This article was solicited for the ACP 20th Anniversary collection. It received positive reviews that very nicely contributed to the ideas and to which the authors responded thoroughly. It is a stimulating read, combining 'big-picture' considerations with more detailed technical discussion of important and illuminating examples.

We very much welcome this useful opinion on ways to strengthen atmospheric research in the Global South.

The article is an interesting and provocative paper that derives its ideas from recently published research results and discoveries about the ambivalent role of atmospheric halogens (chlorine) in controlling the global abundance of tropospheric O3, OH and CH4. It discusses and proposes a possible roadmap to explore how artificially released iron chloride aerosols could reduce the atmospheric CH4 burden to mitigate its climate impact. The authors call for interdisciplinary efforts while keeping in mind ethical concerns and social impacts due to the proposed method.

The acquisition and proliferation of large datasets presents one of the greatest opportunities in environmental science in recent years, particularly with the application of open data principles in dissemination. However, the effective utilisation of these data poses new, ever greater technical and organisational challenges. This article discusses the methods and benefits of the application of the European ENVRI infrastructure to atmospheric science in programmes such as ACTRIS, IAGOS and ICOS, in particular through the application of FAIR principles, and how this will help to push back the frontiers of atmospheric science.

The paper provides an informative and up-to-date overview of the progress made in understanding the evolution of the stratospheric ozone layer since the introduction of the Montreal Protocol to a broad and interested readership. It discusses new evidence that, despite the success of the Montreal Protocol, the stratospheric ozone layer will not necessarily return in the long term to the state it was in before the start of the release of CFCs (before 1970). New challenges arise, due to man-made climate change or natural extreme events like volcanic eruptions that have been shown to impact the ozone layer. The authors' appeal is therefore to continue to closely monitor the state of the stratospheric ozone layer, which is important for life, in the future.

Equilibrium climate sensitivity (ECS), with a specific definition, has been used as a convenient measure, encapsulated in a single number, of the response of the climate to increases in long-lived greenhouse gases. The authors recall some of the history of how ECS has been estimated, by models and observations, including paleoclimate data and note recent progress in reducing uncertainty in the value of ECS. However they also note that there are important aspects of future potential climate change that are not captured by the ECS measure and therefore that there will be limited usefulness in too strong a focus on reducing uncertainty in ECS alone.

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.

Natural aerosols have long been understood to have a substantial effect on Earth’s climate, in particular because they define a natural baseline that affects the magnitude of anthropogenic aerosol radiative forcing. In this Opinion the authors highlight the very large uncertainties in the radiative effects of aerosols particularly from fires and dust, which they estimate to exceed the magnitude of anthropogenic aerosol forcing over the entire industrial period. Rather than just defining a pre-industrial baseline for aerosol, natural emissions have changed substantially during pre-industrial times and through the industrial period, making a very poorly defined contribution to climate change. Based on the estimated large uncertainties, the authors argue for greater research efforts to narrow them.