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Methane is a greenhouse gas that significantly contributes to global warming. Its sources are not well constrained as many point sources are missing in emission inventories that are built based on bottom-up approaches. Emissions include sources caused by human activities (oil/gas, lifestock) but also natural ones, e.g. wetlands. The current paper fills this gap by comprehensively reviewing the capabilities of current and forthcoming satellites as powerful top-down tools to observe atmospheric methane and quantify emissions. Their most important application is to quantify anthropogenic methane sources , where there is substantial interest in identifying hot spots to reduce emissions, closing the methane budget, and to ensure compliance with international climate agreements. This paper is of broad interest for the geoscience community, as it not only presents an overview of the existing discrepancies in the atmospheric methane budget and emissions but also addresses the difficulties in defining its emission inventories on various spatial scales.

The Southern Ocean can be considered a region exhibiting pristine conditions as during the pre-industrial time. Thus, any changes in radiative forcing in this region can be attributed to natural factors. Feedbacks of ocean biological activity on Earth’s radiation budget have been put forward as the CLAW hypothesis (Charlson et al., 1987, https://www.nature.com/articles/326655a0). It implies that emissions of biogenic sulfur-containing compounds result in the formation of cloud condensation nuclei, which lead to higher cloud droplet number concentrations. Such clouds are more reflective and thus lead to a cooling effect. The current study provides satellite-based evidence of the increase droplet number concentrations and cloud reflectivity (‘albedo’) triggered by chlorophyll emissions, as a proxy for biological activity. Specifically, it demonstrates for the first time the extent to which the cloud albedo is modulated by biological factors as a function of latitude along the Antarctic shelf. While the study does not extend to discussing the subsequent feedbacks of cloud reflectivity to biological activity, it clearly demonstrates how biological ocean activity affects cloudiness above the Southern Ocean and thus may regulate temperature.

After a period of near-zero growth of atmospheric methane, a major greenhouse gas, its growth rates have increased, with values in 2020 and 2021 exceeding all prior values since the beginning of systematic measurements in 1983. This recent acceleration, particularly during the covid-19 years 2020 and 2021, raises the question whether the methane increase was due to stronger sources or reduced sinks in the troposphere. A reduction of the tropospheric abundance of the hydroxyl radical (OH), the reaction of which is the main tropospheric methane sink, could be plausibly explained by global-scale reductions in nitrogen oxides due to pandemic-related industry shutdowns. Using an inversion scheme, Feng et al. demonstrate that such a reduction only accounts for about 34% in 2020 and 10% in 2021 of the observed methane rise. Instead, the authors attribute increased methane emissions to hydrological anomalies and microbial sources over the tropics, i.e., Eastern Africa and tropical South America, and temperate North America. The study demonstrates the importance of simultaneously accounting for changes in methane emissions and sinks for an improved quantitative understanding of the evolution of the methane concentrations to assess the role of greenhouse gases in climate change and tropospheric composition.

Wildfires have attracted increasing attention in recent years because of their effects on local air quality, as well as on regional and global climate. Ohneiser et al. documents with impressive accuracy the appearance of wildfire plumes in the stratosphere over Australia in 2020/2021, showing strong influence on stratospheric ozone. We recommend readers to read this paper together with Solomon et al. ("On the stratospheric chemistry of midlatitude wildfire smoke", PNAS 2022,https://doi.org/10.1073/pnas.2117325119). Both studies highlight the importance of wildfires for stratospheric chemistry and the recovery of the ozone layer in a warming climate.

Krüger et al. present observations of the effect of the COVID-19 lockdowns on black carbon emissions using a combination of airborne in situ observations and models. The impact of lockdowns on emissions from transport is by now well established, however the majority of studies to date have focused on NO2 as this is a regulated pollutant and high resolution observations are available in new satellite data products. This work instead uses a state-of-the-art airborne facility (DLR HALO) to report data on black carbon, which has established impacts both as a airborne pollutant damaging to human health, and as a highly potent short-lived climate forcing agent. Data from during the European lockdown period is compared with the prior EMeRGe campaign (https://acp.copernicus.org/articles/special_issue1074.html). The authors found a decrease in BC concentrations of 48% over Western and Southern Europe, of which 7% was likely due to meteorological differences during the two campaigns and 3-9% was due to a long-term downward trend, leading them to conclude that the lockdowns were overall responsible for a reduction in ambient concentrations of 32-38%. By seizing this unique opportunity, this study offers a new and unparalleled insight into the current nature of black carbon emissions from Europe. It also demonstrates how airborne in situ measurements and models can be used in combination to study the emission and transport of pollutants in the troposphere.

Methane is a potent greenhouse gas and an important sink of atmospheric OH radicals, which determine the global atmospheric oxidation capacity. CH4 has increased since pre-industrial times until the year 2000, after which its global concentration has remained relatively stable. Since 2007, it is rapidly increasing again for reasons that are not well understood. The current paper analyses source specific methane emissions that are likely responsible for the recent increase. Global CH4 sources are determined using variational inversion based on measurements of methane and its isotope signatures (δ13C). This latter provide better constraints on the contributions of microbial and fossil fuel CH4 sources than the concentrations alone. The analysis points to a more strongly increasing contribution of microbial sources since 2007 than predicted by previous assessments (Global Carbon Project). These findings have the potential to lead to a major reassessment of the global methane budget.

This paper by Drews et al. reports model simulations of the effect of the 11-year solar cycle on the atmospheric circulation and hence on year-to-year variations in weather patterns, . The physics of the effect of the solar cycle is complex, but one important mechanism is believed to be via the variation in short-wave radiation, which perturbs the ozone distribution in the upper stratosphere. The key development in this study is a good model representation of chemistry, radiation and dynamics and their interactions to enable the dynamical feedback processes, which  potentially communicate the direct physical effects of the solar cycle to the lower part of the atmosphere, to be adequately simulated. An important aspect of the paper is that the  authors exploit an ensemble of simulations that make it possible to distinguish a signal due to solar-cycle effects from natural weather variability. The results convincingly show a solar cycle effect, over the North Atlantic in particular, where variations in the circulation have important implications for the weather experienced in Europe. This is particularly the case in the current period (since 1950 or so) when the 11-year solar variation is strong relative to the entire historical record (starting in about 1850).  The results of this paper suggest that including solar cycle effects in models used for decadal climate predictions can provide worthwhile improvements in the skill of such predictions.

Organic aerosol remain one of the more complex and hard to predict when studying atmospheric aerosols and their influences on air quality, meteorology and climate. Among its many complexities is the phase and viscosity of the organic matter, which dictates how it interacts with other particulate components and the gas phase, in turn affecting growth rates and cloud activation. There have been a number of previous works studying phase separation, where the organic matter becomes immiscible with an aqueous component (containing inorganic salts), but this new letter presents compelling visual evidence that different organic phases are also capable of separation. Different secondary organic aerosol (SOA) mixtures were created and some mixtures exhibited separation, with a factor being the oxygen-to-carbon ratio of the material, likely a surrogate for polarity. If this behaviour is found to be important in atmospheric aerosols this represents a new direction in how these may need to be represented in models.

Datasets defining emissions of greenhouse gases and air pollutants underpin all model assessments of air quality and climate change. Although often emitted from the same source, the inventories have developed along mostly separate lines. This Opinion presents the case for coordinated development of air pollutant and greenhouse gas emission inventories, which would be an important step towards reliably evaluating the potential co-benefit of air pollution control and climate mitigation. The article discusses the similarities and differences between the two inventories, their historical development and applications, as well as the future challenges of a coordinated development of both. It also makes important recommendations for organizations, compilers, and research groups.