Open access publishing is a milestone in the history of academic dissemination, and ACP has been at the forefront of this movement. This paper is co-authored by four scientists who played key roles in initiating ACP, and establishing and maintaining it as a leading journal in atmospheric science. It articulates the vision of 'open science' that motivated the establishment of ACP and provides a comprehensive overview of the publication process of ACP and other EGU journals and how open science is enabled and promoted within that process. Detailed statistics and commentary reveal how open science practices in publication in EGU journals have developed and evolved over the last 25 years, and how this may continue to develop into the future.

Aerosol particles emitted by ships substantially influence the microphysical properties of marine clouds. In 2020, regulatory changes led to a marked reduction in sulphur emissions, resulting in measurable decreases in cloud droplet concentrations. This study exploits a unique “accidental experiment” triggered by military conflict, which caused a rerouting of shipping traffic in the Red Sea. The authors use this event to robustly quantify how cloud droplet concentrations responded to changes in aerosol emissions associated with the changes in regulations.

Following the 2020 global reduction in shipping fuel sulphur, a natural experiment revealed significant changes in marine boundary layer (MBL) cloud properties over the Eastern North Atlantic. These new observations show a ~15% drop in cloud condensation nuclei, leading to fewer but larger cloud droplets. Normally, this would change how clouds reflect sunlight, but an increase in liquid water path (LWP) counteracted these effects. As a result, cloud optical depth and albedo changed very little. Simultaneous shifts in large-scale meteorology, including weaker inversion strength and increased dry air mixing, further complicated attribution of the observed cloud changes. The study suggests that overall, the cooling influence of marine boundary layer clouds seems to be weakening with implications for future climate feedbacks.

Climate models are indispensable tools for quantitatively predicting changes in global mean surface temperature in response to a doubling of atmospheric carbon dioxide concentrations. Despite decades of effort within the modeling community, large uncertainties persist. This opinion paper advocates for prioritizing the use of perturbed parameter ensembles (PPEs) as an effective strategy for reducing uncertainty in climate projections, rather than focusing on increases in model complexity or spatial resolution. Based on a synthesis of the existing literature, the authors evaluate the broad applicability and demonstrated effectiveness of PPEs and propose a strategic agenda for their expanded use within the climate modelling community.

Recent observations show a decrease in global low-level cloudiness, which has implications for the rate of global warming. This study shows that the decrease can be explained by known physical processes – cloud feedback and adjustments to greenhouse gases and aerosols. Global climate models simulate similar trends, providing confidence that current estimates of aerosol forcing and climate sensitivity are consistent with the observational record.