Permafrost is like a giant freezer for carbon: thousands of years worth of plant, animal, and microbe remains mixed with blocks of ice. Historically, only a shallow "active layer" thawed in the short summer. In today's warming Arctic, permafrost is thawing, and the active layer is getting deeper. Warming in the growing season has increased plant growth and allowed plants to remove more carbon dioxide (CO2) from the air during photosynthesis, but decomposition of organic matter by soil microbes is also releasing carbon dioxide and methane. In the past decade, the parts of the Arctic tundra that are routinely observed have become a net source of carbon-containing greenhouse gases because microbial activity is continuing well into winter after plants go dormant. NOAA Climate.gov drawing. Permafrost map from NSIDC.