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  1. Abstract. Northern peatlands have been a large C sink during the Holocene,but whether they will keep being a C sink under future climate change isuncertain. This study simulates the responses of northern peatlands tofuture climate until 2300 with a Peatland version Terrestrial EcosystemModel (PTEM). The simulations are driven with two sets of CMIP5 climate data(IPSL-CM5A-LR and bcc-csm1-1) under three warming scenarios (RCPs 2.6, 4.5 and8.5). Peatland area expansion, shrinkage, and C accumulation anddecomposition are modeled. In the 21st century, northern peatlands areprojected to be a C source of 1.2–13.3 Pg C under all climate scenariosexcept for RCP 2.6 of bcc-csm1-1 (a sink of 0.8 Pg C). During 2100–2300,northern peatlands under all scenarios are a C source under IPSL-CM5A-LRscenarios, being larger sources than bcc-csm1-1 scenarios (5.9–118.3 vs.0.7–87.6 Pg C). C sources are attributed to (1) the peatland water table depth(WTD) becoming deeper and permafrost thaw increasing decomposition rate; (2) net primary production (NPP) not increasing much as climate warms becausepeat drying suppresses net N mineralization; and (3) as WTD deepens,peatlands switching from moss–herbaceous dominated to moss–woody dominated,while woody plants require more N for productivity. Under IPSL-CM5A-LRscenarios, northern peatlands remain as a C sink until the pan-Arctic annualtemperature reaches −2.6 to −2.89 ∘C, while this threshold is −2.09to −2.35 ∘C under bcc-csm1-1 scenarios. This study predicts anorthern peatland sink-to-source shift in around 2050, earlier than previousestimates of after 2100, and emphasizes the vulnerability of northernpeatlands to climate change. 
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  2. null (Ed.)
    Abstract Wildfires are a major disturbance to forest carbon (C) balance through both immediate combustion emissions and post-fire ecosystem dynamics. Here we used a process-based biogeochemistry model, the Terrestrial Ecosystem Model (TEM), to simulate C budget in Alaska and Canada during 1986–2016, as impacted by fire disturbances. We extracted the data of difference Normalized Burn Ratio (dNBR) for fires from Landsat TM/ETM imagery and estimated the proportion of vegetation and soil C combustion. We observed that the region was a C source of 2.74 Pg C during the 31-year period. The observed C loss, 57.1 Tg C year −1 , was attributed to fire emissions, overwhelming the net ecosystem production (1.9 Tg C year −1 ) in the region. Our simulated direct emissions for Alaska and Canada are within the range of field measurements and other model estimates. As burn severity increased, combustion emission tended to switch from vegetation origin towards soil origin. When dNBR is below 300, fires increase soil temperature and decrease soil moisture and thus, enhance soil respiration. However, the post-fire soil respiration decreases for moderate or high burn severity. The proportion of post-fire soil emission in total emissions increased with burn severity. Net nitrogen mineralization gradually recovered after fire, enhancing net primary production. Net ecosystem production recovered fast under higher burn severities. The impact of fire disturbance on the C balance of northern ecosystems and the associated uncertainties can be better characterized with long-term, prior-, during- and post-disturbance data across the geospatial spectrum. Our findings suggest that the regional source of carbon to the atmosphere will persist if the observed forest wildfire occurrence and severity continues into the future. 
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  3. Abstract

    Northern peatlands are a large C stock and often act as a C sink, but are susceptible to climate warming. To understand the role of peatlands in the global carbon‐climate feedback, it is necessary to accurately quantify their C stock changes and decomposition. In this study, a process‐based model, the Peatland Terrestrial Ecosystem Model, is used to simulate pan‐Arctic peatland C dynamics from 15 ka BP to 1990. To improve the accuracy of the simulation, spatially explicit water run‐on and run‐off processes were considered, four different pan‐Arctic peatland extent data sets were used, and a spatially explicit peat basal date data set was developed using a neural network approach. The model was calibrated against 2055 peat thickness observations and the parameters were interpolated to the pan‐Arctic region. Using the model, we estimate that, in 1990, the pan‐Arctic peatlands soil C stock was 396–421 Pg C, and the Holocene average C accumulation rate was 22.9 g C·m−2 yr−1. Our estimated peat permafrost development history generally agrees with multi‐proxy‐based paleo‐climate data sets and core‐derived permafrost areal dynamics. Under Anthropocene warming, in the freeze‐thaw and permafrost‐free regions, the peat C accumulation rate decreased, but it increased in permafrost regions. Our study suggests that if current permafrost regions switch to permafrost‐free conditions in a warming future, the peat C accumulation rate of the entire pan‐Arctic region will decrease, but the sink and source activities of these peatlands are still uncertain.

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  4. null (Ed.)