Greenland's ancient methane reveals a dangerous climate weakness. For years, scientists have been sampling meltwater streams at Greenland's glacier margins, finding methane dissolved in the water. This methane was barely a global concern, as the fluxes seemed small, localized, and dependent on site-specific conditions. But an international team decided to test this picture across the entire western edge of Greenland at once, and what they found was surprising. The age of the gas suggested that the ice had once pulled back far further than anyone assumed.
The gas in question is methane, a potent greenhouse driver more powerful than carbon dioxide. Researchers had already spotted it leaking from glacier fronts at scattered sites worldwide. An earlier paper confirmed methane streaming out of one Greenland outlet. However, no one had checked whether this was an isolated incident or a phenomenon occurring along the entire edge of the ice sheet.
Jade Hatton and her team at Charles University in Prague tackled this question. They sampled 26 streams of meltwater flowing from the western flank of Greenland's ice, stretching over 1,200 miles from the northwest to the southwest. Each meltwater stream carried dissolved subglacial methane and chemical fingerprints of its source.
Field crews drilled over 3,900 feet through the ice to reach the bed, collecting sediment cores and water samples. Down there, oxygen is scarce, forcing microbes to survive without it. In the lab, the team ran two main tests: they analyzed the chemical signature of the gas to determine its origin (microbes or deep geological sources) and used radiocarbon dating to determine its age.
The results were a surprise. The methane was produced by anaerobic microbes breaking down decaying plant material in oxygen-starved sediments, not gas seeping up from deep rock formations. The radiocarbon dating revealed that the samples were 1,500 to 4,400 years old, indicating that microbes have been producing the gas from buried organic matter under the ice for centuries.
This evidence points to a known warm period in Earth's past, the Holocene Thermal Maximum, which occurred around 11,000 to 5,000 years ago when the Arctic was warmer than usual, similar to today's conditions. Greenland's ice sheet was smaller during this warm period, allowing plants to take root. Boreal forest and tundra likely grew on the exposed land, leaving behind carbon-rich sediment.
As the climate cooled, the ice readvanced, covering the exposed land with thousands of feet of frozen weight. Microbes continued to break down the buried carbon in the dark, creating a self-sustaining loop. Modern glacial retreat is now replaying this earlier turn, as the ice margin pulls back, opening new pathways for meltwater to flush out the methane produced by these long-buried microbes.
Western Greenland's land-based glaciers release approximately 790 tons of dissolved methane annually, and the buried organic matter could sustain this flow for at least 200 more years. As Greenland melts, more pathways open beneath the ice, releasing methane with its warming effect, which could accelerate the melt further.
The same setup likely exists at the South Pole on a much larger scale. An older study flagged the possibility of vast methane reservoirs beneath Antarctica, where buried organic matter is far greater than Greenland's. If Antarctic ice continues to thin and its bed becomes more connected by meltwater, the slow methane transport could amplify, dwarfing Greenland's contribution.
The survey covered western Greenland's margin, leaving eastern and northern sectors uncharted, meaning the total flux could be higher than estimated. The 200-year projection also relies on a degradation model, not direct measurement, and carries a wide uncertainty range.
The ultimate irony is that as Greenland retreats, the ice sheet itself further contributes to methane emissions. Existing ice sheet models don't capture this dynamism, and a separate study estimates Greenland is locked into roughly a foot of sea-level rise from melt already committed. The methane evidence suggests that this floor is too low, as Greenland's ice can swing farther than models forecast, feeding the warming that drives more retreat.
"The ultimate irony is that as it retreats, the ice sheet itself further contributes to those methane emissions," said Alun Hubbard, a professor at the University of Oulu and co-author of the study, published in the journal Nature Geoscience.
