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Metabolism is a key property of lake ecosystem functioning, but logistical challenges make it difficult to estimate across remote regions. The steady-state dissolved oxygen (DO) stable isotope method (18O method) estimates metabolism from discrete water samples and thus enables large-scale surveys. However, this method relies on the assumptions that the upper mixed layer DO saturation (DO%) relative to its isotopic composition (δ18ODO) is at a steady state and that an increase in DO% results in a proportional decrease in δ18ODO. The applicability of these assumptions has not been broadly assessed for small, low-productivity lakes with predominantly benthic metabolism. We evaluated the 18O method in these types of systems by surveying 184 Arctic lakes in Sweden and found that the method consistently produces realistic estimates of metabolism in well-mixed conditions and when water temperatures were relatively stable. Under such conditions, results from the 18O method agreed with those from the free-water diel DO method, and rates derived from both methods responded similarly to environmental drivers. In contrast, we found that the 18O method frequently generated unrealistic metabolic rates when temperatures were rising. Increasing temperatures may increase DO% irrespective of δ18ODO in the upper mixed layer and promote lake stratification, both violating the assumptions of the 18O method and preventing benthic metabolism from being integrated by surface water samples. We conclude that the 18O method is a powerful tool for studying metabolism in Arctic lakes across large spatial gradients, provided that temperature dynamics and vertical stratification are considered.

The atmospheric methane (CH4) growth rate surged after 2019, peaking at 16.2 parts per billion per year (ppb year−1) in 2020 before declining to 8.6 ppb year−1 in 2023. using multiple atmospheric inversions constrained by observation- and model-based prescribed hydroxyl radical (OH) fields and CH4 atmospheric data, we show that a drop of OH radicals in 2020–2021, followed by recovery in 2022–2023, accounted for 83% of year-on-year variations in the CH4 growth rate, the rest being explained by wetland and inland water emissions, which increased between 2019 and 2020–2022 [+8.6 ± 2.6 teragrams of CH4 per year (TgCH4 year−1)] and then decreased between 2022 and 2023 (−9.9 ± 3.3 TgCH4 year−1). Most emission changes from 2019 to 2023 occurred in northern tropical wetlands in africa and asia, whereas South american wetlands emissions declined and arctic emissions increased after 2019.

Hydrological disturbances following storm events influence the structure and functioning of headwater streams. However, understanding how these disturbances impact critical processes such as stream metabolism is challenging. We assessed the effect of storm events on the resistance and resilience of gross primary production (GPP) and ecosystem respiration (ER) in a heterotrophic headwater stream. We hypothesized that stream metabolism will show low resistance to storm events because GPP and ER will be either stimulated by inputs of limited resources (small storms) or suppressed by biofilm damage (large storms). We also expected resilience to decrease with the size of the storm event. To test these hypotheses, we hydrologically characterized 53 individual storm events during 4.5 years (period October 2018-February 2023) and estimated metabolic rates prior, during, and after each event. Individual storm events had different duration (from 4 to 32 d), and exhibited contrasting changes in discharge (discharge from 0.6 to 872.4 L s^-1). Due to data and model constraints, we were able to estimate metabolic rates for 35 of these events, for which GPP and ER averaged 1.7 ± 1.8 and -13.4 ± 7 g O₂ m^-2 d^-1, respectively. The two processes showed low resistance to storm events, with magnitudes increasing in 69 % and 86 % of the cases for GPP and ER, respectively. The relationship between hydrological parameters and changes in GPP was not statistically significant, while a positive relation with the magnitude of the storm event was found for ER (R² > 0.37). Similarly, recovery times were positively related to the size of the event only for ER (R² > 0.46). Yet recovery times were always lower than 6 d, suggesting that the positive effect of resource inputs on stream metabolic activity was limited over time. Our findings support the idea that storm events stimulate metabolic activity in headwater streams, especially ER, and highlight how changes in hydrological regimes could impact stream functioning and its role in global biogeochemical cycles.

Inland waters emit large amounts of carbon and are key players in the global carbon budget. Particularly high rates of carbon emissions have been reported in streams draining mountains, tropical regions, and peatlands. However, few studies have examined the spatial variability of CO2 concentrations and fluxes occurring within these systems, particularly as a function of catchment morphology. Here we evaluated spatial patterns of CO2 in three tropical, headwater catchments in relation to the river network and stream geomorphology. We measured dissolved carbon dioxide (pCO2), aquatic CO2 emissions, discharge, and stream depth and width at high spatial resolutions along multiple stream reaches. Confirming previous studies, we found that tropical headwater streams are an important source of CO2 to the atmosphere. More notably, we found marked, predictable spatial organization in aquatic carbon fluxes as a function of landscape position. For example, pCO2 was consistently high (>10,000 ppm) at locations close to groundwater sources and just downstream of hydrologically connected wetlands, but consistently low (<1,000 ppm) in high gradient locations or river segments with larger drainage areas. Taken together, our findings suggest that catchment area and stream slope are important drivers of pCO2 and gas transfer velocity (k) in mountainous streams, and as such they should be considered in catchment-scale assessments of CO2 emissions. Furthermore, our work suggests that accurate estimation of CO2 emissions requires understanding of dynamics across the entire stream network, from the smallest seeps to larger streams.

Climate warming is causing more extreme weather conditions, with both larger and more intense precipitation events as well as extended periods of drought in many regions of the world. The consequence is an alteration of the hydrological regime of streams and rivers, with an increase in the probability of extreme hydrological conditions. Mediterranean-climate regions usually experience extreme hydrological events on a seasonal basis and thus, freshwater Mediterranean ecosystems can be used as natural laboratories for better understanding how climate warming will impact ecosystem structure and functioning elsewhere. In this paper, we revisited and contextualized historical and new datasets collected at Fuirosos, a well-studied Mediterranean intermittent stream naturally experiencing extreme hydrological events, to illustrate how the seasonal alternation of floods and droughts influence hydrology, microbial assemblages, water chemistry, and the potential for biogeochemical processing. Moreover, we revised some of the most influential conceptual and quantitative frameworks in river ecology to assess to what extent they incorporate the occurrence of extreme hydrological events. Based on this exercise, we identified knowledge gaps and challenges to guide future research on freshwater ecosystems under intensification of the hydrological cycle. Ultimately, we aimed to share the lessons learned from ecosystems naturally experiencing extreme hydrological events, which can help to better understand warming-induced impacts on hydrological transport and cycling of matter in fluvial ecosystems.

Understanding and quantifying the global methane (CH4) budget is important for assessing realistic pathways to mitigate climate change. CH4 is the second most important human-influenced greenhouse gas in terms of climate forcing after carbon dioxide (CO2), and both emissions and atmospheric concentrations of CH4 have continued to increase since 2007 after a temporary pause. The relative importance of CH4 emissions compared to those of CO2 for temperature change is related to its shorter atmospheric lifetime, stronger radiative effect, and acceleration in atmospheric growth rate over the past decade, the causes of which are still debated. Two major challenges in quantifying the factors responsible for the observed atmospheric growth rate arise from diverse, geographically overlapping CH4 sources and from the uncertain magnitude and temporal change in the destruction of CH4 by short-lived and highly variable hydroxyl radicals (OH). To address these challenges, we have established a consortium of multidisciplinary scientists under the umbrella of the Global Carbon Project to improve, synthesise, and update the global CH4 budget regularly and to stimulate new research on the methane cycle. Following Saunois et al. (2016, 2020), we present here the third version of the living review paper dedicated to the decadal CH4 budget, integrating results of top-down CH4 emission estimates (based on in situ and Greenhouse Gases Observing SATellite (GOSAT) atmospheric observations and an ensemble of atmospheric inverse-model results) and bottom-up estimates (based on process-based models for estimating land surface emissions and atmospheric chemistry, inventories of anthropogenic emissions, and data-driven extrapolations). We present a budget for the most recent 2010–2019 calendar decade (the latest period for which full data sets are available), for the previous decade of 2000–2009 and for the year 2020.

The revision of the bottom-up budget in this 2025 edition benefits from important progress in estimating inland freshwater emissions, with better counting of emissions from lakes and ponds, reservoirs, and streams and rivers. This budget also reduces double counting across freshwater and wetland emissions and, for the first time, includes an estimate of the potential double counting that may exist (average of 23 Tg CH4 yr−1). Bottom-up approaches show that the combined wetland and inland freshwater emissions average 248 [159–369] Tg CH4 yr−1 for the 2010–2019 decade. Natural fluxes are perturbed by human activities through climate, eutrophication, and land use. In this budget, we also estimate, for the first time, this anthropogenic component contributing to wetland and inland freshwater emissions. Newly available gridded products also allowed us to derive an almost complete latitudinal and regional budget based on bottom-up approaches.

For the 2010–2019 decade, global CH4 emissions are estimated by atmospheric inversions (top-down) to be 575 Tg CH4 yr−1 (range 553–586, corresponding to the minimum and maximum estimates of the model ensemble). Of this amount, 369 Tg CH4 yr−1 or ∼ 65 % is attributed to direct anthropogenic sources in the fossil, agriculture, and waste and anthropogenic biomass burning (range 350–391 Tg CH4 yr−1 or 63 %–68 %). For the 2000–2009 period, the atmospheric inversions give a slightly lower total emission than for 2010–2019, by 32 Tg CH4 yr−1 (range 9–40). The 2020 emission rate is the highest of the period and reaches 608 Tg CH4 yr−1 (range 581–627), which is 12 % higher than the average emissions in the 2000s. Since 2012, global direct anthropogenic CH4 emission trends have been tracking scenarios that assume no or minimal climate mitigation policies proposed by the Intergovernmental Panel on Climate Change (shared socio-economic pathways SSP5 and SSP3). Bottom-up methods suggest 16 % (94 Tg CH4 yr−1) larger global emissions (669 Tg CH4 yr−1, range 512–849) than top-down inversion methods for the 2010–2019 period. The discrepancy between the bottom-up and the top-down budgets has been greatly reduced compared to the previous differences (167 and 156 Tg CH4 yr−1 in Saunois et al. (2016, 2020) respectively), and for the first time uncertainties in bottom-up and top-down budgets overlap. Although differences have been reduced between inversions and bottom-up, the most important source of uncertainty in the global CH4 budget is still attributable to natural emissions, especially those from wetlands and inland freshwaters.

The tropospheric loss of methane, as the main contributor to methane lifetime, has been estimated at 563 [510–663] Tg CH4 yr−1 based on chemistry–climate models. These values are slightly larger than for 2000–2009 due to the impact of the rise in atmospheric methane and remaining large uncertainty (∼ 25 %). The total sink of CH4 is estimated at 633 [507–796] Tg CH4 yr−1 by the bottom-up approaches and at 554 [550–567] Tg CH4 yr−1 by top-down approaches. However, most of the top-down models use the same OH distribution, which introduces less uncertainty to the global budget than is likely justified.

For 2010–2019, agriculture and waste contributed an estimated 228 [213–242] Tg CH4 yr−1 in the top-down budget and 211 [195–231] Tg CH4 yr−1 in the bottom-up budget. Fossil fuel emissions contributed 115 [100–124] Tg CH4 yr−1 in the top-down budget and 120 [117–125] Tg CH4 yr−1 in the bottom-up budget. Biomass and biofuel burning contributed 27 [26–27] Tg CH4 yr−1 in the top-down budget and 28 [21–39] Tg CH4 yr−1 in the bottom-up budget.

We identify five major priorities for improving the CH4 budget: (i) producing a global, high-resolution map of water-saturated soils and inundated areas emitting CH4 based on a robust classification of different types of emitting ecosystems; (ii) further development of process-based models for inland-water emissions; (iii) intensification of CH4 observations at local (e.g. FLUXNET-CH4 measurements, urban-scale monitoring, satellite imagery with pointing capabilities) to regional scales (surface networks and global remote sensing measurements from satellites) to constrain both bottom-up models and atmospheric inversions; (iv) improvements of transport models and the representation of photochemical sinks in top-down inversions; and (v) integration of 3D variational inversion systems using isotopic and/or co-emitted species such as ethane as well as information in the bottom-up inventories on anthropogenic super-emitters detected by remote sensing (mainly oil and gas sector but also coal, agriculture, and landfills) to improve source partitioning.

(Sub)tropical inland waters are important greenhouse gas (GHG) sources, yet limited observations have long hindered broad analyses of GHG variability across this diverse region. Here, through a meta-analysis, we have examined the rates and drivers of GHG emissions from flowing and standing (sub)tropical inland waters. We find considerable spatial variation in fluxes, largely related to differences in hydroclimate, geomorphology, land cover and human disturbance. Flowing waters emit more carbon dioxide (3,3872,1215,702 TgCO2 yr−1, expressing medianfirst quartilethird quartile), methane (10.60.128.8 TgCH4 yr−1) and nitrous oxide (0.620.351.10 TgN2O yr−1) than standing waters (11473219 TgCO2 yr−1, 5.42.19.1 TgCH4 yr−1 and 0.030.020.05 TgN2O yr−1, respectively). (Sub)tropical inland waters release 4,23824737375 TgCO2-equivalents annually, with first- to third-order streams contributing 75% of riverine emissions and lakes larger than 100 km2 contributing 59% of standing water emissions. Our results suggest emissions from (sub)tropical waters are 29–72% lower than earlier estimates, a downward revision with important implications for global GHG budgets.

Accurate accounting of greenhouse-gas (GHG) emissions and removals is central to tracking progress toward climate mitigation and for monitoring potential climate-change feedbacks. GHG budgeting and reporting can follow either the Intergovernmental Panel on Climate Change methodologies for National Greenhouse Gas Inventory (NGHGI) reporting or use atmospheric-based “top-down” (TD) inversions or process-based “bottom-up” (BU) approaches. To help understand and reconcile these approaches, the Second REgional Carbon Cycle Assessment and Processes study (RECCAP2) was established to quantify GHG emissions and removals for carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), for ten-land and five-ocean regions for 2010–2019. Here, we present the results for the North American land region (Canada, the United States, Mexico, Central America and the Caribbean). For 2010–2019, the NGHGI reported total net-GHG emissions of 7,270 TgCO2-eq yr−1 compared to TD estimates of 6,132 ± 1,846 TgCO2-eq yr−1 and BU estimates of 9,060 ± 898 TgCO2-eq yr−1. Reconciling differences between the NGHGI, TD and BU approaches depended on (a) accounting for lateral fluxes of CO2 along the land-ocean-aquatic continuum (LOAC) and trade, (b) correcting land-use CO2 emissions for the loss-of-additional-sink capacity (LASC), (c) avoiding double counting of inland water CH4 emissions, and (d) adjusting area estimates to match the NGHGI definition of the managed-land proxy. Uncertainties remain from inland-water CO2 evasion, the conversion of nitrogen fertilizers to N2O, and from less-frequent NGHGI reporting from non-Annex-1 countries. The RECCAP2 framework plays a key role in reconciling independent GHG-reporting methodologies to support policy commitments while providing insights into biogeochemical processes and responses to climate change.

Reservoirs influence the global climate by exchanging greenhouse gases (GHGs) of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) with the atmosphere. Few studies, however, quantify emissions of all three GHGs from reservoirs, particularly in permafrost-affected mountain regions where ecosystems are highly vulnerable to climate change. This study presents three-year direct measurements of CO₂, CH₄, and N₂O concentrations and fluxes upstream, within, and downstream from two reservoirs draining permafrost catchments on the Qinghai-Tibet Plateau, including periods of reservoir drawdown. Comparing GHG fluxes across space and time exhibits a general pattern of lower fluxes at the two reservoirs relative to up- and downstream channels. Ebullitive fluxes contributed to 36.7% and 9.4% of total CH₄ and N₂O fluxes, respectively. CO₂ has no response to drawdown, but CH₄ and N₂O display synchronous drawdown-associated increase within the reservoir, constituting 57.5% and 32.8% of the annual reservoir emissions in just 2 months, respectively. Riverine emissions from up- and downstream channels accounted for an outsized fraction (55.5% for CH₄, 17.3% for CO₂ and 16.5% for N₂O) of the system-wide GHG budget. Compared with global reservoirs, the two reservoirs have high CO₂ and N₂O but low CH4 fluxes in CO₂ equivalents. Upscaling shows that the two reservoirs emit the same magnitude of carbon as thermokarst lakes, and four times higher N2O than Finnish lakes on an areal basis. This article shows that alpine reservoirs draining permafrost catchments are unrecognized atmospheric sources in current reservoir GHG inventories, but also emphasizes the importance of system-wide emissions when assessing total GHG evasion from reservoir systems.

Methane (CH₄) is a potent greenhouse gas and its concentrations have tripled in the atmosphere since the industrial revolution. There is evidence that global warming has increased CH₄ emissions from freshwater ecosystems 1,2, providing positive feedback to the global climate. Yet for rivers and streams, the controls and the magnitude of CH₄ emissions remain highly uncertain 3,4. Here we report a spatially explicit global estimate of CH₄ emissions from running waters, accounting for 27.9 (16.7–39.7) Tg CH₄ per year and roughly equal in magnitude to those of other freshwater systems 5,6. Riverine CH₄ emissions are not strongly temperature dependent, with low average activation energy (E_M = 0.14 eV) compared with that of lakes and wetlands (E_M = 0.96 eV) 1. By contrast, global patterns of emissions are characterized by large fluxes in high- and low-latitude settings as well as in human-dominated environments. These patterns are explained by edaphic and climate features that are linked to anoxia in and near fluvial habitats, including a high supply of organic matter and water saturation in hydrologically connected soils. Our results highlight the importance of land–water connections in regulating CH₄ supply to running waters, which is vulnerable not only to direct human modifications but also to several climate change responses on land.