Methane’s outsized but short-lived contributions to atmospheric warming have positioned methane mitigation as a cornerstone of actionable, near-term global climate strategy. Yet, cutting anthropogenic methane emissions alone (i.e., from fossil fuels, livestock, rice cultivation, and waste management) will likely not avoid the worst climate outcomes. Natural methane sources, including those sensitive to climate-driven warming (i.e., from wetland and inland waters), account for over one-third of global methane emissions [see supplementary materials (SM) and table S1], making them a substantial component of the global methane budget, critical for understanding and constraining atmospheric methane growth (1–5). Natural sources, however, have been largely ignored over the past few decades in global methane emission reduction initiatives and efforts to strengthen methane source detection to guide mitigation planning. To inform any extension of the Global Methane Pledge (GMP) and other global initiatives, we propose an integrated Global Ecosystem Methane–Observation System (GEM-OS).

The polar ice sheets, their surrounding land fringes and oceans (68 × 10⁶ km²; 13% of Earth's surface) are hot spots for carbon cycle perturbation under future climate change due to glacier retreat, rising meltwater fluxes, reduced sea ice, thawing permafrost, warming land-surfaces and increased precipitation. Here we assess carbon stored and exchanged with the atmosphere (as carbon dioxide and methane) across an expansive bipolar ice-to-ocean domain. We show that the polar regions harbor large reserves of carbon stored in sediments, rocks and the ocean, which differ in their reactivity and turnover times: 5,300–22,200 PgC of organic carbon and 5,600–8,600 PgC of inorganic carbon. These carbon reservoirs include potential reserves of marine and subglacial methane hydrate (80–570 PgC), which could become destabilized under future warming scenarios. Oceans (270–360 PgC) and ice sheets (14–96 PgC Greenland, 5,000–21,000 PgC Antarctica) dominate organic carbon stores, with smaller (but regionally important) stocks found in ice sheet land fringes (13–58 PgC). Estimates of natural CO₂ and CH₄ fluxes from these polar regions to the atmosphere present high uncertainty but highlight oceanic CO₂ sinks in Greenland (−110 to −49 TgC-CO₂ a⁻¹) and in the ICE and SPSS biomes of the Southern Ocean (−480 to 55 TgC-CO₂ a⁻¹), with potential CH₄ sources associated with the Greenland Ice Sheet. Such high uncertainty in polar carbon reservoirs and fluxes is important to resolve if future feedbacks between the polar regions, Earth's carbon cycle and climate are to be conclusively determined.

Permafrost soils are particularly vulnerable to climate change. To assess and improve estimations of carbon (C) and nitrogen (N) budgets it is necessary to accurately map soil carbon and nitrogen in the permafrost region. In particular, soil organic carbon (SOC) stocks have been predicted and mapped by many studies from local to pan-Arctic scales. Several studies have been carried out at the Canadian Beaufort Sea coast, though no regional maps of terrestrial carbon stocks based on spatial modelling has been conducted yet. This study combines available field data from the Canadian Yukon coastal plain and uses it to map regional SOC and N stocks using the machine learning algorithm random forest and environmental variables based on remote sensing data. We developed models using the data for the entire region and separate models for the coastal mainland area and Qikiqtaruk Herschel Islandand. Each model was used to map SOC and N stocks for its respective area. We assessed the performance of the different random forest models by using crossvalidation. We further assessed model results using the Area of Applicability (AOA) method and the quantile regression forest approach, comparing the results and discussing their implications within the context of both methods. We explore local differences in soil properties and how soil data distribution across the region affects the accuracy of the predictions of SOC and N stocks. The estimated SOC stock for the upper metre is 48.7 +/- 6.6 kg m-2 and the N stock 3.03 +/- 0.30 kg m-2. The average SOC stocks vary significantly when creating separate models for subsets of the data. Qikiqtaruk Herschel Island is geologically different from the coastal mainland and has on average lower SOC stocks. Including Qikiqtaruk Herschel Island soil data to predict SOC stocks at the mainland has large impact on the results. Differences in N stocks were not as dependent on the location as SOC stocks and rather differences between individual studies occurred. The results of the separate models show 38.0 +/- 5.6 kg C m-2 and 2.87 +/- 0.34 kg N m-2 for Qikiqtaruk Herschel Island and 52.5 +/- 6.3 kg C m-2 and 3.15 +/- 0.32 kg N m-2 for the mainland. These estimates refer to the entire study area, without masking out regions outside the Area of Applicability (AOA). Our results indicate that the spatial perspective, whether regional or local (whole area vs. mainland and Qikiqtaruk Herschel Island separately), can affect the patterns and values represented in the resulting maps, highlighting the importance of scale when interpreting SOC and N stock distributions. Using a more densely sampled, region-specific dataset, our study captures finer regional-scale patterns than previous lower-resolution or pan-Arctic analyses.

Rising Arctic temperatures are making polygonal tundra increasingly vulnerable, primarily due to high ground ice contents. These landscapes form through soil-hydrology interactions, leading to ice wedge formation and degradation. Understanding the future of ice wedge polygon (IWP) landscapes requires detailed land cover classifications, as soil properties vary significantly across IWP sub-features like rims and centers. Existing classifications often distinguish between high-centered (HCP) and low-centered (LCP) polygons but fail to capture finer sub-feature distributions. This study provides high-resolution land cover datasets for two IWP sites on the Canadian Beaufort coast using WorldView-3 imagery. Ptarmigan Bay features well-defined landforms, while Komakuk Beach exhibits greater permafrost degradation. We compare two land cover-mapping approaches: object-based image analysis (OBIA) with segmentation and random forest classification, and a deep learning U-net model. Results show that the OBIA-random forest method performed better, and substantial differences in the landform type distribution between study areas and methods exist. Both methods identify 60% of HCP centers at Ptarmigan Bay and 50% at Komakuk Beach, but mapped IWP sub-feature proportions (HCP troughs, LCP centers, LCP rims) vary across areas and methods, reflecting classification uncertainties. Furthermore, the transferability of models between regions is constrained when there are pronounced differences in degradation of landforms.

Boreal forests provide considerable global land carbon storage and uptake, but they are being rapidly transformed to managed secondary forests, with poorly quantified implications for ecosystem carbon storage. Here we present data from extensive mapping and field inventories of carbon storage in primary forests in Sweden and use multiple methods to show that primary forests store similar to 72% (70 to 74% across methods) more carbon than managed secondary forests in vegetation, deadwood, soils, and harvested wood products combined. Soils constitute both the largest carbon store and the largest difference between these forest types. The total carbon storage difference between primary and managed secondary forests is 2.7 to 8.0 times larger than previous estimates. Our results challenge estimated past and future contributions of boreal forest management to atmospheric carbon dioxide concentrations.

Understanding how soil respiration responds to fluctuations in soil moisture and temperature is increasingly important as climate change intensifies drying and rewetting events and causes soil warming. In this study, we investigated whether the temperature sensitivity of soil respiration (Q₁₀) and its temperature dependence differ between drying and rewetting periods. We collated high-frequency field data on soil respiration, soil moisture, and soil temperature from the COSORE dataset (28 sites), and partitioned them into dry-down and rewetting periods. Next, random forest regressions and accumulated local effects plots were used to assess the main effect of temperature on soil respiration for both periods. This approach enables estimation of the temperature dependence of Q₁₀ without the confounding effect of soil moisture variations. Results showed that the temperature dependence of Q₁₀ differs between the drying and rewetting periods; Q₁₀ is lower during rewetting than during drying in soils with temperatures below 15 °C, but higher in soils with temperatures higher than 15 °C. No differences were found in the temperature dependences of Q₁₀ between rewetting events followed by large and small respiration pulses. We conclude that the effects of warming on soil respiration differ between dry-down and rewetting periods.

Arctic lowland tundra is characterized by pronounced spatial heterogeneity that introduces uncertainty into predictions of permafrost soil carbon dynamics. In these ecosystems, edaphic variability is primarily structured along two spatial axes: ice wedge polygon microtopography at the terrain scale and soil layers at the pedon scale. Here, we investigated how polygon types (low-, flat-, and high-centered polygons) and major soil layers (organic topsoil, mineral subsoil, cryoturbated material, and upper permafrost) jointly shape soil organic matter pools, microbial community composition, and potential extracellular enzyme activities.Polygon-specific patterns in soil organic matter characteristics and microbial communities persisted across all soil layers, and soil-layer specific differences were consistent across polygon types, while interactive effects were comparatively minor. Low centered polygons showed reduced organic matter bioavailability, lower microbial abundances, and diminished hydrolytic enzyme potential compared to flat- and high-centered polygons. Organic topsoils emerged as pronounced microbial and enzymatic hotspots. The upper permafrost contained substantial amounts of relatively undecomposed organic matter and indicated a considerable potential for hydrolytic degradation upon thaw. Across both spatial axes, patterns in soil organic matter pools, and microbial communities were largely structured along gradients in organic matter inputs and redox conditions, which themselves arise from interactions in surface microtopography, hydrology, and vegetation.Overall, our findings demonstrate that a limited number of spatial units captures a disproportionate share of edaphic, microbial, and biogeochemical variability in Arctic lowland tundra soils. Explicitly accounting for polygon morphologies and major soil layers therefore provides a tractable framework for upscaling soil processes across spatially heterogeneous ecosystems and improving climate-relevant biogeochemical projections.

Permafrost thaw due to climate change raises concerns about the potential remobilisation of organic pollutants such as Polycyclic Aromatic Hydrocarbons (PAHs) in Arctic environments. This study aims to identify environmental controls and create maps of PAH storage in two catchment-scale study areas, Komakuk Beach and Ptarmigan Bay, on the Canadian Beaufort coast (Yukon), while dealing with the lack of data concerning organic contaminants in Arctic soils. Soil samples were collected using a stratified random sampling design based on the catchment area and quaternary geology. The samples were analysed for 22 different PAHs, including 16 PAHs classified as priority pollutants by the US EPA, using accelerated solvent extraction and gas chromatography – triple quadrupole mass spectrometry. The dataset included measurements of PAHs, soil organic carbon content (SOC %), depth, and landform types from permafrost sediment and soil samples. Significant differences in PAH concentrations were found across soil depths and between low-centered polygon (LCP) and high-centered polygon (HCP) landforms, reflecting varying biogeochemical and hydrological dynamics in permafrost degradation forms. Random Forest models were used to predict spatial PAHs distributions in the study areas, divided by molecular weight and depth intervals, using SOC% data from digital soil mapping as an environmental support variable. The modelling approach showed promise for intermediate soil layers but faced challenges in the surface and deep layers owing to data limitations. This study addresses a critical knowledge gap and demonstrates the potential of upscaling PAH distribution data in Arctic permafrost regions using limited ground data. These findings highlight the complex relationships between PAHs, soil carbon, and permafrost landforms, emphasising the need to consider these factors when assessing contaminant dynamics in thawing permafrost. This study identifies spatial patterns and landscape-level factors for PAH upscaling, which can support further studies across Arctic permafrost regions.

Non-Technical Summary  This review highlights 10 recent advances in climate change research with high policy relevance, spanning diverse topics: (1) the global temperature jump of 2023–2024; (2) sea surface warming and marine heatwaves; (3) land carbon sinks; (4) interactions between climate change and biodiversity loss; (5) accelerated groundwater decline; (6) global dengue incidence; (7) income and labour productivity loss; (8) strategic considerations for scaling carbon dioxide removal (CDR); (9) integrity of carbon credit markets; and (10) policy mixes for climate change mitigation.

Technical Summary  Interdisciplinary understanding is vital for delivering sound climate policy advice. However, navigating the ever-growing and increasingly diverse scholarly literature on climate change is challenging for any individual researcher. This annual synthesis highlights and explains recent advances across a variety of fields of climate change research. This year, the 10 insights focus on: (1) the record-warmth of 2023/2024 and the elevated Earth energy imbalance; (2) acceleration of ocean warming and intensifying marine heatwaves; (3) northern land carbon sinks under strain; (4) reinforcing feedback between biodiversity loss and climate change; (5) accelerated depletion of groundwater; (6) global dengue incidence; (7) global income losses and labour productivity declines; (8) strategic scaling of CDR; (9) integrity challenges in carbon credit markets and emerging responses; and (10) effective policy mixes for emissions reductions. The insights have been written to be accessible to researchers from different fields, serving as entry-points to specific topics, as well as providing an overview of the evolving landscape of climate change research. In the final section, the insights are used to develop overarching policy-relevant messages. This paper provides the basis for a science-policy report that was shared with all Party delegations ahead of COP30 in Belém, Brazil.

Purpose of Review  We review how ‘abrupt thaw’ has been used in published studies, compare these definitions to abrupt processes in other Earth science disciplines, and provide a definitive framework for how abrupt thaw should be used in the context of permafrost science.

Recent Findings  We address several aspects of permafrost systems necessary for abrupt thaw to occur and propose a framework for classifying permafrost processes as abrupt thaw in the future. Based on a literature review and our collective expertise, we propose that abrupt thaw refers to thaw processes that lead to a substantial persistent environmental change within a few decades. Abrupt thaw typically occurs in ice-rich permafrost but may be initiated in ice-poor permafrost by external factors such as hydrologic change (i.e., increased streamflow, soil moisture fluctuations, altered groundwater recharge) or wildfire.

Summary  Permafrost thaw alters greenhouse gas emissions, soil and vegetation properties, and hydrologic flow, threatening infrastructure and the cultures and livelihoods of northern communities. The term ‘abrupt thaw’ has emerged in scientific discourse over the past two decades to differentiate processes that rapidly impact large depths of permafrost, such as thermokarst, from more gradual, top-down thaw processes that impact centimeters of near-surface permafrost over years to decades. However, there has been no formal definition for abrupt thaw and its use in the scientific literature has varied considerably. Our standardized definition of abrupt thaw offers a path forward to better understand drivers and patterns of abrupt thaw and its consequences for global greenhouse gas budgets, impacts to infrastructure and land-use, and Arctic policy- and decision-making.