Page 35 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

compared to peatlands in temperate climates, such as in the Netherlands further decelerate organic matter decomposition rates. This favors carbon sequestration in the long term and makes them important global carbon sinks (Moore and Basiliko 2006).
In a warming world, there is a strong connection between boreal peatlands and thawing permafrost (Camill 2005). Permafrost thaw in peatlands leads to an increase in peatland area and is linked to both carbon fixation and carbon losses (Heffernan et al. 2020). This makes it challenging to quantify their role in a warming world. A general observation is that polar ecosystems are warming faster than other areas on the globe (Christensen et al. 2013). Global climate models predict up to an 8°C rise in mean annual temperature in polar regions by 2100, compared to a global average of 1.4–5.8°C (Camill 2005; Allan et al. 2014). As a consequence, the role of peatlands in permafrost ecosystems will likely intensify.
Permafrost comprises a large temperature-sensitive greenhouse gas source
Permafrost is soil, sediment, or rock material that is permanently exposed to subzero temperatures for at least two consecutive years (Tamocai et al. 2009). It is present beneath 24% (23 × 106 km2) of the Northern Hemisphere land surface and is a significant part of boreal, Arctic, and alpine ecosystems (Zhang et al. 1999; Camill 2005). In the Southern Hemisphere, permafrost is mostly limited to a few alpine areas and is studied more in the context of geohazards than C storage. Permafrost soils contain ~50% of global terrestrial belowground organic C stocks (equivalent to 1,330–1,580 Pg C), which is roughly twice the amount of C currently present in the atmosphere as either CO2 or CH4 (Tamocai et al. 2009; Hugelius et al. 2014; Schuur et al. 2015).
Permafrost ecosystems are currently considered a net C sink, taking into account CH4 emissions and the CO2 sink of tundra ecosystems (Schaefer et al. 2011; Kirschke et al. 2013; Parmentier et al. 2013). As a result of rising temperatures driving permafrost thaw, it is estimated that tundra ecosystems will shift toward a net C source by the mid-2020s (Schuur et al. 2015). Estimates of permafrost loss by the end of the 21st century have ranged from 20 to 70% (Schaefer et al. 2011; Schuur and Abbott 2011; Wisser et al. 2011; Lawrence, Slater and Swenson 2012), but more recent conservative estimates are closer to 5–15% (Schuur et al. 2015), stabilizing at 60% of the current permafrost extent by 2300 if emission targets limit the global climate to a 2°C warming (Chadburn et al. 2017).
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