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Abstract The Arctic tundra is a significant source of CH₄generated through anaerobic methanogenesis, and vegetation explains much of the heterogeneity in CH₄fluxes across the Arctic landscape. Wet sedge-dominated tundra exhibits particularly high rates of CH₄flux due to the presence of aerenchyma and therefore, plant-mediated transport of CH₄. However, the response of CH₄to future CO₂concentrations remains largely uncertain. Model predictions assumed that elevated CO₂(eCO₂) will increase primary production and, as a result, CH₄emissions, but this has been poorly tested given the challenges of conducting eCO₂experiments in the Arctic. To address this knowledge gap, we tested the response of CH₄fluxes in soil and vegetation cores from a wet-sedge tundra ecosystem in Alaska to eCO₂. The eCO₂stimulated plant productivity and increased CH₄emissions in a short-term treatment. The instantaneous response suggests a direct contribution of plant-derived labile carbon input in stimulating methanogenesis despite the large soil carbon store in these tundra ecosystems. 

Abstract Freeze-thaw cycles, hydrology, soil mixing, and permafrost degradation shape landscape patterns in permafrost regions, forming unvegetated or sparsely vegetated landforms with varied topography. These features, which we refer to as unvegetated thermokarst polygons, result from permafrost thaw and may play a critical role in Arctic greenhouse gas (GHG) dynamics by unlocking vast amounts of carbon stored in permafrost regions. However, their spatial extent remains uncertain due to their variable size (∼0.5–15 m) and the challenges of detecting them in heterogeneous landscapes such as polygonal tundra. This study assesses the extent of unvegetated thermokarst polygons near Utqiaġvik, Alaska, using imaging spectroscopy and lidar data with a 1 m spatial resolution from the National Ecological Observing Network airborne observation platform on the joint-basis of a linear spectral mixture analysis and lidar-based relief. Based on a confusion matrix evaluating threshold models of our analysis against field-based results, the model that performed with the greatestF1 score (0.63), with balanced precision (0.71) and recall (0.57), estimated a 9.55% cover of unvegetated thermokarst polygons, or 380 000 m²total area of unvegetated thermokarst polygons within the study extent. Ground truth data from 52 sites revealed that the spectral mixture model correctly mapped ∼81% of unvegetated thermokarst polygons. Our results highlight the challenges and potential of detecting unvegetated thermokarst polygons in complex Arctic landscapes with high-resolution remote sensing. Although these features currently cover a small area in the study site, their relevance may grow with increased landscape changes and GHG dynamics under warm conditions. Moreover, their extent could be greater in areas with more active permafrost degradation, emphasizing the need for detection methods to monitor these critical landforms. 

Abstract Arctic headwater streams play a crucial role in the carbon cycle, contributing disproportionately to carbon dioxide (CO₂) evasion despite their small size and surface area. In this study, we examined how seasonal hydrologic patterns, such as spring snowmelt (freshet), mid-summer low-flows (baseflow) and the late summer period in August and early September (late-season), influence CO₂evasion in Arctic headwater streams (1–9 km²basin size; 1–4 stream orders) near Utqiagvik, Alaska. During three summers, we measured key environmental parameters (CO₂partial pressure, water temperature, discharge, dissolved organic carbon (DOC), gas transfer velocity) and CO₂evasion using a floating chamber. CO₂evasion followed distinct seasonal patterns with the highest evasion rates during freshet, followed by the late-season, and the lowest during baseflow. Despite its short duration (10 d), freshet accounted for 22% more total CO₂evasion than the 36 d late-season period. The descending peakflow within freshet had the highest median evasion rate (2486 mg C–CO₂m⁻²d⁻¹) of the entire season, while the ascending portion of the peakflow had the lowest (448 mg C–CO₂m⁻²d⁻¹). These two freshet periods showed a statistically significant difference (p< 0.05). Water temperature, DOC, andpCO₂explained 35%, 30% and 67% of freshet evasion variability, respectively. No measured parameters reliably predicted CO₂evasion for the baseflow or late-season periods, suggesting increased hydrologic and biological complexity. The stream showed supply-limitation during freshet but transitioned to chemostatic behavior thereafter. By synthesizing our measurements with previously published data, we show that small streams (<1 km²watersheds) exhibit ∼10-times the CO₂evasion rates than large waterways (>1000 km²). We also find that across the Arctic, only ∼15% of riverine CO₂measurements capture the freshet period. Together, the omission of small streams and the underrepresentation of freshet likely lead to a systematic underestimation of CO₂losses from Arctic surface waters to the atmosphere. 

Abstract Global atmospheric concentrations of nitrous oxide have been increasing over previous decades with emerging research suggesting the Arctic as a notable contributor. Thermokarst processes, increasing temperature, and changes in drainage can cause degradation of polygonal tundra landscape features resulting in elevated, well-drained, unvegetated soil surfaces that exhibit large nitrous oxide emissions. Here, we outline the magnitude and some of the dominant factors controlling variability in emissions for these thermokarst landscape features in the North Slope of Alaska. We measured strong nitrous oxide emissions during the growing season from unvegetated high centered polygons (median (mean) = 104.7 (187.7) µg N₂O-N m⁻² h⁻¹), substantially higher than mean rates associated with Arctic tundra wetlands and of similar magnitude to unvegetated hotspots in peat plateaus and palsa mires. In the absence of vegetation, isotopic enrichment of¹⁵N in these thermokarst features indicates a greater influence of microbial processes, (denitrification and nitrification) from barren soil. Findings reveal that the thermokarst features discussed here (~1.5% of the study area) are likely a notable source of nitrous oxide emissions, as inferred from chamber-based estimates. Growing season emissions, estimated at 16 (28) mg N₂O-N ha⁻¹ h⁻¹, may be large enough to affect landscape-level greenhouse gas budgets. 

Abstract The Arctic–Boreal Zone is rapidly warming, impacting its large soil carbon stocks. Here we use a new compilation of terrestrial ecosystem CO₂fluxes, geospatial datasets and random forest models to show that although the Arctic–Boreal Zone was overall an increasing terrestrial CO₂sink from 2001 to 2020 (mean ± standard deviation in net ecosystem exchange, −548 ± 140 Tg C yr⁻¹; trend, −14 Tg C yr⁻¹;P < 0.001), more than 30% of the region was a net CO₂source. Tundra regions may have already started to function on average as CO₂sources, demonstrating a shift in carbon dynamics. When fire emissions are factored in, the increasing Arctic–Boreal Zone sink is no longer statistically significant (budget, −319 ± 140 Tg C yr⁻¹; trend, −9 Tg C yr⁻¹), and the permafrost region becomes CO₂neutral (budget, −24 ± 123 Tg C yr⁻¹; trend, −3 Tg C yr⁻¹), underscoring the importance of fire in this region. 

Abstract The timing and progression of the spring thaw transition in high northern latitudes (HNL) coincides with warmer temperatures and landscape thawing, promoting increased soil moisture and growing season onset of gross primary productivity (GPP), heterotrophic respiration (HR), and evapotranspiration (ET). However, the relative order and spatial pattern of these events is uncertain due to vast size and remoteness of the HNL. We utilized satellite environmental data records (EDRs) derived from complementary passive microwave and optical sensors to assess the progression of spring transition events across Alaska and Northern Canada from 2016 to 2020. Selected EDRs included land surface and soil freeze‐thaw status, solar‐induced chlorophyll fluorescence (SIF) signifying canopy photosynthesis, root zone soil moisture (RZSM), and GPP, HR, and ET as indicators of ecosystem carbon and water‐energy fluxes. The EDR spring transition maps showed thawing as a precursor to rising RZSM and growing season onset. Thaw timing was closely associated with ecosystem activation from winter dormancy, including seasonal increases in SIF, GPP, and ET. The HR onset occurred closer to soil thawing and prior to GPP activation, reducing spring carbon (CO₂) sink potential. The mean duration of the spring transition spanned ∼6 ± 1.5 weeks between initial and final onset events. Spring thaw timing and maximum RZSM were closely related to active layer thickness in HNL permafrost zones, with deeper active layers showing generally earlier thawing and greater RZSM. Our results confirm the utility of combined satellite EDRs for regional monitoring and better understanding of the complexity of the spring transition. 

Abstract The changing thermal state of permafrost is an important indicator of climate change in northern high latitude ecosystems. The seasonally thawed soil active layer thickness (ALT) overlying permafrost may be deepening as a consequence of enhanced polar warming and widespread permafrost thaw in northern permafrost regions (NPRs). The associated increase in ALT may have cascading effects on ecological and hydrological processes that impact climate feedback. However, past NPR studies have only provided a limited understanding of the spatially continuous patterns and trends of ALT due to a lack of long-term high spatial resolution ALT data across the NPR. Using a suite of observational biophysical variables and machine learning (ML) techniques trained with availablein situALT network measurements (n= 2966 site-years), we produced annual estimates of ALT at 1 km resolution over the NPR from 2003 to 2020. Our ML-derived ALT dataset showed high accuracy (R²= 0.97) and low bias when compared within situALT observations. We found the ALT distribution to be most strongly affected by local soil properties, followed by topographic elevation and land surface temperatures. Pair-wise site-level evaluation between our data-driven ALT with Circumpolar Active Layer Monitoring data indicated that about 80% of sites had a deepening ALT trend from 2003 to 2020. Based on our long-term gridded ALT data, about 65% of the NPR showed a deepening ALT trend, while the entire NPR showed a mean deepening trend of 0.11 ± 0.35 cm yr⁻¹[25%–75% quantile: (−0.035, 0.204) cm yr⁻¹]. The estimated ALT trends were also sensitive to fire disturbance. Our new gridded ALT product provides an observationally constrained, updated understanding of the progression of thawing and the thermal state of permafrost in the NPR, as well as the underlying environmental drivers of these trends. 

Abstract Significant uncertainties persist concerning how Arctic soil tundra carbon emission responds to environmental changes. In this study, 24 cores were sampled from drier (high centre polygons and rims) and wetter (low centre polygons and troughs) permafrost tundra ecosystems. We examined how soil CO₂and CH₄fluxes responded to laboratory-based manipulations of soil temperature (and associated thaw depth) and water table depth, representing current and projected conditions in the Arctic. Similar soil CO₂respiration rates occurred in both the drier and the wetter sites, suggesting that a significant proportion of soil CO₂emission occurs via anaerobic respiration under water-saturated conditions in these Arctic tundra ecosystems. In the absence of vegetation, soil CO₂respiration rates decreased sharply within the first 7 weeks of the experiment, while CH₄emissions remained stable for the entire 26 weeks of the experiment. These patterns suggest that soil CO₂emission is more related to plant input than CH₄production and emission. The stable and substantial CH₄emission observed over the entire course of the experiment suggests that temperature limitations, rather than labile carbon limitations, play a predominant role in CH₄production in deeper soil layers. This is likely due to the presence of a substantial source of labile carbon in these carbon-rich soils. The small soil temperature difference (a median difference of 1 °C) and a more substantial thaw depth difference (a median difference of 6 cm) between the high and low temperature treatments resulted in a non-significant difference between soil CO₂and CH₄emissions. Although hydrology continued to be the primary factor influencing CH₄emissions, these emissions remained low in the drier ecosystem, even with a water table at the surface. This result suggests the potential absence of a methanogenic microbial community in high-centre polygon and rim ecosystems. Overall, our results suggest that the temperature increases reported for these Arctic regions are not responsible for increases in carbon losses. Instead, it is the changes in hydrology that exert significant control over soil CO₂and CH₄emissions. 

Abstract Boreal‐Arctic regions are key stores of organic carbon (C) and play a major role in the greenhouse gas balance of high‐latitude ecosystems. The carbon‐climate (C‐climate) feedback potential of northern high‐latitude ecosystems remains poorly understood due to uncertainty in temperature and precipitation controls on carbon dioxide (CO₂) uptake and the decomposition of soil C into CO₂and methane (CH₄) fluxes. While CH₄fluxes account for a smaller component of the C balance, the climatic impact of CH₄outweighs CO₂(28–34 times larger global warming potential on a 100‐year scale), highlighting the need to jointly resolve the climatic sensitivities of both CO₂and CH₄. Here, we jointly constrain a terrestrial biosphere model with in situ CO₂and CH₄flux observations at seven eddy covariance sites using a data‐model integration approach to resolve the integrated environmental controls on land‐atmosphere CO₂and CH₄exchanges in Alaska. Based on the combined CO₂and CH₄flux responses to climate variables, we find that 1970‐present climate trends will induce positive C‐climate feedback at all tundra sites, and negative C‐climate feedback at the boreal and shrub fen sites. The positive C‐climate feedback at the tundra sites is predominantly driven by increased CH₄emissions while the negative C‐climate feedback at the boreal site is predominantly driven by increased CO₂uptake (80% from decreased heterotrophic respiration, and 20% from increased photosynthesis). Our study demonstrates the need for joint observational constraints on CO₂and CH₄biogeochemical processes—and their associated climatic sensitivities—for resolving the sign and magnitude of high‐latitude ecosystem C‐climate feedback in the coming decades.