C3S Climate Bulletin is a key climate monitoring product, implemented The European Centre for Medium-Range Weather Forecasts (ECMWF) as part of the program Copernicus of the European Union, which serves as Europe's eyes on Earth. Its history began to be written in September 2015 with the publication of the first monthly bulletin (then called the “monthly summary”), which covered temperature statistics for August 2015 for both global and European regions and placed them in historical context. This edition kicked off a decade of continuous climate monitoring by C3S, which has since expanded to include other well-known products such as the European State of the Climate (ESOTC) and Global Climate Highlights (GCH).

Purpose and content development

The climate bulletin provides monthly report key variables and statistics just a few days after the end of the given month. The timely nature of the product focuses on areas where the suitability of the basic data sets has been verified by comparison with other data sets. The core of the output is the summaries presented on the C3S website, while Summaries for hydrological variables and sea ice were introduced in 2017. The content of the newsletter is constantly evolving; the templates have been updated to include statistics for meteorological seasons from August 2019 a sea surface temperatures from March 2024. The introduction of new products is a deliberate process; they are often tested in the context of other publications such as ESOTC before becoming a regular part of the bulletin. Examples include the daily sea ice maps that appeared in ESOTC 2019 and were fully integrated into the bulletin from July 2024, or the daily sea surface temperature maps that were first developed in the interactive web application Climate Pulse (February 2024) and subsequently introduced into the bulletin in May 2024. In addition to the content, improvements are also being made to user experience and visual identity, for example by introducing interactive time series graphics from July 2024.

Production process overview

Behind every monthly newsletter is long and precisely defined production chain involving development, discussions and implementation. This process follows a pre-established workflow and schedule with operational software suites and distributed tasks across the team. The basic steps have remained the same since the beginning: raw data is transformed into graphs and statistics using automated software. These are verified and inserted into templates for writing and review. The final summaries are then distilled into key messages, which are used to create communication products. Over the years, the processes have become more efficient and sophisticated, including further automation. The data sources have also changed, moving from a single ERA-Interim dataset to ERA5, ERA5 Land and EUMETSAT OSISAF Sea Ice Index depending on the variable.

Challenges and adaptation

One of the key challenges was change of standard reference period to 1991-2020 in January 2021. Although the change of reference period is computationally relatively simple for datasets such as reanalyses, it required additional analysis, evaluation and documentation. Communication challenge was how to present the results effectively, as changing the reference period affects the relative scale of the anomalies presented. C3S decided to refer in the text to the new period 1991-2020, while graphs for both the old and new periods were available during the transition period, while global temperatures continued to be compared to pre-industrial temperatures. It was also ensured transparency and traceability by incorporating climatologies and anomalies for both reference periods into the relevant catalogue.

Growing reach and influence

The success of the Climate Bulletin is demonstrated by its enormous increase in popularity, which is visible in the constantly growing number of mentions in the media. Data from the bulletin is regularly downloaded data journalists and other media to illustrate findings or provide a complementary perspective. Examples include Financial Times and BBC, which used data from the bulletin to highlight the exceptional global temperatures of 2024. Public products now include more refined graphics, additional visualizations for print and social media, as well as direct access to processed data.

The C3S Climate Bulletin is thus a key climate monitoring tool that is constantly evolving and adapting to provide timely, accurate and transparent information on the state of our planet. JRi