Black carbon (BC) aerosols are short-lived climate pollutants with important, but uncertain, climate impacts. In this Review, we synthesize observations of atmospheric BC concentrations, sources, optical properties, lifetimes and climate effects, drawing comparisons with atmospheric model simulations. Isotopic fingerprinting reveals regional differences in BC sources, with biomass burning contributing 93 ± 3% in sub-Saharan Africa, 56 ± 7% in South Asia and 28 ± 5% in East Asia. Atmospheric BC loadings have declined in South America, East Asia, Europe and North America, and stabilized in Africa and South Asia owing to clean air policies and advances in technology and practices. The optical properties of BC influence its climate effects. The global-mean mass absorption coefficient (MAC550) of atmospheric BC is 12.3 ± 5.8 m2 g−1, being highest in Africa, Europe and South Asia. MAC550 is enhanced near universally by 1.6 ± 0.4 owing to ageing during long-range transport. In major emission regions, the aerosol absorption optical depth and the direct aerosol radiative forcing ratio between the bottom and the top of the atmosphere are lower in model simulations than in observations by factors of 2 and 1.5, respectively. Relative to long-term observations, model simulations estimate higher BC deposition fluxes but lower concentrations and sunlight absorption. These discrepancies have implications for the accuracy of model representations of humidity, clouds, precipitation and climate forcing. Future research should prioritize comparisons of emission inventory and model estimates with observations to enhance model accuracy and guide mitigation efforts.

Elevated methane concentrations in seawater have been reported over extensive areas of the East Siberian Arctic Seas, overlying thawing subsea permafrost. However, observed methane concentrations of the ephemeral seawater are highly variable across both space and time, compromised by both the timing of rare measurements and storm-driven exchanges to the atmosphere. Here, we applied time-integrated signals of the δ¹³C-composition of specific C₃₀ hopanoids (diploptene, hop-17(21)-ene, neohop-13(18)-ene and diplopterol) in surface sediments to trace aerobic methane oxidation as a proxy for enhanced methane cycling. Interpretations of hopanoids and possible sources were further assessed by 16S-rRNA analyses in the surface sediments. The consistently low δ¹³C-C₃₀ hopenes signals, ranging between −57.5 ‰ to −37.1 ‰ (n=23) across the Laptev Sea shelf indicated aerobic methane oxidation. This suggests ubiquitous methane cycling with the most pronounced intensities in the outer shelf region, broadly consistent with the observed methane concentrations. Notably, depleted δ¹³C-C₃₀ hopenes were also found in the mid-shelf region of the Laptev Sea, earlier thought to be an area of comparatively low methane cycling. High methane concentrations were also observed in the vicinity of the Lena River delta, yet the isotopically heavier δ¹³C-C₃₀ hopenes may here reflect a combination of lower aerobic methane oxidation, a greater relative abundance of type II methanotrophs (lower isotope fractionation during hopanoid production) and isotope dilution from non-methanotrophic sources. While this complicates the biomarker interpretation in the unique setting near the Lena River delta, the δ¹³C-C₃₀ hopenes were still much lower than δ¹³C-organic carbon, indicating aerobic methane oxidation and a clear methane cycling signal also in this regime. Taken together, the results unravel the wider cross-shelf patterns of enhanced methane cycling in the Laptev Sea through probing of methane fossilised in membrane lipids of aerobic methanotrophs, with the molecular-isotopic pattern being preserved in the sedimentary archive.

The shallow East Siberian Arctic Shelf (ESAS) shows high methane concentration in seawater; however, methane oxidation processes for the ESAS are poorly constrained. Here, we examined porewater geochemical profiles, microbial lipids, and their carbon isotope compositions in multiple sediment cores to explore molecular-isotopic diagnostics of microbial methane oxidation in the outer Laptev Sea. There was covariation between methane and sulfate profiles in only one core in the studied upper 20 cm. Across all cores, concentrations of microbial fatty acids (C₁₂–C₁₈) decreased sharply below the surface sediment with carbon isotope compositions (δ¹³C) ranging from −31 to −20‰. Notably, anaerobic methane oxidizing archaea (ANME)-derived lipid biomarkers, e.g., 2,6,10,15,19-pentamethylicosane, archaeal and hydroxyarchaeol, were absent or below detection limits, indicating that anaerobic methane oxidation (AOM) was not a dominant process in the studied sediment. However, the tetraether-based Methane Index increased from 0.03 to 0.8 in the core where sulfate and methane covaried, implying either a developing AOM microbial community with presently low activity or an episodic AOM event occurring within the past decades to a century. In contrast, hop-17(21)-ene was present throughout the cores, exhibiting strong ¹³C depletion, with values ranging from −66.1 to −44.0‰. The concentration and δ¹³C values of hop-17(21)-ene were distinct from the terrestrially-derived nC₃₁ alkane, which showed relatively constant δ¹³C values of −34.9 ± 1.1‰. This suggests that hop-17(21)-ene in the sediments is primarily derived from marine microbes. Given the oxic seawater in this system (< 60 m), the ¹³C-depleted hop-17(21)-ene likely reflects aerobic methane oxidation (AeOM) occurring in the water column and/or in surface sediments. AeOM may play a significant role in regulating methane in this subsea permafrost seepage system.

The East Siberian Arctic Shelf (ESAS) is a significant source of atmospheric methane, and the coastal Laptev Sea near the Lena River Delta is a documented hotspot for methane seepage. While anaerobic oxidation of methane (AOM) acts as a critical biofilter, the specific pathways and electron acceptors driving this process in the unique river-influenced settings of the Arctic remain poorly understood. The Lena River delivers substantial terrigenous nutrients to this region, contributing approximately 10 % of the total riverine nitrogen and 17 % of the iron input to the Arctic Ocean. This study examines the hypothesis that these nutrient fluxes play a crucial role in regulating AOM, a key process mitigating methane emissions. We compared the geochemical conditions at active methane seep sites and background areas, with a special focus on nitrogen and iron compounds in bottom and pore waters.Our geochemical investigation reveals three key findings: (1) Methane concentrations were substantially elevated at seep sites (up to 3118 nM) compared to background areas (as low as 5.5 nM) and showed negative correlations with nitrate (NO₃⁻) and iron (Fe) concentrations. (2) A clear decoupling of the FeMn cycles at seep sites, which was absent in background sediments, indicates the preferential utilization of reactive iron oxides in AOM (Fe-AOM). (3) A severe depletion of bioavailable nitrogen species was observed in seep zones, where nitrate and nitrite concentrations were 5.5-fold and 2-fold lower, respectively, than in background areas, suggesting active nitrate/nitrite-dependent AOM (N-AOM).These findings demonstrate that on the nutrient-rich, river-influenced Laptev Sea shelf, AOM is primarily driven not by canonical sulfate reduction, but by the reduction of nitrogen species and iron. This newly identified coupling between riverine nutrient fluxes and AOM pathways represents a critical, yet previously overlooked, mechanism that could significantly suppress methane emissions from climate-sensitive Arctic shelves. To our knowledge, this study provides the first comprehensive investigation of nitrogen and iron transformation mechanisms coupled with methane oxidation at the water-sediment interface in the cold seeps of the coastal Laptev Sea shelf.

Sulfate aerosols cool the climate and thus temporarily mask climate warming, but at a cost to air quality. Their short atmospheric lifetime leads to heterogeneous global coverage, with sulfate concentrations over South Asia being especially elevated and continuing to increase. It remains challenging to constrain the relative importance of different emission sources due to poor observational coverage and uncertainties in bottom-up technology-based emission estimates. The stable sulfur isotope composition (delta 34S-SO42-) quantitatively distinguishes natural and anthropogenic sources. This study aimed to constrain the sources of sulfate arriving at the Maldives Climate Observatory Hanimaadhoo (MCOH), which is ideally situated for intercepting the outflow from airsheds over the Indian subcontinent. The results show that anthropogenic sources of sulfate contributed 93 +/- 14 %, 87 +/- 10 %, and 66 +/- 12 % in winter (post-monsoon), spring (pre-monsoon), and summer (monsoon), respectively. There was also a moderate to strong correlation (r2= 0.75, p << 0.05, n=7) between continental anthropogenic (winter and spring) sulfate (delta 34S) and black carbon aerosols from fossil fuel combustion (pinpointed by Delta 14C). This study provides improved constraints on sulfate sources for South Asia - a key region for aerosol pollution and aerosol masking of climate warming.

Elevated methane concentrations are observed in the shallow water column above subsea permafrost on the East Siberian Arctic Shelf, including the inner Laptev Sea. The subsea source of this methane is poorly understood, yet crucial for predicting future methane emissions. Here, we combine analyses of dissolved methane concentrations, triple-isotopic fingerprinting, and Bayesian Markov Chain Monte Carlo statistics to constrain the source of high-concentration methane sampled during four expeditions, 2016-2020. In contrast to earlier findings of predominantly thermogenic methane release in the outer Laptev Sea, our results for the major methane release region of the inner Laptev Sea suggest that old microbial methane (radiocarbon age >48000 y BP) is being released from preformed methane pools stored within the subsea permafrost system. This observation reveals that several sources contribute to the elevated methane on the shelf and thus the necessity to consider a diversity of sources when estimating future methane release trajectories.

Air pollution is the leading environmental cause of premature death and an impediment to sustainable development in Africa, where exposure levels are high, yet data are scarce. This study provides year-round speciation and source identification for PM_2.5-bound polycyclic aromatic hydrocarbons (PAH) in the East African city of Nairobi. The ∑₁₉PAH concentrations ranged between 5–20 ng m^–3 with an average of 11 ± 4 ng m^–3 in the urban background and were dominated by heavy molecular weight compounds (four or more fused benzene rings). The PAH loadings in Nairobi stayed rather invariant through the year with limited seasonal variability. The observed PAH concentrations in Nairobi were higher than those reported in cities with more stringent emission controls. Furthermore, the calculated benzo[a]pyrene equivalent in Nairobi’s background atmosphere exceeded 1 ng m^–3, signaling a severe health concern. Source identification using molecular diagnostic ratio analysis suggests the PAH in Nairobi are predominantly of pyrogenic (combustion of fossil fuels and biomass burning) origins. Overall, this study provides a baseline reference data set for future local and regional studies and contributes to scientific underpinnings to motivate the urgent need to develop emission control initiatives in Nairobi, among other rapidly growing African cities.

Meiofauna (all invertebrates smaller than 1 mm) are not only sensitive to environmental changes but also contribute significantly to nutrient cycling and energy transfer to higher trophic levels. Despite their importance, meiofauna distribution and ecology in the Siberian seas remain understudied. Here, we employ sediment environmental DNA metabarcoding to characterize meiofauna diversity across the unexplored Siberian seas. We show that meiofauna community structure is primarily driven by river discharge and coastal erosion, which are heavily influenced by climate change, rather than geographical distinctions between the seas. We observed higher meiofauna diversity in nearshore areas where river plumes promoted colonizer nematode communities that are resilient to disturbances. Yet, their dominance may lead to decreased ecosystem stability in the future. This study provides a valuable baseline for meiofauna diversity in remote Siberian seas undergoing rapid environmental change, which will be useful for assessing the future direction and pace of benthic ecological trajectories.

Organic matter stored in Arctic permafrost represents a key component of the carbon cycle, yet its reactivity across heterogeneous continent-scale permafrost regions remains poorly understood. Here, we leverage the four shelf seas of the Eurasian Arctic as integrative receptor systems to evaluate terrestrial organic matter reactivity, assessed by examining organic carbon preservation as a function of ¹⁴C-constrained cross-shelf transport time. Our findings reveal higher reactivity of terrestrial organic matter released to the Laptev Sea and the eastern East Siberian Sea, lower reactivity in the western East Siberian Sea, and no deducible degradation in the Kara Sea. The reactivity of terrestrial organic matter is primarily determined by the degradation status and composition of its source, alongside potential microbiological controls during transport. This study reveals the heterogeneity of terrestrial organic matter reactivity across the Eurasian Arctic margin and highlights the need for detailed assessments of region-specific carbon release and modeling parameterization.