Page 135 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

The warming Arctic faces permafrost degradation and thermokarst formation
Permafrost covers a quarter of the northern hemisphere land surface (Camill 2005; Gruber
2012). Its carbon pools are estimated at 1300 Pg C (range 1100-1500 Pg C) which equals twice
the amount of carbon that is present in the atmosphere (Schuur et al. 2009; Hugelius et al.
2014). The major fraction, about 1000 Pg, is present in the near surface (upper 3 m) that is vulnerable to warming (Hugelius et al. 2014). Warming leads to destabilization and degradation
of permafrost landscapes (Schneider von Deimling et al. 2015b). The Arctic Climate Impact Assessment (ACIA) reported a two times higher rise in Arctic air surface temperatures compared with the global average (ACIA 2005; Tenenbaum 2005), resulting in an average permafrost soil and sediment warming of 0.29 ± 0.12°C between 2007 and 2016 (Biskaborn et
al. 2019). This disproportionate near-surface warming is known as the “Arctic amplification” (Graversen et al. 2008). It has a pronounced effect on near-surface permafrost which is expected
to be reduced by over 90% at the end of the 21st century (Lawrence and Slater 2005). Upon 6 thaw, the increased biological availability of carbon leads to enhanced microbial greenhouse
gas (GHG) production (Lupascu et al. 2012; Schuur et al. 2015; Dean et al. 2018; Knoblauch et al. 2018) and net GHG emissions which consist mainly of CO2 and CH4 (Koven et al. 2011; Schaefer et al. 2011; Hayes et al. 2014).
One consequence of a warming Arctic and permafrost thaw is the formation of thermokarst (see Glossary) landscapes. These landscapes are associated with pingos, thermokarst troughs and pits that can collapse to form thermokarst lakes (Kokelj and Jorgenson 2013). GHG emissions from these lakes can have disproportionate climate effects due to the rapid release of long-term stored carbon into the atmosphere, which initiates a strong positive climate feedback (Walter Anthony, Smith and Stuart Chapin 2007; Schuur et al. 2008; Schaefer et al. 2011; Walter Anthony et al. 2016, 2018). This review focuses on the role of thermokarst lakes in a warming world and the microbial mechanisms that underlie their contributions to the global GHG budget.
Thermokarst lakes are a feature of thawing Yedoma deposits
About 40% (500 Pg) of the permafrost carbon stocks are found in ice-rich Yedoma deposits which have an organic carbon content ranging from 2% to 5% (Zimov, Schuur and Chapin III
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