The rapid increase in global warming in 2023 has sparked fears that Earth has entered a new warm state. Possible explanations for a warming spike have included transient changes in radiative forcing or deficiencies in climate models. This study shows that the primary cause is a mode of natural climate variability – the El Niño/Southern Oscillation (ENSO). The authors show that, while the magnitude of global warming caused by ENSO was particularly large in 2023, it is not unprecedented in the historical record. The implication is that global warming is continuing at a steady average rate without dramatic acceleration.

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.

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.

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.

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.

The Royal Society report on the Krakatoa eruption included some marvelous paintings of extraordinary and highly unusual green sunsets. This article provides a novel explanation employing a detailed physical model that emphasizes the necessity for large sizes and amounts of aerosols.

The field of climate prediction has been bedeviled by the problem of how to represent the enormous complexity of clouds. The usual strategy is to peform deterministic simulations with advanced cloud models. The study outlined here concentrates on a statistical approach that is arguably better suited to determining the mean climatological state. The presented observations from a wide range of satellite platforms show that a power-law well describes frequencies of occurence of cloud sizes across a very wide range of scales, and that the exponent is robust to local climatological characteristics as surface temperature, aerosol loading, Coriolis forces, or dominant cloud type. Instead, the distribution of cloud sizes emerge simply from a competition for energy and air that occurs due to small-scale cloud mixing processes at cloud edge.

This paper presents a timely and topical research. It directly compares the marine cloud brightening (MCB) with the stratospheric aerosol injection (SAI) effects by utilizing the fully coupled atmosphere-ocean simulations. The study also reveals some new side effects by the MCB geoengineering, such as the locking of the climate into a permanent La Nina state and an increase in sea-level over the south Pacific Ocean. Those effects should be taken into account when developing geoengineering plans.

This personal tribute, authored by close colleagues and friends, is an homage to Paul Crutzen's profound and multifaceted impact in the field of atmospheric chemistry and physics. The article summarizes not only the exceptional breadth of research ranging from tropospheric ozone formation to stratospheric ozone loss, and coining the term ‘anthropocene’ but it also describes Paul Crutzen’s instrumental role in shaping the next generations of atmospheric scientists, many of whom now are leaders in the field. Furthermore, Crutzen’s legacy is manifested by his contributions to the creation of the journal Atmospheric Chemistry and Physics (ACP), that laid the foundation for EGU's pioneering open-access publication model, fostering public peer review and interactive scientific discourse.

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.

Bromine chemistry in polar regions is important for the composition of the atmosphere as well as climate. Reactive bromine strongly affects the oxidation capacity and the local ozone budget, and through the export to lower latitudes, it affects the ozone budget and the atmosphere's radiative properties outside polar regions. The newly identified bromine reservoir changes our understanding of the chemical budgets of polar halogens which will have implications for the ozone and mercury removal cycles.

For well over a century, many advances in tropospheric chemistry and physics have been driven by in situ observations, with measurements tending to be either specific measurements over multi-year periods, often as part of a network, or short term ‘intensive’ observations over of a more limited duration, typically 1-2 months. But in recent decades, many major developments in atmospheric science have been fueled through long-term comprehensive observations, and the Finnish SMEAR project is a major pioneering example of this. As part of the 20th anniversary Special Issue of Atmospheric Chemistry and Physics, this opinion piece reflects on the manifold and sustained contributions this has made to atmospheric science.

The core element of climate change is the sensitivity of the Earth's climate system to the balance between incoming solar radiation and outgoing terrestrial radiation at the top of the atmosphere. CO2 is one of the key atmospheric parameters controlling the outgoing longwave radiation. In this paper, hyper-spectral satellite data of the outgoing long wave radiation from a decade of observations are successfully used for the first time to disentangle the radiative effect of CO2 increase from those of other relevant parameters such as temperature and water vapour variations. The paper demonstrates convincingly that the observed radiative changes agree excellently with theoretical predictions of outgoing long wave radiation changes due to the observed CO2 increase. While solely based on observations from space, these results confirm a fundamental theoretical underpinning of the science of global warming.

The question of the climate sensitivity of CMIP models (used for IPCC) is a central question regarding the reliability of climate projections. Aerosol aspects are central here

While it has been established for decades that particulate matter, especially the PM2.5 fraction, can be quantitatively linked to a variety of negative health outcomes from both long- and short-term exposure, our ability to conclusively link this to specific particle components or types has proved stubbornly elusive. If particular sources or chemical components were found to be the driving factors, these could lead to more effective interventions and regulations. Because of recent advances in our observational capabilities and fundamental understanding of particle composition and toxicity, this opinion piece argues that we are at a pivotal moment in this ongoing area of study, and summarises some recent advances in this area.