Introduction Background and RationalePeatlands are organic-rich wetlands that provide important ecosystem services at a range of spatial scales (Kimmel & Mander, 2010). Local hydrological setting is of central importance in determining the characteristics and functions of these ecosystems (Siegel & Glaser, 2006). Peatlands are characterized by waterlogged, anoxic conditions that suppress microbial decomposition, causing carbon to accumulate slowly but persistently over thousands of years in the form of partially decomposed plant detritus (Yu et al., 2010). Peatlands cover less than 3% of the Earth's land surface (Xu et al., 2018b) yet they are thought to store between approximately 500 and 600 Gt (5-6 × 10 17 g) of carbon (Müller & Joos, 2020;Page et al., 2011;Yu, 2011Yu, , 2012, equivalent to between approximately one sixth and one third of global soil carbon (Scharlemann et al., 2014). As well as being long-term carbon sinks, peatlands also emit greenhouse gases, particularly carbon dioxide (CO 2 ) and methane. Peatland greenhouse gas budgets are highly sensitive to surface wetness, and even modest changes in water-table depths can cause peatlands to switch between being net sinks and sources of greenhouse gases when measured in CO 2 -equivalent units (Evans et al., 2021;Günther et al., 2020). In some locations, water that drains from peat

Human-induced climate warming by 2100 is expected to thaw large expanses of northern permafrost peatlands. However, the spatio-temporal dynamics of permafrost peatland thaw remain uncertain due to complex permafrost-climate interactions, the insulating properties of peat soils, and variation in model projections of future climate. Here we show that permafrost peatlands in Europe and Western Siberia will soon surpass a climatic tipping point under scenarios of moderate-to-high warming (SSP2-4.5, SSP3-7.0, and SSP5-8.5). The total peatland area affected under these scenarios contains 37.0-39.5 Gt carbon (equivalent to twice the amount of carbon stored in European forests). Our bioclimatic models indicate that all of Fennoscandia will become climatically unsuitable for peatland permafrost by 2040. Strong action to reduce emissions (SSP1-2.6) by the 2090s could retain suitable climates for permafrost peatlands storing 13.9 Gt carbon in northernmost Western Siberia, indicating that socioeconomic policies will determine the rate and extent of permafrost peatland thaw. Main

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<p>Human-induced climate change during the 21<sup>st</sup> century is expected to thaw large expanses of permafrost peatlands - one of Earth’s largest terrestrial carbon stores. Whilst frozen, peatland carbon fluxes are inhibited by cold temperatures, but emissions of carbon dioxide (CO<sub>2</sub>) and methane (CH<sub>4</sub>) are expected to substantially increase post-thaw. Peatland permafrost is often characterised by the presence of frost mounds, termed palsas/peat plateaus, or by ice-wedge polygons in more northerly regions. The spatio-temporal dynamics of future permafrost peatland thaw remain highly uncertain due to incomplete mapping of their modern distribution, the insulating properties of organic soils, and the variation in model projections of future climate.</p><p>Here, we present simulations of the modern and future climate envelopes of permafrost peatlands in Europe and Western Siberia. We collated > 2,000<strong> </strong>site observations from across the northern hemisphere to quantify the modern distributions of palsas/peat plateaus and polygon mires. We fitted novel climate envelope models by relating landform distributions to modern climate data. We forced our climate envelope models with decadal projections of future climate under four Shared Socioeconomic Pathway (SSP) scenarios from 2020–2090, taken from an ensemble of 12 general circulation models included in the Coupled Model Intercomparison Project 6 (CMIP6). We then combined our simulations with recent soil organic carbon maps to estimate the total peat carbon stocks that may be at risk from future losses of suitable climate space.</p><p>Our simulations indicate that permafrost peatlands in Europe and Western Siberia will soon surpass a climatic tipping point under scenarios of moderate-to-high warming (SSP2-4.5, SSP3-7.0, and SSP5-8.5). We show that permafrost peatlands in Fennoscandia currently exist under warmer, wetter climates than those in Western Siberia. Our projections suggest that Fennoscandia will no longer be climatically suitable for peatland permafrost by 2040. Projected climate space losses by 2100 under these scenarios would affect peatlands containing 37.0–39.5 Gt carbon in Europe and Western Siberia (equivalent to twice the amount of carbon stored in European forests). Under a scenario with strong climate change mitigation (SSP1-2.6), our analyses show that permafrost peatlands storing 13.9 Gt carbon in the northernmost parts of Western Siberia would remain climatically supported by the 2090s. These results indicate that the rate and extent of 21st century permafrost peatland thaw will be determined by near-future socioeconomic developments.</p>

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