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In 2020, a new international law was imposed that placed strong restrictions on sulfur emissions from the international shipping industry. In addition to reducing air pollution, an anticipated side effect was reduction of the climate cooling effect that is often associated with "ship-tracks". Aerosol pollutant emissions from ships, when they rise into overlying clouds, lead to higher cloud droplet number concentrations, smaller cloud droplet sizes, and clouds that are more reflective to incoming sunlight, easily seen in satellite imagery as long bright lines downwind of ships. Past studies into whether the new law has led to darker clouds have been equivocal. For this study, the authors used sophisticated statistical techniques to compare cloud droplet size and reflectivity before and after the law was implemented focusing on a shipping corridor in the southeast Atlantic. They found strong evidence that droplet sizes have indeed increased, and that clouds have darkened with a significant local climate warming. Globally, the impact is much smaller, but may still represent an important consideration for assessments of the total summed effect of aerosols on climate.

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.

Methane is a potent greenhouse gas with a global warming potential much greater than carbon dioxide. In order to understand and limit its global warming effect, it is important to have have observation systems to estimate its anthropogenic emissions. This paper analyses in a novel way TROPOMI satellite observations to track methane emissions from the largest oil production basin in the United States over a 2-year period. The analysis shows that emission variability and trends are driven by multiple factors, among which new well development and natural gas spot price are the most significant ones. The work is an excellent demonstration of the potential of satellite observations for near-real-time monitoring of methane emissions. It opens a broad range of applications, helping science and policy to understand and mitigate climate change.

The term "nuclear winter" was commonly used in the 1980s and represented a scenario where a nuclear war could profoundly alter the earth's climate through injection of aerosols into the stratosphere, which could be considered a highly extreme example of human activity altering the earth system. Paul Crutzen, in his Nobel Prize speech, said that this concept is politically important as it "magnifies and highlights the dangers of a nuclear war and convinces me that in the long run mankind can only escape such horrific consequences if nuclear weapons are totally abolished by international agreement". While nuclear war is hopefully something that we will never experience, it is highly interesting to return to this topic four decades later, after the major advancements in the science of aerosol-climate interactions that have occurred in the meantime (partly motivated by an exploration of geoengineering concepts), and improvements in our abilities to model the agricultural and economic impacts of climate change. The conclusions of the paper by Robock et al. offer a sobering reminder of the manifold dangers of this type of warfare to the earth system and society.

Climate is changing, and the evidence for this fact increases every day. Therefore, research is addressing the question of how the impact of climate change could be alleviated by influencing the climate of the Earth through geoengineering. Research on such issues has increased tremendously since the seminal paper on this topic by Paul Crutzen in 2006. An entire project, the Geoengineering Model Intercomparison Project (GeoMIP), was started in 2010 aimed at understanding the physical processes and projected impacts of solar geoengineering. The paper by Visioni et al. discusses the successes and missed opportunities of the GeoMIP project and gives recommendations regarding both future model experiments and more general activities. The paper will be without doubt a focus and a nucleation point of future discussions on the aspect of geoengineering.

The role of chemical reactions within the atmospheric multiphase system, i.e. aerosol particles and cloud droplets surrounded by a gas phase, has been recognized for several decades to affect the atmospheric composition. In this article, significant advancements in measurement techniques during the past 20 years are briefly described that led to the identification of thousands of organic compounds and molecular-scale processes within the atmospheric multiphase system. Similarly, the complexity of multiphase chemistry models has been increasing to include in detail the underlying chemical processes and phase transfers between gas, aqueous, and organic phases. The authors propose how complementary techniques, such as machine learning and molecular dynamics, can be used to constrain the resulting model complexity. They also identify chemical parameters to be further constrained in lab and field studies. Overall, the article highlights the need and future directions of increasingly interdisciplinary efforts to characterize the atmospheric multiphase chemistry system and its impacts on climate-chemistry interactions, atmospheric oxidation capacity and human health.

This article offers a wide ranging review of current understanding of the role various tropical cirrus cloud types play in the redistribution of water within the atmosphere and how they affect the changing Earth's energy balance by reflecting sunlight and preventing the escape of thermal energy to outer space. Improved understanding of these dynamics has been identified as critical for predicting whether such clouds may amplify or slow future global climate change. A clear exposition is provided of the various methods used to study tropical cirrus cloud characteristics and processes, including remote sensing, in situ measurements, modeling and laboratory work. Key questions include identifying how small-scale microphysical processes affect larger cloud structure, and how cirrus altitude and extent responds to changing radiation and thermodynamic profiles. A call is made not only for improved representations of cloud processes at finer scales, but also for a more holistic approach using a hierarchy of model detail. Such efforts would enable new knowledge obtained from studying even the smallest scales to be more readily placed within a broader context applicable to climate studies.

Ship tracks are enhanced regions of cloud brightness that trail behind ships that are known to be created by their effluent. They are widely used as observational test beds to deepen understanding of how pollution might affect cloud microphysical processes and climate at larger regional and global scales.  This study uses satellite observations of cloud properties and records of ship position and their effluent to assess perturbations from ship emissions on cloud droplet number and liquid water path. The study found that, as expected, the droplet number perturbation in shiptracks scales with ship emission rates of aerosol particles. Surprisingly, however, the liquid water path in drizzling clouds increased by an amount that was nearly fixed. The observation points to novel non-linear threshold behaviours of relevance to representations of aerosol indirect effects in climate models.

The international Montreal Protocol was signed in 1987 in order to protect the atmospheric ozone layer by phasing out the production of halogenated hydrocarbons that deplete stratospheric ozone. The protocol was successfully implemented and, over the years, amendments and adjustments of the protocol were essential to its success. Ultimately, the protocol has resulted in a reduced halogen loading of the atmosphere since the mid-1990s. Trifluoromethane (HFC-23) is one of the substances regulated by the Montreal protocol since the Kigali amendment in 2016. HFC-23 does not deplete stratospheric ozone but is a very potent greenhouse gas. Commitments were made to reduce emissions of HFC-23 during the production of HCFC-22 as part of agreements in the protocol. However, the data presented and analysed in this paper indicate that in China more than the agreed amount of HFC-23 has been emitted since 2015, resulting either from unsuccessful factory-level HFC-23 abatement and/or inaccurate quantification of emission reductions. The analysis provides valuable data of atmospheric HFC-23. The study is also a good example of how compliance with the Montreal Protocol can be monitored.

This provocative opinion piece examines the theoretical, numerical, and observational evidence in support of two highly cited proposed mechanisms for invigorating deep convective clouds through higher aerosol concentrations. Both start with high concentrations of water droplets. Through cold-phase invigoration, precipitation is reduced allowing for greater release of latent heat from freezing higher up in clouds. With warm-phase invigoration, increased latent heating occurs lower down due to accelerated liquid condensation. In both cases, the article persuasively argues from a variety of standpoints that the evidence to support the importance of the effects is weak, particularly once the full complexity of clouds and their interactions with their environment is fully taken into account. Concrete suggestions are made for improving definition, observations, and modeling of the problem, but also an admonishment that attention in the field might be better directed towards more fruitful aspects of the aerosol-cloud interaction problem.

The Amazon's role in the tropical and global carbon cycle is highly significant. Usually considered as the "lung of the planet", it is mandatory to monitor if this role is kept, or if the large rainforest even turns into a source of carbon dioxide. The study by Basso et al. finds that during the analysed years, from 2010 to 2018, the Amazon is a small net source of carbon to the atmosphere. They find that fire is the primary driver of the Amazonian source, while drought years intensify the carbon emissions. The study also examined the contributions of different regions to the Amazonian carbon budget and found that emissions in the eastern Amazon were greater than those in the western region, primarily due to fires. These findings are of high relevance - and concern - to the larger geosciences community and indicate how important it is to stop slash-and-burn in the large rainforests.

The influence aerosols have on climate has long been recognised, but our ability to observe and quantify them on global scales came of age with the use of new satellite-based instruments and data products in recent decades. As instrumentation technologies improve alongside our abilities to handle large quantities of data and process using the latest data science approaches, the data products available to atmospheric science will only improve in decades to come. This opinion article tries to anticipate what tools and insights the community can look forward to in the near future.