U.S. scientists report that they have discovered new sources of methane percolating up from underground reservoirs as glaciers, ice caps, and permafrost melt in the Arctic. University of Alaska researchers, conducting aerial and ground surveys, said they have discovered 150,000 methane seeps in Alaska alone near the margins of retreating glaciers or thawing permafrost. In Greenland, the seeps tended to be concentrated around the margins of ice caps that have been retreating for the past 150 years, the scientists said. Katey M. Walter Anthony, lead author of the study, published in Nature Geoscience, said these seeps in the earth’s frozen zones, or cryosphere, are not currently a major source of methane emissions. But, she added, “As the cryosphere degrades further, it could be a really big source.” Methane is a far more potent greenhouse gas than carbon dioxide, and researchers are concerned rapid warming of the Arctic could trigger a methane “time bomb” as thawing permafrost, vegetation, and land ice result in the release of huge quantities of methane.