Report The Panel at its 49th Session in May 2019 adopted Decision IPCC-XLIX-7 that the IPCC Task Force on National Greenhouse Gas Inventories (TFI) should produce a Methodology Report on Short-Lived Climate Forcers (SLCF). Preparatory work for the Methodology Report is starting in the sixth assessment cycle and continue with further methodological development in the seventh assessment cycle. Within the sixth assessment cycle, three to four expert meetings will produce a series of supporting materials which will inform a scoping meeting on the Methodology Report on SLCF. The scoping meeting where the outline was drafted considered the work on SLCF from the Sixth Assessment Reports of Working Group I and III. The scoping meeting took place from 26 – 28 February 2024 in Brisbane, Australia. The Panel agreed on the outline ( Decision IPCC-LXI-7) for the 2027 IPCC Methodology Report on Inventories for Short-lived Climate Forcers at the 61st Session of the Panel that took place from 27 July to 2 August 2024 in Sofia, Bulgaria. Preparatory Work The TFI has rescheduled the expert meetings planned for 2020 because of disruption caused by the COVID-19 pandemic. Expert meetings are now planned for: 2021 TBD, FAO, Rome, Italy – Expert meeting on SLCF emissions from sources in the Agriculture, Forestry and Other Land Use (AFOLU) and Waste sectors 15-17 December 2020, Istanbul, Turkey – Expert meeting on SLCF emissions from sources in the Energy and Industrial Processes and Product Use (IPPU) sectors. The TFI Technical Support Unit is compiling source categories of SLCF emissions and producing a comparative analysis of IPCC methodologies for greenhouse gas emissions/removals and available methodologies for estimation of SLCF emissions to assess their degree of integration, as well as any gaps. Draft analysis for all sectors, i.e. Energy, IPPU, AFOLU and Waste, have already been shared with organizations that produce SLCF methodologies to collect their feedback. Participants of both expert meetings have been selected by the TFI Bureau taking into account scientific and technical expertise, geographical and gender balance to the extent possible in line with Appendix A to the Principles Governing IPCC Work.

Report The IPCC is currently in its seventh assessment cycle which formally began in July 2023 following the elections of the new Chair and new IPCC and TFI Bureaus. The Panel at its Forty-third Session (Nairobi, Kenya, 11–13 April 2016) in its decision on the products of the sixth assessment cycle, decided that the seventh assessment cycle would include a Special Report on Climate Change and Cities. The scoping meeting to draft the outline was held in April 2024 in Riga, Latvia. The report’s outline (Decision IPCC-LXI- 5 ) was agreed the Panel during its 61st Session held in Sofia, Bulgaria from 27 July to 2 August 2024. The Special Report will be developed under the joint scientific leadership of Working Groups I, II and III with support from the Working Group II Technical Support Unit. Scoping A scoping meeting to determine the scope and outline of the report was held from 16 to 19 April 2024 in Riga, Latvia. We thank the government of Latvia for hosting the meeting. The meeting prepared a draft outline for the report as well as details of how it will be prepared and its timeline. A letter soliciting nominations of experts to attend the meeting was sent to governments and observer organizations on 20 October 2023. Participants in the Scoping Meeting collectively had expertise in the following areas: Biophysical (expertise examples include urban meteorology and climatology, urban meteorological/energy/water/carbon/air quality modelling and observational monitoring, urban carbon cycle, urban hydrology, urban biodiversity, land-atmosphere interactions); Impacts and Risks, including (i) Economic and Non-economic Losses and Damages and (ii) Compounding and Cascading Aspects (expertise examples include statistical climatology, detection and attribution of climate extremes, heat islands and urban overheating, air pollution, inland and coastal flooding, critical infrastructure including power, digital communication and transport, water/energy/food/health nexus, food security, health, supply chain, vulnerability, losses and damages, risk management, disaster risk reduction, risk modelling); Sectoral Development, Adaptation, Mitigation and Responses to Losses and Damages (expertise examples include built environment, urban planning, building design and materials, tourism, urbanization trends, informal settlements, migration and urban poverty, water management, energy systems, infrastructure, ecosystems and biodiversity, nature based solutions/ecosystem based adaptation, livelihood and communities perspectives issues, urban scenarios/pathways, transportation systems and mobility services, industry, urban agriculture and food production, waste management, climate services and early warning systems, environmental psychology); Energy and Emissions (expertise examples include emission inventory, urban and embodied emissions, urban energy demand and services, energy mix, urban energy management, power grid layout, standards and regulations, carbon accounting, urban carbon sequestration, renewable energy, life cycle assessment); Governance, Policy, Institutions, Planning and Finance (expertise examples include typology of cities and decision frameworks, urban planning, architecture, smart cities, mitigation / adaptation policies, energy security, water security and sanitation, barriers and enabling conditions for urban climate finance, insurance, livable cities, role of normative principles in governance, science, technology and innovation for sustainable cities, articulation between local and national development priorities, climate resilient development in cities, Sustainable Development Goals, environmental, social and governance reporting); Civil Society (expertise examples include social coherence, justice, equity, ethics, gender, intersectionality science, decision making, science/policy interaction, communication, digital/cyber security, indigenous knowledge systems, diversity of urban stakeholders involved in climate responses, city networks / alliances, environmental advocacy). Scientific Steering Committee Members The Scientific Steering Committee is chaired by IPCC Vice-Chair Diana Ürge-Vorsatz. Scientific Steering Committee members include: Robert Vautard (Co-Chair of Working Group I) Xiaoye Zhang (Co-Chair of Working Group I) Winston Chow (Co-Chair of Working Group II) Bart van den Hurk (Co-Chair of Working Group II) Katherine Calvin (Co-Chair of Working Group III) Joy Pereira (Co-Chair of Working Group III) Nana Ama Browne Klutse (Vice-Chair of Working Group I) Ines Camillioni (Vice-Chair of Working Group I) Laura Gallardo (Vice-Chair of Working Group II) Zinta Zommers (Vice-Chair of Working Group II) Malak Al Nory (Vice-Chair of Working Group III) Şiir Kilkis (Vice-Chair of Working Group III) Pre-scoping The Scientific Steering Committee for the Scoping of the IPCC Special Report on Climate Change and Cities organized pre-scoping webinars to help inform the scoping meeting. The pre-scoping webinars were open to all IPCC member government Focal Points and all experts nominated for participation in the Scoping Meeting not attending the Riga meeting. The webinars followed a format that collected suggestions for key elements that could be considered for inclusion in the outline of the IPCC Special Report on Climate Change and Cities. To accommodate a maximum number of participants from all the different time zones, the webinar which were identical in nature, took place on: 18 March from 15:00 to 17:00 PM CET, 19 March from 9:00 to 11:00 AM CET, and 19:00 to 21:00 PM CET

Mineral aerosols are one of the most important ice nucleating particles (INPs) because their efficiency in nucleating ice, wide transport and largest mass contribution to particulate matter in the atmosphere. They are sourced from the arid regions of the world. In this context, this work evaluates the INP potential of fourteen topsoil samples collected from subtropical South American deserts, the major source of mineral aerosols in South America, in the immersion freezing mode. Samples were obtained from three distinct regions located in the South American Arid Diagonal and recognized as potential dust source areas: the Puna-Altiplano Plateau in the north, the central-west of Argentina, and Patagonia in the south. In general, results reveal that samples from the Puna-Altiplano and Patagonia regions, and the central-west of Argentina region exhibit the highest and lowest INP abilities, respectively. The active sites per unit surface area for a given temperature were calculated and compared with previously reported values. The results demonstrate that soil mineral particles from the region of study exhibit ice nucleating abilities comparable to the inorganic fraction of agricultural soils of central Argentina. No direct relationship was identified between INP ability and the major minerals observed in the samples. This study is the first to analyze the ice nucleation properties of soil samples collected along the South American Arid Diagonal and one of the few in South America. Since the analyzed topsoil particles were collected from potential dust source regions, this work contributes to understanding the role of aerosols in initiating atmospheric ice formation, providing valuable data for empirical parameterizations. This could contribute to the improvement in the performance of climate models, as the obtained results suggest that the underestimation of coarse and super-coarse aerosols at altitudes relevant for cloud formation may lead to underestimations in INP concentrations, particularly in regions near to the emission sources.

Increasing heavy rainfall poses significant challenges in the Yangtze-Huaihe River Valley (YHRV). There is a need for more specific insights into the vertical microphysical structures and their influence on heavy rainfall to enhance the accuracy of numerical simulations and forecasts. Using data from the Dual-frequency Precipitation Radar (DPR) on the Global Precipitation Measurement (GPM) satellite from 2014 to 2023, this study investigated the vertical microphysical structures of different types of heavy summer rainfall (> 8 mm/h) and elucidated their impacts on the rain rate in the YHRV. Based on the radar reflectivity thresholds at different altitudes, heavy summer rainfall was classified into four types: deep convective, shallow convective, stratiform rainfall, and warm rainfall. In the YHRV region, shallow convective rainfall contributed the most to total heavy rainfall (39.1 %) and had the highest occurrence (44.7 %) of extreme rainfall (>50 mm/h). Stratiform rainfall occurred most frequently but decreased rapidly with increasing rain rates, while warm rainfall contributed little to heavy rainfall. For the vertical microphysical structure of heavy rainfall, deep convective rainfall exhibited rapid growth of large particles above the melting layer, resulting in the largest average mass-weighted diameter (Dm) near the surface (2.2 mm), but the smallest average droplet concentration (recorded as dBNw in the decibel scale), approximately 37. Below the melting layer, the Dm of small particles in the shallow convective rainfall increased rapidly, and the impact of coalescence was much greater than that of break-up. Except for warm rainfall, the average Dm for other types of heavy rainfall remained relatively high, exceeding 1.5 mm both within and below the melting layer. The average dBNw increased consistently as altitude decreased. As rainfall intensified to extreme rainfall, the average rain rate of shallow convective rainfall slightly surpassed that of deep convective rainfall. This was due to a decrease in average dBNw for deep convective rainfall, while the average dBNw of shallow convective rainfall continued to increase.

Defining regions with similar characteristics for extreme precipitation is crucial for understanding the impacts of climate change, planning and managing water resources, and designing hydraulic structures. However, studies on the regionalization of extreme precipitation for Türkiye are limited, and regional extreme precipitation characteristics are not well defined. In this study, motivated by the need to contribute to this field, homogenous regions for extreme precipitation across Türkiye were determined using the latest version (V07) of Integrated Multi-satellitE Retrievals for GPM (IMERG). We initially validated IMERG V07 estimates using data from 214 ground-based stations and compared the results with its predecessor V06. The results revealed that IMERG showed some notable improvements from V06 to V07 for all seasons, especially in winter. During this season, the correlation coefficient increased from 0.57 to 0.64, the mean absolute bias decreased from 78.22 % to 69.27 %, and the RMSE decreased from 11.10 mm/day to 9.70 mm/day. In V07, while the trend of decreasing accuracy with increasing elevation observed in V06 continues, it has been shown that some notable improvements were achieved in continuous and categorical metrics. We then applied widely used non-hierarchical (K-means) and hierarchical (Ward's method) clustering techniques. To perform this, we first applied Principal Component Analysis (PCA) to reduce the number of variables related to extreme precipitation (e.g. amount, frequency, standard deviation, and seasonality) and geographic characteristics to identify the most significant variables for analysis. The K-means method delineated Türkiye into eight extreme precipitation regions, while the Ward's method resulted in six distinct extreme precipitation regions. We evaluated the results based on the existing extreme precipitation climatology literature for Türkiye and by associating them to known precipitation dynamics, and as a result, we recommended eight precipitation regions determined by the K-means. The identified precipitation regions are expected to contribute to future studies analyzing the effects of climate change and to regional studies on natural disasters resulting from extreme precipitation.

The present study identifies a close linkage between spring (MAM) sea ice concentration (SIC) anomalies in the Greenland-Barents (GB) Seas and the tropical cyclone (TC) genesis frequency over the eastern North Pacific (ENP) in the subsequent summer and fall (JJASON) during 1979–2022. An increase in MAM GB SIC anomalies results in a decrease in subsequent JJASON ENP TC genesis frequency. The physical process for the influence of Arctic sea ice anomalies on TC formation is further examined. Detailed dynamical diagnosis reveals that a higher GB SIC during MAM results in an increase in upward shortwave radiation, leading to sea surface temperature (SST) cooling. This SST cooling triggers a teleconnection atmospheric wave train, traversing Eurasia, the northern Pacific and the northern America and reaching the northern Atlantic. The associated anomalous cyclone over mid-latitude northern Atlantic is accompanied by anomalous southwesterly winds over the subtropics, leading to SST warming in the subtropical northern Atlantic through weakening total wind speed and upward surface latent heat flux. SST warming in the subtropical northern Atlantic extends southward into the tropical Atlantic via wind-evaporation-SST feedback during the subsequent summer and autumn, which induces an anomalous zonal-vertical circulation with descending motion over the ENP. This descending motion reduces relative humidity and weakens local convection over the ENP, and thus is unfavorable for TC genesis there. This study suggests that the spring GB SIC could serve as a potential predictor of JJASON ENP TC genesis.

Snowfall plays a crucial role in the mountainous cryosphere cycle and is significantly influenced by climate change. This study utilizes the global climate models (GCMs) from Coupled Model Intercomparison Project phase 6 (CMIP6) with multivariate bias correction (MBC) to explore potential future variations in snowfall and its elevation dependency across the Tibetan Plateau (TP). Findings indicate a consistent decline in annual snowfall across the majority of the TP by the end of the century, except for certain high-elevation regions in the northwest. The decreasing trend is projected to intensify with strengthen Shared Socioeconomic Pathway (SSP) scenarios and exhibits elevation dependency below 5000 m. Specifically, under the SSP5–8.5 scenario, snowfall over the TP is expected to decrease by 39.74 % in the far future (2071–2100), with the elevation zone below 2000 m experiencing the most intense decline of approximately 62 %. This trend is largely attributed to the significant warming, which reduces the snow fraction as more precipitation falls as rain rather than snow. This shift is evidenced by the identification of turning points in snow fraction in the mid-2040s to 2050s, coinciding with rapid temperature increases. Furthermore, substantial decreases in future (heavy) snowfall days contribute to the overall reduction in snowfall. However, complex interplay between increased precipitation and temperature effects results in a slight increase in snowfall over high elevation areas in the northern edge. Uncertainty analysis indicates model uncertainty as the dominant source in snowfall projections, accounting for over 50 % of total variance. The projected declines in snowfall and snow fraction, as well as shortened snowfall days could considerably impact the cryosphere, hydrological and ecological systems of the TP.

Since 2013, heavy pollution episodes have occurred frequently over the North China Plain after unprecedented efforts to reduce primary pollutants. In this study, a pollution process in Tianjin, a typical city in North China, was selected to investigate the impact of 3-D meteorological patterns on PM2.5 and meteorological element profiles obtained by tethered balloons and meteorological towers. The pollution episode lasted 4 days with hourly PM2.5 concentrations exceeding 150 μg·m−3 for 81 h and a peak concentration of 377 μg·m−3. In the early stages of the first pollution period, wind speed with height showed an almost opposite trend to PM2.5 concentrations. In the vertical direction, weak winds were frequently accompanied by PM2.5 peaks, whereas strong winds were favourable for the diffusion of pollutants. In the later stage, a stable boundary layer with a height of approximately 600–700 m, thermal inversion layer capping the boundary layer, uniformly high-humidity atmosphere (>80 %), and relatively uniform distribution of wind speed across heights contributed to the high PM2.5, which remained within the boundary layer, and the continuous growth of surface PM2.5 concentrations. In the secondary pollution period, the successive regional transport of particles from Beijing and Baoding was the main reason for the two surface PM2.5 peaks in Tianjin. Different regional sources elevate PM2.5 levels, further extending the duration of haze pollution. The results reveal that 3-D meteorological conditions are the key reason for heavy pollution occurrence in the context of pollution reduction.

Recently, the frequency of heatwaves has increased worldwide, with significant implications for public health, the environment, and socio-economic stability. In densely populated urban areas such as Hong Kong, heatwave events are of particular concern owing to their potential to exacerbate air pollution and modulate weather patterns, including extreme precipitation events. Despite extensive research on these phenomena, there exists a gap in understanding the interconnections of heatwaves, pollution, and extreme precipitation in subtropical urban climates. Utilising in-situ meteorological data from the Hong Kong Observatory (HKO), we investigated the characteristics of heatwaves and explored their impact on pollution and local rainfall. We observed a significant increase in the frequency, intensity, and duration of heatwaves in Hong Kong, particularly after 2010. Synoptic analysis indicates that these heatwaves are often associated with persistent high-pressure systems or tropical cyclones periphery downdrafts, as these patterns could induce subsidence and reduce cloud cover, which contributes to temperature increase. Additionally, heatwaves significantly elevated concentrations of ground-level ozone and particulate matter owing to increased photochemical reactions and stagnant air conditions. These pollution spikes coincide with heatwaves, aggravating public health risks. Furthermore, the increased frequency of heatwaves has altered the composition of local precipitation, with heatwave-following extreme precipitation events occurring more frequently, suggesting a thermal-driven amplification of the hydrological cycle. Our results highlight the urgent need for integrated urban planning and health policies that address the compounding effects of heatwaves, pollution, and subsequent extreme precipitation, underscoring the importance of adapting to and mitigating these linked phenomena amid a changing climate.

The mechanism of climate and vegetation change induced dust variations has been a phenomenal environmental concern in East Asia. However, the extent to which climate and vegetation cover separately affect the interannual variations of dust activity, is little known. Here in our study, the dust interannual variations and the contributions of climate and vegetation changes on dust variations were investigated and quantified through a regional climate model (RegCM4.9.5) and GeoDetector analysis. It is indicated that the dust aerosol optical depth, column burden and emission flux over East Asia all showed fluctuating downward trends followed by subsequent increases during 2000–2018. Climatic factors dominated the interannual variation of dust over East Asia, particularly the modulation of El Niño Southern Oscillation events. Vegetation improvement has occurred in most dust source regions across East Asia, playing a secondary yet positive role in dust variations, with an overall contribution rate of approximately 37 %. The spatial heterogeneity of dust variations in different regions was shaped by the effects of climate change and improved vegetation conditions.