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Mercury emissions from artisanal and small-scale gold mining throughout the Global South exceed coal combustion as the largest global source of mercury. We examined mercury deposition and storage in an area of the Peruvian Amazon heavily impacted by artisanal gold mining. Intact forests in the Peruvian Amazon near gold mining receive extremely high inputs of mercury and experience elevated total mercury and methylmercury in the atmosphere, canopy foliage, and soils. Here we show for the first time that an intact forest canopy near artisanal gold mining intercepts large amounts of particulate and gaseous mercury, at a rate proportional with total leaf area. We document substantial mercury accumulation in soils, biomass, and resident songbirds in some of the Amazon’s most protected and biodiverse areas, raising important questions about how mercury pollution may constrain modern and future conservation efforts in these tropical ecosystems. The Peruvian Amazon is facing the highest known input of mercury pollution of any ecosystem globally. Intact forests located near artisanal gold mining are particularly at risk from this toxin.

Stay-at-home policies invoked in response to COVID-19 have led to well-publicized drops in some air pollutants. The extent to which such reductions translate to improved air quality is dictated by not only emissions and meteorology, but also chemical transformations in the atmosphere.

Ensuring that global warming remains <2 °C requires rapid CO 2 emissions reduction. Additionally, 100–900 gigatons CO 2 must be removed from the atmosphere by 2100 using a portfolio of CO 2 removal (CDR) methods. Ocean afforestation, CDR through basin-scale seaweed farming in the open ocean, is seen as a key component of the marine portfolio. Here, we analyse the CDR potential of recent re-occurring trans-basin belts of the floating seaweed Sargassum in the (sub)tropical North Atlantic as a natural analogue for ocean afforestation. We show that two biogeochemical feedbacks, nutrient reallocation and calcification by encrusting marine life, reduce the CDR efficacy of Sargassum by 20–100%. Atmospheric CO 2 influx into the surface seawater, after CO 2 -fixation by Sargassum , takes 2.5–18 times longer than the CO 2 -deficient seawater remains in contact with the atmosphere, potentially hindering CDR verification. Furthermore, we estimate that increased ocean albedo, due to floating Sargassum , could influence climate radiative forcing more than Sargassum -CDR. Our analysis shows that multifaceted Earth-system feedbacks determine the efficacy of ocean afforestation. Ocean afforestation is considered as an important method to remove gigatons of CO 2 from the atmosphere. Here the authors use the Great Atlantic Sargassum Belt as a natural analogue to show that the efficacy of ocean afforestation is determined by complicated feedbacks with the Earth system.

Mercury is a potent neurotoxin that poses health risks to the global population. Anthropogenic mercury emissions to the atmosphere are projected to decrease in the future due to enhanced policy efforts such as the Minamata Convention, a legally-binding international treaty entered into force in 2017. Here, we report the development of a comprehensive climate-atmosphere-land-ocean-ecosystem and exposure-risk model framework for mercury and its application to project the health effects of future atmospheric emissions. Our results show that the accumulated health effects associated with mercury exposure during 2010–2050 are $19 (95% confidence interval: 4.7–54) trillion (2020 USD) realized to 2050 (3% discount rate) for the current policy scenario. Our results suggest a substantial increase in global human health cost if emission reduction actions are delayed. This comprehensive modeling approach provides a much-needed tool to help parties to evaluate the effectiveness of Hg emission controls as required by the Minamata Convention. Mercury is a neurotoxin and pollutant with enhanced emissions from anthropogenic activities. Here, the authors develop a global emissions, transport, and human risk model and find substantial future losses in revenue and public health if emission reductions proposed by the Minamata Convention are delayed.

Observational evidence shows the ubiquitous presence of ocean-emitted short-lived halogens in the global atmosphere 1 – 3 . Natural emissions of these chemical compounds have been anthropogenically amplified since pre-industrial times 4 – 6 , while, in addition, anthropogenic short-lived halocarbons are currently being emitted to the atmosphere 7 , 8 . Despite their widespread distribution in the atmosphere, the combined impact of these species on Earth’s radiative balance remains unknown. Here we show that short-lived halogens exert a substantial indirect cooling effect at present (−0.13 ± 0.03 watts per square metre) that arises from halogen-mediated radiative perturbations of ozone (−0.24 ± 0.02 watts per square metre), compensated by those from methane (+0.09 ± 0.01 watts per square metre), aerosols (+0.03 ± 0.01 watts per square metre) and stratospheric water vapour (+0.011 ± 0.001 watts per square metre). Importantly, this substantial cooling effect has increased since 1750 by −0.05 ± 0.03 watts per square metre (61 per cent), driven by the anthropogenic amplification of natural halogen emissions, and is projected to change further (18–31 per cent by 2100) depending on climate warming projections and socioeconomic development. We conclude that the indirect radiative effect due to short-lived halogens should now be incorporated into climate models to provide a more realistic natural baseline of Earth’s climate system. Short-lived halogens have a substantial indirect cooling effect on climate and this cooling effect has increased since pre-industrial times owing to anthropogenic amplification of natural halogen emissions.

The Inter-Sectoral Impact Model Intercomparison Project offers a framework to compare climate impact projections in different sectors and at different scales. Consistent climate and socio-economic input data provide the basis for a cross-sectoral integration of impact projections. The project is designed to enable quantitative synthesis of climate change impacts at different levels of global warming. This report briefly outlines the objectives and framework of the first, fast-tracked phase of Inter-Sectoral Impact Model Intercomparison Project, based on global impact models, and provides an overview of the participating models, input data, and scenario set-up.

Between about 1998 and 2012, a time that coincided with political negotiations for preventing climate change, the surface of Earth seemed hardly to warm. This phenomenon, often termed the ‘global warming hiatus’, caused doubt in the public mind about how well anthropogenic climate change and natural variability are understood. Here we show that apparently contradictory conclusions stem from different definitions of ‘hiatus’ and from different datasets. A combination of changes in forcing, uptake of heat by the oceans, natural variability and incomplete observational coverage reconciles models and data. Combined with stronger recent warming trends in newer datasets, we are now more confident than ever that human influence is dominant in long-term warming. Apparently contradictory conclusions regarding the ‘global warming hiatus’ are reconciled, strengthening the current scientific understanding that long-term global warming is extremely likely to be of anthropogenic origin. Analysis of a global warming hiatus After a spike in global-mean temperature associated with the 1998 El Niño, the climate system experienced several years of reduced warming, and perhaps even slight cooling. This period, variously termed the 'hiatus', 'pause' or 'slowdown', should have come as no surprise given our understanding of El Niño and natural climate variability. However, soon after the recognition of the reduced warming, it appeared that models and observations were diverging, raising the question of whether the models were missing important processes. Although global warming has since recommenced, the hiatus sparked an enormous research effort. Iselin Medhaug et al . synthesize the literature and reassess the model and observational evidence. Their assessment reconciles the apparent contradictions between models and data and obviates the need to revise our understanding of the underlying physics of climate systems. The hiatus was an episode of natural variability after all.

Hydrogen (H 2 ) is expected to play a crucial role in reducing greenhouse gas emissions. However, hydrogen losses to the atmosphere impact atmospheric chemistry, including positive feedback on methane (CH 4 ), the second most important greenhouse gas. Here we investigate through a minimalist model the response of atmospheric methane to fossil fuel displacement by hydrogen. We find that CH 4 concentration may increase or decrease depending on the amount of hydrogen lost to the atmosphere and the methane emissions associated with hydrogen production. Green H 2 can mitigate atmospheric methane if hydrogen losses throughout the value chain are below 9 ± 3%. Blue H 2 can reduce methane emissions only if methane losses are below 1%. We address and discuss the main uncertainties in our results and the implications for the decarbonization of the energy sector. H 2 has the potential to become the green, low-carbon fuel of the future. However, hydrogen emissions impact atmospheric methane (CH 4 ). Bertagni et al. investigate the fate of atmospheric CH 4 in scenarios of H 2 economy.

Long-term time series of surface ocean-atmosphere heat fluxes show that the mid-latitude North Atlantic ocean may influence atmospheric variability on multidecadal timescales. Long-term climate effects of the oceans In 1964 the Norwegian–American meteorologist Jacob Bjerknes conjectured that interactions between the atmosphere and the ocean have opposing effects, depending on the timescale of the interaction: at interannual scales, the atmosphere controls the sea surface temperature, but at multidecadal scales, the ocean is the main driver of atmospheric variability. The former is now accepted and modelling work has been used to support the latter, but until now no firm observational evidence has been offered in support of oceanic control of the atmosphere. Sergey Gulev et al . have now assembled the available observational evidence and demonstrate that, at least in the North Atlantic, the ocean does drive multidecadal variability in surface heat fluxes. Nearly 50 years ago Bjerknes 1 suggested that the character of large-scale air–sea interaction over the mid-latitude North Atlantic Ocean differs with timescales: the atmosphere was thought to drive directly most short-term—interannual—sea surface temperature (SST) variability, and the ocean to contribute significantly to long-term—multidecadal—SST and potentially atmospheric variability. Although the conjecture for short timescales is well accepted, understanding Atlantic multidecadal variability (AMV) of SST 2 , 3 remains a challenge as a result of limited ocean observations. AMV is nonetheless of major socio-economic importance because it is linked to important climate phenomena such as Atlantic hurricane activity and Sahel rainfall, and it hinders the detection of anthropogenic signals in the North Atlantic sector 4 , 5 , 6 . Direct evidence of the oceanic influence of AMV can only be provided by surface heat fluxes, the language of ocean–atmosphere communication. Here we provide observational evidence that in the mid-latitude North Atlantic and on timescales longer than 10 years, surface turbulent heat fluxes are indeed driven by the ocean and may force the atmosphere, whereas on shorter timescales the converse is true, thereby confirming the Bjerknes conjecture. This result, although strongest in boreal winter, is found in all seasons. Our findings suggest that the predictability of mid-latitude North Atlantic air–sea interaction could extend beyond the ocean to the climate of surrounding continents.

A three-dimensional global climate model shows that the loss of a planet’s oceans through complete vaporization or evaporative escape to space will occur at considerably higher insolation than previously thought, owing to stabilizing atmospheric effects. A longer wait for a 'runaway greenhouse' The Sun is gradually increasing in brightness on a geological timescale. This could lead eventually to a 'runaway greenhouse' effect on Earth, a state that occurs when a planet absorbs more energy from the Sun than it can radiate back to space. The oceans would evaporate and the climate would warm to Venus-like temperatures. Jérémy Leconte et al . use a three-dimensional climate model to show that the threshold for the initiation of a runaway greenhouse is considerably higher than that previously estimated by simplified one-dimensional models. A crucial factor is cooling caused by changes in atmospheric circulation that more than offset cloud-induced warming. This finding is of importance in relation to extrasolar planets, since it extends the size of the habitable zone around other stars. The increase in solar luminosity over geological timescales should warm the Earth’s climate, increasing water evaporation, which will in turn enhance the atmospheric greenhouse effect. Above a certain critical insolation, this destabilizing greenhouse feedback can ‘run away’ until the oceans have completely evaporated 1 , 2 , 3 , 4 . Through increases in stratospheric humidity, warming may also cause evaporative loss of the oceans to space before the runaway greenhouse state occurs 5 , 6 . The critical insolation thresholds for these processes, however, remain uncertain because they have so far been evaluated using one-dimensional models that cannot account for the dynamical and cloud feedback effects that are key stabilizing features of the Earth’s climate. Here we use a three-dimensional global climate model to show that the insolation threshold for the runaway greenhouse state to occur is about 375 W m −2 , which is significantly higher than previously thought 6 , 7 . Our model is specifically developed to quantify the climate response of Earth-like planets to increased insolation in hot and extremely moist atmospheres. In contrast with previous studies, we find that clouds have a destabilizing feedback effect on the long-term warming. However, subsident, unsaturated regions created by the Hadley circulation have a stabilizing effect that is strong enough to shift the runaway greenhouse limit to higher values of insolation than are inferred from one-dimensional models. Furthermore, because of wavelength-dependent radiative effects, the stratosphere remains sufficiently cold and dry to hamper the escape of atmospheric water, even at large fluxes. This has strong implications for the possibility of liquid water existing on Venus early in its history, and extends the size of the habitable zone around other stars.