responsive “barometer” to environmental pressures, making it more susceptible to temperature swings.
How will massive, continental glaciers known as ice sheets respond to climate change in the future? Surprising new research from a team including Assistant Professor of Earth & Environmental Science Jon Hawkings offers an unsettling look.
Published recently in Nature Geoscience, the findings link widespread methane emissions from beneath the Greenland Ice Sheet to a warming period dating back just a few thousand years. According to Hawkings, that “strongly suggests that the Greenland Ice Sheet responds much more quickly to climate change than we previously assumed.”
Prior warm periods are a good indication of how the world’s two contemporary ice sheets—which cover Greenland and Antarctica—will react to the effects of modern climate change. The researchers dated the methane emerging from beneath the Greenland Ice Sheet back around 5,000 years ago, where temperatures similar to those in the present day sparked the Holocene Thermal Maximum period.
These methane data indicate that during that period, the ice sheet had retreated well beyond its present extent. That allowed vegetation and soils to develop in its place before the ice advanced and covered it again. “This means that the temperature increases we’ve seen over the last few decades and the warming expected in the next century will very likely lead to a smaller Greenland Ice Sheet, potentially much smaller if worse-case scenarios play out,” Hawkings says.
Charles University in Prague led the broader international project, with Hawkings’ Penn BiCycles Lab team serving as collaborators. The study team, including former postdoc Jack Murphy and Hawkings, spent several months in remote Greenland collecting samples and data from 26 meltwater streams emerging from the western margin of the Greenland Ice Sheet.
The project marks the first time a team has investigated multiple sites along an ice sheet margin—because of the region’s remoteness. But Hawkings says that’s just the tip of the ice when it comes to the study’s findings. Here are three big takeaways:
1. This is the first time ice sheet-derived methane has been “dated.”
Among other groundbreaking achievements, the project is the first of its kind to establish the age of methane emerging from an ice sheet, through stable isotope analysis and radiocarbon dating. “The carbon in the methane was 1,500 to 4,500 years old,” says Hawkings, noting that this finding helped the researchers link Greenland’s large stores of methane to the Holocene Thermal Maximum.
For years, methane—a potent greenhouse gas—has been detected at glacier margins, sometimes in very high concentrations. But questions remained as to whether these readings were anomalies or outliers. Figuring out the methane’s age helps scientists better understand where beneath the ice sheet methane originates, how it is produced, and how future scenarios might look.
Dating methane requires measuring levels of carbon-14 (also known as radiocarbon) in methane molecules. Carbon-14 is a rare and unstable radioactive isotope of carbon that decays at a steady rate over time. It also serves as a vital time stamp—the more it has decayed, the older the sample is.
Hawkings, whose lab scrutinizes the role of glaciers and ice sheets in global biogeochemical cycles, says he was surprised by how “young” Greenland’s methane turned out to be. “The age suggests the ice sheet retreated significantly from its present margin during the Holocene and that plants and soils developed in a region that is now ice coverage again,” he explains, referencing the epoch encompassing the last 12,000 years.
2. The Greenland Ice Sheet emits methane everywhere—sometimes in big amounts.
Across almost all areas of the Greenland Ice Sheet the team sampled, Hawkings says one thing was pretty consistent: “You can find methane higher than the atmospheric background … and sometimes at very high concentrations.”
Greenland’s buried vegetation from prior warming periods is fueling the methane production and release, driven by tiny microbes known as methanogens. Given how prevalent methane emissions are at glacier margins, scientists worry that rapidly retreating ice could ramp up emissions, producing a warming feedback—what Hawkings calls “positive feedback to the climate system, analogous to methane release from melting permafrost.”
It’s not all bad news, though. The total amount of methane coming from the Greenland Ice Sheet is relatively small when considered on a global scale. But Hawkings says better understanding emissions is still a major focus for researchers scrutinizing the role of ice sheets in the climate system.
“We now know more about methane production and export under the ice sheet, but we still know relatively little about other greenhouse gases, like carbon dioxide and nitrous oxide,” he adds. “We also don’t know how much of a feedback prior methane release from ice sheets had on climate systems in the past.”
3. The ice sheet is extremely sensitive to environmental changes.
One of the most disconcerting findings, Hawkings says, is that Greenland’s ice seems to be “much more dynamic” than scientists have long assumed. That means the ice sheet is a highly responsive “barometer” to environmental pressures, making it more susceptible to temperature swings.
That could prove to be a massive problem. In addition to disrupting ecosystems and longstanding ways of life in Greenland, ice melt also accelerates sea-level rise across the globe, fueling challenges ranging from coastal erosion to habitat loss and population displacement. Hawkings notes that the Greenland Ice Sheet’s sensitivity has “clear implications for its future behavior as the climate warms.”
Still, the more scientists learn, the better equipped they are to tackle major problems. Hawkings says the new findings are enormously informative, even if they’ve raised some new puzzles—like why the Greenland Ice Sheet appears to be so much more susceptible to temperature change than previously assumed. “Answering this question,” he says, “will be a big community effort.”
