More Snow Accelerates Legacy Carbon Emissions From Arctic Permafrost

Snow is critically important to the energy budget, biogeochemistry, ecology, and people of the Arctic. While climate change continues to shorten the duration of the snow cover period, snow mass (the depth of the snow pack) has been increasing in many parts of the Arctic. Previous work has shown that deeper snow can rapidly thaw permafrost and expose the large amounts of ancient (legacy) organic matter contained within it to microbial decomposition. This process releases carbonaceous greenhouse gases but also nutrients, which promote plant growth and carbon sequestration. The net effect of increased snow depth on greenhouse gas emissions from Arctic ecosystems remains uncertain. Here we show that 25 years of snow addition turned tussock tundra, one of the most spatially extensive Arctic ecosystems, into a year‐round source of ancient carbon dioxide. More snow quadrupled the amount of organic matter available to microbial decomposition, much of it previously preserved in permafrost, due to deeper seasonal thaw, soil compaction and subsidence as well as the proliferation of deciduous shrubs that lead to 10% greater carbon uptake during the growing season. However, more snow also sustained warmer soil temperatures, causing greater carbon loss during winter (+200% from October to May) and year‐round. We find that increasing snow mass will accelerate the ongoing transformation of Arctic ecosystems and cause earlier‐than‐expected losses of climate‐warming legacy carbon from permafrost. Plain Language Summary Northern ecosystems are shaped by snow. With climate change, the duration of the snow cover period in the Arctic has been decreasing while the amount of snow falling has been increasing. It is not clear how more snow will affect Arctic ecosystems, specifically greenhouse gas emissions from thawing permafrost. We know that deeper snow can rapidly thaw permafrost and the large amounts of ancient organic matter contained within it. The decomposition of this material by soil microbes releases climate‐warming carbon dioxide, however, it also stimulates the growth of plants which sequester carbon dioxide from the atmosphere through photosynthesis. Here, we discuss the results of a climate change experiment where more snow was added to a typical and widely‐distributed tundra ecosystem in northern Alaska for 25 years. We found that more snow thawed permafrost and led to a four‐fold increase in the amount of organic matter available for microbial decomposition. While