I decided to resurrect this thread for a science paper on Permafrot melt, rather than post it on the Permafrost topic which is much more about current observations. I also changed the name for this post from "Study on Permafrost Melt: Arctic Methane Emissions ‘Certain to Trigger Warming'" to
Permafrost and Greenhouse gases- Science Papers as the current title is too specific and the Permafrost melting contribution to greenhouse gas (CO2, CH4, N2O) emissions is a matter of sometimes fierce debate.
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All the climate models predict a warmer wetter Arctic and the high laitude regions resulting in less snowcover days but more snow depth.
This study concludes that this will lead to faster permafrost melt and that that means more greenhouse gases in winter but less in summer but overall an increase in emissions. To quote....
Our unique long-term climate manipulation experiment in Northern Alaska demonstrates that we urgently need a comprehensive observation system to quantify legacy carbon emissions from permafrost and can no longer afford to ignore the Arctic in climate change projections and mitigation policy.https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023AV000942More Snow Accelerates Legacy Carbon Emissions From Arctic Permafrost
Abstract
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.
Key Points
Twenty-five years of snow addition to Arctic tundra thawed permafrost and increased carbon and nitrogen available for microbial decomposition 4-fold
More snow sustained ancient carbon emissions year-round, despite greater productivity associated with a shift from graminoid to shrub tundra
In the rapidly warming Arctic, increases in snow mass will lead to earlier-than-expected losses of 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 this stimulated the growth of plants (specifically that of deciduous shrubs) and soil carbon sequestration, microbial decomposition of previously frozen organic matter outpaced the benefits. Our study demonstrates that greater snowfall will cause earlier-than-expected losses of ancient carbon from permafrost and further accelerate climate change.
1 Introduction
Arctic soils contain large amounts of carbon (1,035 ± 150 Pg C (Hugelius et al., 2014)) and nitrogen (22–106 Pg N (Strauss et al., 2022)) in the form of frozen organic matter (0–3 m), much of which was sequestered during the Pleistocene and early Holocene with radiocarbon ages ≥5,000 years before present (BP) (Miner et al., 2022). Rapid climate change and permafrost thaw (Box et al., 2019; Rantanen et al., 2022) renders this “legacy” carbon and nitrogen vulnerable to microbial decomposition, and its emission as carbon dioxide (CO2), methane, or nitrous oxide will further increase greenhouse gas concentrations in the atmosphere and accelerate climate change (Miner et al., 2022; Schuur et al., 2022; Voigt et al., 2020).
4 Results and Discussion
4.1 More Snow Transforms Tussock Tundra Into a Shrubland
Twenty-five years of deeper snow has transformed the tundra from a graminoid- to a deciduous shrub-dominated ecosystem, which coincides with increases in plant-available nitrogen, leaf-level photosynthesis, and active layer depth (ALD) (Figure 1). Between 1994 and 2021, deciduous shrubs have expanded in both zones, from 16% to 20% cover under ambient snow and to more than 26% under deeper snow (Figure S2 in Supporting Information S1) (Leffler et al., 2016; Leffler & Welker, 2013). This shift in vegetation raised the productivity of the tundra (GPP was 45% greater in +Snow in 2021, P < 0.05) and resulted in approximately 6%–13% greater carbon sequestration during the growing season (Figure 2a–2c), when NEE was −229 ± 4 g C m−2 in +Snow (weeks 25–38) versus −203 ± 4 or −217 ± 4 g C m−2 in Control during weeks 25–38 or the total snow free period (weeks 22–38), respectively.
see attached image
Transformation of Arctic tundra under ambient climate (Control) and in response to long-term snow addition (+Snow). Fraction of soil organic carbon (ave. ±SE) as a function of depth below the soil surface, shaded by bulk soil age (radiocarbon content (Δ14C)) and density. Legacy carbon has Δ14C < −470‰ (radiocarbon ages >5,000 years before present), modern carbon has Δ14C ≥ 0‰ (CO2 assimilated by photosynthesis from the atmosphere since 1950). Dashed horizontal lines indicate the interface of organic and mineral soil and solid lines with shading the depth of the seasonally thawed active layer (Aug. ave. ±SD, 1995–2022).
4.2 More Snow Accelerates Legacy Carbon Emissions From Permafrost
Our year-round CO2 efflux observations show that more snow resulted in three times greater carbon loss during the winter (October–May 2021), when Reco was about 267 g C m−2 in +Snow versus 87 g C m−2 in Control (Figure 2a–2c). Our cold season estimates are slightly higher than previous estimates for tussock tundra of 20–70 g C m−2 (Sullivan et al., 2008). Our results also indicate that the deeper snow turned the tundra into a year-round carbon source (Figure 2a–2c).
Continuous monitoring of the age of soil CO2 (Pedron et al., 2021) reveals that the higher CO2 emissions under deeper snow during the cold season are fueled by newly exposed legacy carbon that is being actively decomposed year-round (Figure 2g–2i). At similar depths, pore space CO2 was significantly older (3.5 times lower Δ14CO2 under +Snow (−350‰) than Control (−100‰), P < 0.001). While both treatments follow the expected seasonal trend toward younger CO2 during the growing season (Pedron et al., 2022), legacy carbon is a dominant fraction of the CO2 produced under deeper snow during the growing season. These data prove that legacy carbon is readily metabolized, possibly because more fresh seasonal carbon is also available (Keuper et al., 2020).
Most of the legacy carbon is emitted, however, during the fall and winter, when seasonal inputs of fresh carbon have ceased. Cold season emissions are amplified under deeper snow, where emissions of CO2 were larger (Figure 2a–2c) and much older (Figure 2g–2i). As such, our study provides further evidence that microbial decomposition of soil organic matter during fall and winter drives the losses of (legacy) carbon from permafrost soils (Natali et al., 2019; Pedron et al., 2022).
Previous research at this site documented the slow, but lengthy loss of CO2 during the winter and suggested that these systems may be net carbon emitters (Fahnestock et al., 1999; Sullivan et al., 2008; Welker et al., 2000). Yet, it is only with this study that we can attribute these cold season emissions to the decomposition of legacy carbon that, like the combustion of fossil fuels, are injecting ancient carbon into the modern atmosphere and contributing to climate change.
5 Conclusions
Twenty-five years of snow addition to Arctic tundra reveal that increases in snow mass associated with the ongoing wetting of the Arctic climate (Box et al., 2019) will significantly accelerate the thaw of permafrost with severe implications for Arctic ecosystems, communities, and global climate. Deeper snow liberates plant nutrients (nitrogen) and promotes the expansion of deciduous shrubs (Sturm et al., 2005; W. Xu et al., 2021), which results in greater carbon uptake by the ecosystem during the growing season. Greater carbon sequestration in woody biomass and the topsoil, however, is accompanied by microbial decomposition of legacy carbon at depth year-round and converts the tundra into a year-round source of climate-warming legacy carbon. Our unique long-term climate manipulation experiment in Northern Alaska demonstrates that we urgently need a comprehensive observation system to quantify legacy carbon emissions from permafrost and can no longer afford to ignore the Arctic in climate change projections and mitigation policy.
