Northern peatland initiation lagged abrupt increases in deglacial atmospheric CH4

Climatic controls on the dynamic lateral expansion of northern peatlands and its potential implication for the 'anomalous' atmospheric CH4 rise since the mid-Holocene

Understanding the dynamic changes in peatland area during the Holocene is essential for unraveling the connections between northern peatland development and global carbon budgets. However, studies investigating the centennial to millennial-scale process of peatland expansion and its climate and environmental drivers are still limited. In this study, we present a reconstruction of the peatland area and lateral peatland expansion rate of a peatland complex in northern Sweden since the mid-Holocene, based on Ground Penetrating Radar measurements of peat thickness supported by radiocarbon (14C) dates from four peat cores. Based on this analysis, lateral expansion of the peatland followed a northwest-southeast directionality, constrained by the undulating post-glacial topography. The areal extent of peat has increased non-linearly since the mid-Holocene, and the peatland lateral expansion rate has generally been on the rise, with intensified expansion occurring after around 3500 cal yr BP. Abrupt declines in lateral expansion rates were synchronized with the decreases in total solar irradiance superimposed on the millennial ice-rafted debris events in the northern high latitudes. Supported by the temporal evolution of peatland extent in four other Fennoscandian peatlands, it appears that the northern peatland areal extent during the early to middle Holocene was much smaller compared to previous empirical model reconstructions based on basal age compilations. Interestingly, our reconstruction shows the increments of peat area since the mid-Holocene coincide with the rise in atmospheric CH4 concentration, and that abrupt variations in atmospheric CH4 on decadal to centennial timescales could be synchronized with peatland lateral expansion rates. Based on our analysis we put forward the hypothesis that lateral expansion of northern peatlands is a significant driver of dynamics in the late Holocene atmospheric CH4 budget. We strongly urge for more empirical data to quantify lateral expansion rates and test such hypotheses.

Climate-induced Arctic-boreal peatland fire and carbon loss in the 21st century

Boreal peatlands are increasingly vulnerable to wildfires as climate change continues accelerating. Fires consume substantial quantities of organic soils and rapidly transfer large stocks of terrestrial carbon to the atmosphere. Herein, we quantify the minimum environmental temperature from -45 °C to 45 °C that allows the moist peat to smolder, as the fire threshold of peatlands. We then apply a typical vertical soil temperature profile to estimate the future depth of burn and carbon emissions from boreal peatland fires under the impact of global warming. If the boreal region continues warming at a rate of 0.44 °C/decade, we estimate the carbon loss from the boreal peat fires on a warmer soil layer may increase from 143 Mt. in 2015 to 544 Mt. in 2100 and reach a total of 28 Gt in the 21st century. If the global human efforts successfully reduce the boreal warming rate to 0.3 °C/decade, the peat fire carbon loss would drop by 21% to 22 Gt in the 21st century. This work helps understand the vulnerability of boreal peatland to more frequent and severer wildfires driven by global warming and estimate climate-induced carbon emissions from boreal peatland fires in the 21st century.

Modelling past and future peatland carbon dynamics across the pan-Arctic

The majority of northern peatlands were initiated during the Holocene. Owing to their mass imbalance, they have sequestered huge amounts of carbon in terrestrial ecosystems. Although recent syntheses have filled some knowledge gaps, the extent and remoteness of many peatlands pose challenges to developing reliable regional carbon accumulation estimates from observations. In this work, we employed an individual- and patch-based dynamic global vegetation model (LPJ-GUESS) with peatland and permafrost functionality to quantify long-term carbon accumulation rates in northern peatlands and to assess the effects of historical and projected future climate change on peatland carbon balance. We combined published datasets of peat basal age to form an up-to-date peat inception surface for the pan-Arctic region which we then used to constrain the model. We divided our analysis into two parts, with a focus both on the carbon accumulation changes detected within the observed peatland boundary and at pan-Arctic scale under two contrasting warming scenarios (representative concentration pathway-RCP8.5 and RCP2.6). We found that peatlands continue to act as carbon sinks under both warming scenarios, but their sink capacity will be substantially reduced under the high-warming (RCP8.5) scenario after 2050. Areas where peat production was initially hampered by permafrost and low productivity were found to accumulate more carbon because of the initial warming and moisture-rich environment due to permafrost thaw, higher precipitation and elevated CO₂ levels. On the other hand, we project that areas which will experience reduced precipitation rates and those without permafrost will lose more carbon in the near future, particularly peatlands located in the European region and between 45 and 55°N latitude. Overall, we found that rapid global warming could reduce the carbon sink capacity of the northern peatlands in the coming decades.

Old carbon reservoirs were not important in the deglacial methane budget

Permafrost and methane hydrates are large, climate-sensitive old carbon reservoirs that have the potential to emit large quantities of methane, a potent greenhouse gas, as the Earth continues to warm. We present ice core isotopic measurements of methane (Δ¹⁴C, δ¹³C, and δD) from the last deglaciation, which is a partial analog for modern warming. Our results show that methane emissions from old carbon reservoirs in response to deglacial warming were small (<19 teragrams of methane per year, 95% confidence interval) and argue against similar methane emissions in response to future warming. Our results also indicate that methane emissions from biomass burning in the pre-Industrial Holocene were 22 to 56 teragrams of methane per year (95% confidence interval), which is comparable to today.

Global peatland initiation driven by regionally asynchronous warming

Widespread establishment of peatlands since the Last Glacial Maximum represents the activation of a globally important carbon sink, but the drivers of peat initiation are unclear. The role of climate in peat initiation is particularly poorly understood. We used a general circulation model to simulate local changes in climate during the initiation of 1,097 peatlands around the world. We find that peat initiation in deglaciated landscapes in both hemispheres was driven primarily by warming growing seasons, likely through enhanced plant productivity, rather than by any increase in effective precipitation. In Western Siberia, which remained ice-free throughout the last glacial period, the initiation of the world's largest peatland complex was globally unique in that it was triggered by an increase in effective precipitation that inhibited soil respiration and allowed wetland plant communities to establish. Peat initiation in the tropics was only weakly related to climate change, and appears to have been driven primarily by nonclimatic mechanisms such as waterlogging due to tectonic subsidence. Our findings shed light on the genesis and Holocene climate space of one of the world's most carbon-dense ecosystem types, with implications for understanding trajectories of ecological change under changing future climates.

Tropical/Subtropical Peatland Development and Global CH4 during the Last Glaciation

Knowledge of peatland development over the tropical/subtropical zone during the last glaciation is critical for understanding the glacial global methane cycle. Here we present a well-dated ‘peat deposit-lake sediment’ alternate sequence at Tengchong, southwestern China, and discuss the peatland development and its linkage to the global glacial methane cycle. Peat layers were formed during the cold Marine Isotope Stage (MIS)-2 and -4, whereas lake sediments coincided with the relatively warm MIS-3, which is possibly related to the orbital/suborbital variations in both temperature and Asian summer monsoon intensity. The Tengchong peatland formation pattern is broadly synchronous with those over subtropical southern China and other tropical/subtropical areas, but it is clearly in contrast to those over the mid-high Northern Hemisphere. The results of this work suggest that the shifts of peatland development between the tropical/subtropical zone and mid-high Northern Hemisphere may have played important roles in the glacial/interglacial global atmospheric CH₄ cycles.

A synthesis of methane emissions from 71 northern, temperate, and subtropical wetlands

Wetlands are the largest natural source of atmospheric methane. Here, we assess controls on methane flux using a database of approximately 19 000 instantaneous measurements from 71 wetland sites located across subtropical, temperate, and northern high latitude regions. Our analyses confirm general controls on wetland methane emissions from soil temperature, water table, and vegetation, but also show that these relationships are modified depending on wetland type (bog, fen, or swamp), region (subarctic to temperate), and disturbance. Fen methane flux was more sensitive to vegetation and less sensitive to temperature than bog or swamp fluxes. The optimal water table for methane flux was consistently below the peat surface in bogs, close to the peat surface in poor fens, and above the peat surface in rich fens. However, the largest flux in bogs occurred when dry 30-day averaged antecedent conditions were followed by wet conditions, while in fens and swamps, the largest flux occurred when both 30-day averaged antecedent and current conditions were wet. Drained wetlands exhibited distinct characteristics, e.g. the absence of large flux following wet and warm conditions, suggesting that the same functional relationships between methane flux and environmental conditions cannot be used across pristine and disturbed wetlands. Together, our results suggest that water table and temperature are dominant controls on methane flux in pristine bogs and swamps, while other processes, such as vascular transport in pristine fens, have the potential to partially override the effect of these controls in other wetland types. Because wetland types vary in methane emissions and have distinct controls, these ecosystems need to be considered separately to yield reliable estimates of global wetland methane release.