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Mercury is a potent neurotoxic substance and accounts for 250,000 intellectual disabilities annually. Worldwide, coastal fisheries contribute the majority of human exposure to mercury through fish consumption. Recent global mercury cycling and risk models attribute all the mercury loading to the ocean to atmospheric deposition. Nevertheless, new regional research has noted that the riverine mercury export to coastal oceans may also be significant to the oceanic burden of mercury. Here we construct an unprecedented high-spatial-resolution dataset estimating global river mercury and methylmercury exports. We find that rivers annually deliver 1,000 (minimum–maximum: 893–1,224) Mg mercury to coastal oceans, threefold greater than atmospheric deposition. Furthermore, high flow events, which are becoming more common with climate change, are responsible for a disproportionately large percentage of the export. Coastal oceans constitute 0.2% of the entire ocean volume but receive 27% of the external mercury input to the ocean. We estimate that the river mercury export could be responsible for a net annual export of 350 (interquartile range: 52–640) Mg mercury across the coastal–open-ocean boundary, although there is still high uncertainty around this estimate. Our results show that river export is the largest source of mercury to coastal oceans worldwide, and continued mercury risk modelling should incorporate the impact of rivers. Rivers transport about 1,000 Mg mercury annually to coastal oceans, which is threefold greater than the amount delivered by atmospheric deposition, according to a global analysis of mercury measurements in rivers.

Lentic systems (lakes and reservoirs) are emission hotpots of nitrous oxide (N 2 O), a potent greenhouse gas; however, this has not been well quantified yet. Here we examine how multiple environmental forcings have affected N 2 O emissions from global lentic systems since the pre-industrial period. Our results show that global lentic systems emitted 64.6 ± 12.1 Gg N 2 O-N yr −1 in the 2010s, increased by 126% since the 1850s. The significance of small lentic systems on mitigating N 2 O emissions is highlighted due to their substantial emission rates and response to terrestrial environmental changes. Incorporated with riverine emissions, this study indicates that N 2 O emissions from global inland waters in the 2010s was 319.6 ± 58.2 Gg N yr −1 . This suggests a global emission factor of 0.051% for inland water N 2 O emissions relative to agricultural nitrogen applications and provides the country-level emission factors (ranging from 0 to 0.341%) for improving the methodology for national greenhouse gas emission inventories. Modeling shows that N 2 O emissions from global lakes and reservoirs have doubled since the pre-industrial era, this was mainly caused by widespread agricultural nitrogen application.

More than 70,000 large dams have been built worldwide. With growing water stress and demand for energy, this number will continue to increase in the foreseeable future. Damming greatly modifies the ecological functioning of river systems. In particular, dam reservoirs sequester nutrient elements and, hence, reduce downstream transfer of nutrients to floodplains, lakes, wetlands, and coastal marine environments. Here, we quantify the global impact of dams on the riverine fluxes and speciation of the limiting nutrient phosphorus (P), using a mechanistic modeling approach that accounts for the in-reservoir biogeochemical transformations of P. According to the model calculations, the mass of total P (TP) trapped in reservoirs nearly doubled between 1970 and 2000, reaching 42 Gmol y−1, or 12% of the global river TP load in 2000. Because of the current surge in dam building, we project that by 2030, about 17% of the global river TP load will be sequestered in reservoir sediments. The largest projected increases in TP and reactive P (RP) retention by damming will take place in Asia and South America, especially in the Yangtze, Mekong, and Amazon drainage basins. Despite the large P retention capacity of reservoirs, the export of RP from watersheds will continue to grow unless additional measures are taken to curb anthropogenic P emissions.

Public perception of anthropogenic environmental impacts, including climate change, is primarily driven by exposure to different forms of media. Here, we show how Pokémon, the largest multimedia franchise worldwide, mirrors public discourse in the video games' narratives with regard to human impacts on environmental change. Pokémon demonstrates a trajectory towards greater acknowledgement of climate change and anthropogenic impacts in each released game and presents a hopeful vision for how society can adapt.

There is a growing understanding of the role that bedrock weathering can play as a source of nitrogen (N) to soils, groundwater and river systems. The significance is particularly apparent in mountainous environments where weathering fluxes can be large. However, our understanding of the relative contributions of rock-derived, or geogenic, N to the total N supply of mountainous watersheds remains poorly understood. In this study, we develop the High-Altitude Nitrogen Suite of Models (HAN-SoMo), a watershed-scale ensemble of process-based models to quantify the relative sources, transformations, and sinks of geogenic and atmospheric N through a mountain watershed. Our study is based in the East River Watershed (ERW) in the Upper Colorado River Basin. The East River is a near-pristine headwater watershed underlain primarily by an N-rich Mancos Shale bedrock, enabling the timing and magnitude of geogenic and atmospheric contributions to watershed scale dissolved N-exports to be quantified. Several calibration scenarios were developed to explore equifinality using >1600 N concentration measurements from streams, groundwater, and vadose zone samples collected over the course of four years across the watershed. When accounting for recycling of N through plant litter turnover, rock weathering accounts for approximately 12% of the annual dissolved N sources to the watershed in the most probable calibration scenario (0–31% in other scenarios), and 21% (0–44% in other scenarios) when considering only “new” N sources (i.e. geogenic and atmospheric). On an annual scale, instream dissolved N elimination, plant turnover (including cattle grazing) and atmospheric deposition are the most important controls on N cycling.

An accurate quantification of global methane sources and sinks is imperative for assessing realistic pathways to mitigate climate change. A key challenge of quantifying the Global Methane Budget (Saunois et al., 2020, https://doi.org/10.5194/essd‐12‐1561‐2020) is the lack of consistency in uncertainties between sectors. Here we provide a new perspective on bottom‐up (BU) and top‐down (TD) methane uncertainties by using an expert opinion analysis based on a questionnaire conducted in 2021. Expectedly, experts rank highest uncertainty and lowest confidence levels in the Global Methane Budget related to natural sources in BU budgets. Here, we further reveal specific uncertainty types and introduce a ranking system for uncertainties in each sector. We find that natural source uncertainty is related particularly to driver data uncertainty in freshwater, vegetation, and coastal/ocean sources, as well as parameter uncertainty in wetland models. Reducing uncertainties, most notably in aquatic and wetland sources will help balance future BU and TD global methane budgets. We suggest a new methane source partitioning over gradients of human disturbance and demonstrate that 76.3% (75.8%–79.4%) or 561 (443–700) Tg CH4 yr−1 of global emissions can be attributed to moderately impacted, man‐made, artificial, or fully anthropogenic sources and 23.7% (20.6%–24.2%) or 174 (115–223) Tg CH4 yr−1 to natural and low impacted methane sources. Finally, we identify current research gaps and provide a plan of action to reduce current uncertainties in the Global Methane Budget. Plain Language Summary To effectively address climate change, it's crucial to gain a better understanding of the difficulties involved in estimating global methane sources and sinks. One of the key challenges in this process are the varying levels of uncertainty associated with different sectors responsible for methane emissions and methane uptake. In this study, we conducted a survey to gather expert opinions regarding the uncertainty of methane data. The experts highlighted that the Global Methane Budget has the highest uncertainty and lowest confidence levels when it comes to natural sources in the bottom‐up budgets. This uncertainty primarily stems from uncertain data related to natural sources like freshwater, vegetation, and coastal/ocean areas, as well as the parameters used in wetland models. We show that 76.3% of all global methane emissions are related to human‐impact or fully man‐made. Additionally, we identify existing gaps in research and lay out a plan of action to reduce the current uncertainties associated with the Global Methane Budget. This effort will contribute to a more accurate understanding of methane's role in climate change mitigation. Key Points Highest uncertainty and lowest confidence levels were identified in bottom‐up natural emissions in the Global Methane Budget High driver data uncertainties are associated with freshwater, vegetation, and coastal/ocean methane sources New methane source partitioning reveals 76% of global emissions are related to moderate to high levels of anthropogenic impacts

River networks represent the largest biogeochemical nexus between the continents, ocean and atmosphere. Our current understanding of the role of rivers in the global carbon cycle remains limited, which makes it difficult to predict how global change may alter the timing and spatial distribution of riverine carbon sequestration and greenhouse gas emissions. Here we review the state of river ecosystem metabolism research and synthesize the current best available estimates of river ecosystem metabolism. We quantify the organic and inorganic carbon flux from land to global rivers and show that their net ecosystem production and carbon dioxide emissions shift the organic to inorganic carbon balance en route from land to the coastal ocean. Furthermore, we discuss how global change may affect river ecosystem metabolism and related carbon fluxes and identify research directions that can help to develop better predictions of the effects of global change on riverine ecosystem processes. We argue that a global river observing system will play a key role in understanding river networks and their future evolution in the context of the global carbon budget. A review of current river ecosystem metabolism research quantifies the organic and inorganic carbon flux from land to global rivers and demonstrates that the carbon balance can be influenced by a changing world.