Methane (CH₄) is produced in many natural systems that are vulnerable to change under a warming climate, yet current CH₄ budgets, as well as future shifts in CH₄ emissions, have high uncertainties. Climate change has the potential to increase CH₄ emissions from critical systems such as wetlands, marine and freshwater systems, permafrost, and methane hydrates, through shifts in temperature, hydrology, vegetation, landscape disturbance, and sea level rise. Increased CH₄ emissions from these systems would in turn induce further climate change, resulting in a positive climate feedback. Here we synthesize biological, geochemical, and physically focused CH₄ climate feedback literature, bringing together the key findings of these disciplines. We discuss environment‐specific feedback processes, including the microbial, physical, and geochemical interlinkages and the timescales on which they operate, and present the current state of knowledge of CH₄ climate feedbacks in the immediate and distant future. The important linkages between microbial activity and climate warming are discussed with the aim to better constrain the sensitivity of the CH₄ cycle to future climate predictions. We determine that wetlands will form the majority of the CH₄ climate feedback up to 2100. Beyond this timescale, CH₄ emissions from marine and freshwater systems and permafrost environments could become more important. Significant CH₄ emissions to the atmosphere from the dissociation of methane hydrates are not expected in the near future. Our key findings highlight the importance of quantifying whether CH₄ consumption can counterbalance CH₄ production under future climate scenarios.

It is now established that microplastics are a pervasive presence in aquatic and terrestrial ecosystems. The same is assumed to be true for nanoplastics but data are lacking due to technical difficulties associated with sample analysis. Here, we measured nanoplastics in waterbodies at two contrasting sites: remote Siberian Arctic tundra and a forest landscape in southern Sweden. Nanoplastics were detected in all sampled Swedish lakes (n = 7) and streams (n = 4) (mean concentration = 563 µg l⁻¹) and four polymer types were identified (polyethylene, polyvinyl chloride (PVC), polypropylene, polyethylene terephthalate). In Siberia nanoplastics were detected in 7/12 sampled lakes, ponds and surface flooding, but only two polymer types were detected (PVC and polystyrene) and concentrations were lower (mean 51 µg l⁻¹). Based on back-calculation of air mass trajectories and particle dispersion, we infer that nanoplastics arrive at both sites by aerial deposition from local and regional sources. Our results suggest that nanoplastics may be a near-ubiquitous presence even in remote ecosystems.

The magnitude of future emissions of greenhouse gases from the northern permafrost region depends crucially on the mineralization of soil organic carbon (SOC) that has accumulated over millennia in these perennially frozen soils. Many recent studies have used radiocarbon (¹⁴C) to quantify the release of this “old” SOC as CO₂ or CH₄ to the atmosphere or as dissolved and particulate organic carbon (DOC and POC) to surface waters. We compiled ~1,900 ¹⁴C measurements from 51 sites in the northern permafrost region to assess the vulnerability of thawing SOC in tundra, forest, peatland, lake, and river ecosystems. We found that growing season soil ¹⁴C‐CO₂ emissions generally had a modern (post‐1950s) signature, but that well‐drained, oxic soils had increased CO₂ emissions derived from older sources following recent thaw. The age of CO₂ and CH₄ emitted from lakes depended primarily on the age and quantity of SOC in sediments and on the mode of emission, and indicated substantial losses of previously frozen SOC from actively expanding thermokarst lakes. Increased fluvial export of aged DOC and POC occurred from sites where permafrost thaw caused soil thermal erosion. There was limited evidence supporting release of previously frozen SOC as CO₂, CH₄, and DOC from thawing peatlands with anoxic soils. This synthesis thus suggests widespread but not universal release of permafrost SOC following thaw. We show that different definitions of “old” sources among studies hamper the comparison of vulnerability of permafrost SOC across ecosystems and disturbances. We also highlight opportunities for future ¹⁴C studies in the permafrost region.

Inland waters (rivers, lakes and ponds) are important conduits for the emission of terrestrial carbon in Arctic permafrost landscapes. These emissions are driven by turnover of contemporary terrestrial carbon and additional pre-aged (Holocene and late-Pleistocene) carbon released from thawing permafrost soils, but the magnitude of these source contributions to total inland water carbon fluxes remains unknown. Here we present unique simultaneous radiocarbon age measurements of inland water CO₂, CH₄and dissolved and particulate organic carbon in northeast Siberia during summer. We show that >80% of total inland water carbon was contemporary in age, but pre-aged carbon contributed >50% at sites strongly affected by permafrost thaw. CO₂and CH₄were younger than dissolved and particulate organic carbon, suggesting emissions were primarily fuelled by contemporary carbon decomposition. Our findings reveal that inland water carbon emissions from permafrost landscapes may be more sensitive to changes in contemporary carbon turnover than the release of pre-aged carbon from thawing permafrost.

Summary: Arctic permafrost soils store large amounts of organic matter that is sensitive to temperature increases and subsequent microbial degradation to methane (CH ₄ ) and carbon dioxide (CO ₂ ). Here, we studied methanogenic and methanotrophic activity and community composition in thermokarst lake sediments from Utqiag˙vik (formerly Barrow), Alaska. This experiment was carried out under in situ temperature conditions (4°C) and the IPCC 2013 Arctic climate change scenario (10°C) after addition of methanogenic and methanotrophic substrates for nearly a year. Trimethylamine (TMA) amendment with warming showed highest maximum CH ₄ production rates, being 30% higher at 10°C than at 4°C. Maximum methanotrophic rates increased by up to 57% at 10°C compared to 4°C. 16S rRNA gene sequencing indicated high relative abundance of Methanosarcinaceae in TMA amended incubations, and for methanotrophic incubations Methylococcaeae were highly enriched. Anaerobic methanotrophic activity with nitrite or nitrate as electron acceptor was not detected. This study indicates that the methane cycling microbial community can adapt to temperature increases and that their activity is highly dependent on substrate availability.

Despite the importance of high-latitude surface energy budgets (SEBs) for land-climate interactions in the rapidly changing Arctic, uncertainties in their prediction persist. Here, we harmonize SEB observations across a network of vegetated and glaciated sites at circumpolar scale (1994–2021). Our variance-partitioning analysis identifies vegetation type as an important predictor for SEB-components during Arctic summer (June-August), compared to other SEB-drivers including climate, latitude and permafrost characteristics. Differences among vegetation types can be of similar magnitude as between vegetation and glacier surfaces and are especially high for summer sensible and latent heat fluxes. The timing of SEB-flux summer-regimes (when daily mean values exceed 0 Wm⁻²) relative to snow-free and -onset dates varies substantially depending on vegetation type, implying vegetation controls on snow-cover and SEB-flux seasonality. Our results indicate complex shifts in surface energy fluxes with land-cover transitions and a lengthening summer season, and highlight the potential for improving future Earth system models via a refined representation of Arctic vegetation types.