This paper shows that current climate policies risk triggering tipping points like permafrost thaw and rainforest loss. While these changes add some extra warming, human emissions remain the biggest driver. Lower emissions reduce the risk, but reaching tipping points is still possible.

Strong reduction of sulfur emission from shipping since 2020 provides a rare opportunity to examine the response of climate system to anthropogenic forcing. Using a global climate model, this study estimates a global aerosol effective radiative forcing of 0.13 W m-2 from ship emission reduction. This emission reduction leads to a global mean warming of 0.04 K in 2020-2049 with larger warming at regional scales. The warming may not be evident at present day because of the climate variability, but can represent a significant fraction (17%) of the remaining warming to 1.5 K target.

This review is topical and of potential interest to a broad audience

The Coupled Model Intercomparison Project lies at the core of global climate prediction. This paper details the Coupled Model Intercomparison Project phase 7 (CMIP7) and its Fast Track initiative. By transitioning into a continuous climate modeling program with enhanced coordination and federated planning, CMIP7 aims to address key climate questions more effectively. The expansion of the Diagnostic, Evaluation, and Characterization of Klima (DECK) experiments—including the addition of historical simulations, effective radiative forcing assessments, and CO₂-emissions-driven experiments—strengthens the foundation for climate model evaluation and projection. Additionally, the AR7 Fast Track ensures timely delivery of critical climate simulation data to support the upcoming 7th Intergovernmental Panel on Climate Change Assessment Report. This paper highlights how these advancements in experimental protocols and infrastructure support not only scientific understanding but also inform policy-making and climate services, ultimately contributing to global efforts in climate adaptation and mitigation.

The explosive Hunga Tonga eruption in 2022 had several characteristics that are unique in the recent observational record. Its effects on the atmosphere are of great scientific interest and have included rapidly propagating patterns of Lamb and gravity waves and long-lived anomalies in stratospheric water vapour. This modelling study demonstrates how, through a coupled sequence of chemical, radiative and dynamical mechanisms, the additional water vapour in the tropical upper stratosphere is likely to have had a significant effect on the wintertime circulation of the stratosphere and hence the troposphere in Northern Hemisphere mid- and high latitudes. These results on the fascinating Hunga Tonga event have wider relevance to perturbations of the climate system where the initial impact is stratospheric but important secondary effects are felt elsewhere, including at the surface.

Our understanding of the impact of aerosol particles on deep-convective clouds is still incomplete. Earlier research found increased lightning above shipping lanes and hypothesized that the ship emissions support the development of deep-convective cloud systems. The present study investigates the effect of a sevenfold reduction in sulphur content of shipping fuel, implemented following the International Marine Organisation 2020 regulation. A significant decrease in lightning activity is found over shipping lanes in combination with a reduction of both the concentration and the size of the emitted particles, providing further evidence of a strong connection between shipping missions, deep-convective cloud development, and lightning activity. This highlights the need to resolve the still unclear acting mechanisms.

The manuscript investigates the future stability of the West Antarctic Ice Sheet (WAIS) exploring how ice-sheet models may be initialised in a way that incorporates present-day satellite observations of ice mass loss. The study shows that with present-day ocean water temperatures large parts of WAIS will deglaciate, with major implications for global sea levels, particularly in Europe. Model simulations also show that current ocean-driven melting trends will destabilize the Thwaites and Pine Island glaciers over the next several centuries even in the absence of further change, leading to rapid ice loss.

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.

This work highlights the importance of data collected through citizen science for studying extreme geomagnetic storms. The Gannon storm was an unprecedented opportunity to test the limitations of citizen science data collection and to offer solutions to enable similar studies in the future. This study has great potential to be of interest to the wider public.

This paper is the first to present a mechanism for greening of the Sahara that relies on westerlies intensity as the primary mechanism. This mechanism, which is based on cutting edge analysis and observations, has the potential to move the 30 year-long debate on African Humid Period forward. This interdisciplinary study joins several fields, including climatology, paleoclimate, remote sensing, and hydrology.

This paper reports the alarming increase in wildfires in the Amazon rainforest during the severe 2023/2024 drought leading to an increase in large-scale forest degradation. It is highly relevant as the fire-driven forest degradation impacted also intact forests and released large amounts of carbon dioxide to the atmosphere pushing the Amazon rainforest closer to a potential tipping point.

This paper represents a state-of-the-art review of tipping points in the physical climate system. It should be of great value and use to inform the community about the current status of research on tipping points and global climate change.

Volcanic aerosol can persist in the stratosphere for many months and have a significant cooling effect on the troposphere. The effects of the eruption of Mount Pinatubo in 1991 are an important recent example. The time that aerosol remains in the stratosphere is determined by the combined effects of transport and sedimentation. This paper sets out a simple conceptual framework for capturing these effects and combines it with observations and modelling to give robust estimates of key timescales. This framework will be valuable in studies of future eruptions and in studies of artificial stratospheric aerosol injection as a possible approach to solar radiation modification.

This paper provides a critical evaluation of decadal trends in wetland methane emissions, offering robust insights into how climate change is driving increases in this potent greenhouse gas. By utilizing an ensemble of sixteen models alongside ground-based measurements, the paper provides a robust quantification of emission trends and associated uncertainties. As an international collaboration, this work contributes a comprehensive analysis that not only quantifies recent trends but also identifies temperature as the primary driver, offering essential information for predicting and managing future methane feedbacks.

Measurement datasets of cloud condensation nuclei (CCN) are vital for our understanding of aerosol-cloud interaction and reliable climate modelling. This study analyses and compares the only two global CCN datasets derived from satellite and reanalysis data. These key datasets are found to disagree over pristine oceans in terms of their climatology as well as seasonal and annual variations. Given the importance of CCN as a fundamental property in climate model simulations, further research is needed to reconcile these differences and to produce an observation-based dataset that can be confidently used to evaluate our understanding.

Aerosols moderate climate change by modifying cloud properties to make them more reflective to sunlight, but the magnitude of the effect remains very uncertain, with important implications for future climate change. This study combines satellite observations and reanalysis data to calculate a much weaker radiative forcing due to aerosol-cloud interactions than previous studies. The implication is that aerosols may play a smaller role in climate change than has been widely supposed.

Ice streams are dynamical features that control the ice flow from an ice sheet's interior to its margin. Due to their dynamical nature, ice streams may change flow speed and direction in response to external forcing or due to internal variability. This study investigates the interplay between neighbouring ice streams that feed the Dotson and Crosson Ice Shelves in West Antarctica. The authors use satellite observations to measure the change in ice speed and flow direction from 2005 to 2022, and their observations reveal a highly complex pattern of dynamical changes and the redirection of ice flow from one stream to another. These observations reveal previously undocumented impacts of spatially varying ice flow, which may influence the ice shelf and ice sheet.

This paper provides a guideline The Role of AerChemMIP and will therefore be helpful both for future assessments and for the development of the MIPs in general. Rolf Müller

For most countries dealing with the consequences of sea-level rise, a constructive discussion about actionable science is critical. There is a need to strengthen the lines of evidence for sea-level projections and at the same time there is a strong need for practitioners to understand which science they should rely on to plan adaptation actions. This manuscript outlines when scientific results may be considered actionable and discusses the risks in using novel results to inform decision-making. The case study discussed in the manuscript is also valid for other climate-change-related fields.

The paper presents the first quantification of the freezing behaviour of siver iodide (AgI) containg seed particles derived from glaciogenic cloud seeding experiments in the real atmosphere. The study uses a unique approach by releasing the seeding particles from an unmanned airborne vehicle in supercooled low stratus clouds and measuring the formed ice crystals by a tethered balloon placed downwind. Based on their unique observation data the authors propose a novel freezing mechanism based on the hygroscopic growth of the particles in to solution droplets, subsequent freezing of droplets and further growth of ice crystals while ruling out competing freezing mechanisms like contact freezing.

Noctilucent clouds form in the extremely cold temperatures in the high-latitude summer mesosphere (altitudes of 75-85km). Their formation requires the right combination of water vapour concentrations and temperatures. It has been speculated for example, that increasing frequency of occurrence of such clouds might result from increases in methane concentrations, with the methane being converted into water vapour in the upper stratosphere and mesosphere. This paper reports observations of noctilucent clouds, made using an automated lidar system in southern Argentina, at unexpectedly low latitudes compared to previous Southern Hemisphere observations. Possible explanations, including systematic moistening of the mesosphere by space traffic, are discussed.

This study integrates deep learning techniques into hydrological modelling to reconstruct runoff data. The extended reconstruction of runoff spanning over six decades (1962-2023) provides an unprecedented data basis to study long-term runoff patterns and trends in Switzerland. The findings spotlight a shift towards less frequent wet years and more frequent dry conditions in Switzerland. This insight is also relevant given the current situation of extreme droughts and floods in Europe.

Effusive, long-lasting volcanic eruptions impact climate through emission of gases and subsequent production of aerosols. Although previous studies have shown that the sulphate aerosol produced by these eruptions cools Earth's climate, this Earth system modelling study shows that high-latitude effusive eruptions can cause Arctic warming during the fall and wintertime. This warming effect is caused by the enhancement of downward longwave radiation from very optically thin clouds. The results have implications for our understanding of future Arctic climate change as well as any efforts to deliberately modify the climate through solar radiation management.

Graphical representations of modelling processes play an important role in communicating the nature of models to their users, and to the downstream users of the outputs of those models. This paper takes an innovative approach to developing and disseminating graphical model representations in a consistent manner across a whole research community. By publishing these together as a journal paper, the rigour and accuracy of these representations has been subjected to peer review. The quality of the diagrams themselves is high, and the process undertaken to produce them is an examplar of good practice in disseminating science.

The cause of fluctuations of the stratospheric quasi-biennial oscillation (QBO) period has been debated since its discovery. Explanations for variations in the QBO period are of great interest given the relevance of the QBO as one of the major modes of atmospheric interannual variability that affects not only the tropics, but also on the surface weather and climate in the extra-tropics. This study analyses the latest versions of atmospheric reanalyses, which resolve a part of the gravity wave spectrum, to show that the period fluctuations can be explained by seasonal variations in the forcing by small-scale (gravity) waves in the equatorial region. In doing so, the study improves our understanding of one of the fundamental modes of stratospheric variability and suggests that improvements in our ability to capture multi-scale interactions in the models can improve the accuracy of seasonal forecasts and the reliability of future climate projections.

It is widely accepted that secondary organic aerosols formed from the atmospheric processing of biogenic and anthropogenic VOCs represent a major component of particulate matter globally, affecting air quality, meteorology and climate, but predictive skill in atmospheric models remains poor. It has previously been theorised that water vapour may assist organic molecules forming particulates by condensing together, but until now this process has not generally been implemented in models. This paper demonstrates that by including this process using a new efficient parameterisation in the popular GEOS-CHEM global chemistry model, the effects on predicted aerosol formation can be substantial, so this effect warrants consideration going forward.

This study explores how choice of glacier and GCM model can impact prediction of future runoff for 75 of the world's major river basins. It is a very timely piece of research given ongoing pressures on river flow in glaciated regions which are on track to worsen, and offers a framework from which modellers can produce policy-relevant information related to future contribution of glaciers to freshwater resources, including consideration of uncertainty.

An important parameter for calculation of the impact of cirrus clouds on climate is the asymmetry parameter. The asymmetry parameter captures the degree to which light is scattered by individual ice crystals towards the ground rather than reflected upwards to space. The measurements presented in this study show that, for a common type of ice crystal called bullet rosettes, cirrus clouds may be far more efficient at reflecting sunlight than is typically assumed in the radiative transfer models used to represent the role of clouds in climate.

The Northeast Greenland is one of the most remote and inaccessible areas in the world. This study presents a comprehensive overview of numerous observations from the region as well as results from numerical model simulations. The results highlight the dynamics of Greenland's largest marine-terminating glacier, revealing key differences between the two neighbouring glacier outlets. The authors also discuss the impact of glacier melt on the ocean, including the potential impact on the Atlantic meridional overturning circulation - a key topic of concern in the community and the general public.

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.

The Aral Sea represents both an example of large-scale environmental degradation caused by human activity and a message of hope through its partial restoration. This follow-up study emphasizes that while the system appears healthy at present, it lacks resilience and small perturbations could significantly alter the entire ecosystem. The topic of the paper is of broad interest and the results of the analysis are also important for the management of the region.