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Limiting global warming to well below 2°C and pursuing efforts to limit it to 1.5°C, as agreed in the 2015 Paris Agreement, requires global carbon neutrality by mid-century at the latest. The corresponding carbon budget is decreasing steadily and significantly. To phase out carbon emissions in line with the specified temperature target, countries are formulating their mitigation efforts in their long-term low greenhouse gas emission development strategies (LT-LEDS). However, there are no standardized specifications for preparing these strategies, which is why the reports published to date differ widely in terms of structure and scope. To consider the multiple facets of reaching net-zero from a systemic perspective as comprehensively as possible, the authors propose the Net-Zero-2050 System: A novel, transferrable systems approach that supports the development of national endeavors toward carbon neutrality. The Net-Zero-2050 System is defined by three interconnected components: The Carbon-Emission-Based System, the surrounding Framing System and a set of system boundaries. For both systems levels, IPCC approaches were used as a basis and were then adjusted and supplemented by Net-Zero-2050. We suggest applying the Net-Zero-2050 System—beyond the project environment—in carbon emission based contexts at different levels. Especially at the national level, this would improve the comparability of the different national strategies to achieve carbon neutrality.

Human-induced climate change is one of the most pressing challenges of our time. The Helmholtz Association is making essential research contributions to mitigate the causes and impacts of climate change and find ways to adapt. The “Net-Zero-2050” project, the Cluster I of the Helmholtz Climate Initiative, scientifically investigates and evaluates strategies and new ways to reduce, extract and permanently store carbon emissions. Two digital knowledge transfer products (DKTPs) were developed to present the complex research results comprehensively: (1) the “Net-Zero-2050 Web-Atlas” provides information on methods and technologies for CO2 reduction and possible reduction paths; (2) the “Soil Carbon App” provides simulated soil carbon data to estimate climate protection potentials through different land management methods. Both formats intend to support users in making informed decisions and developing appropriate climate neutrality strategies. During the two DKTPs development, common main challenges were identified regarding concepts and stakeholder involvement. Along with that, specific approaches to solving the tasks could be distilled for each product. In the still-evolving arena of digital knowledge transfer, no standard methods can be applied. At the same time, communication of climate research results to decision-makers is becoming more and more relevant. This paper extracts the challenges and gives approaches to facilitate a transfer of the gained experience to future similar projects.

The goal of an interdisciplinary team of scientists at the Climate Service Center Germany (GERICS) was to make the findings of the special report IPCC SR1.5 more accessible to the citizens of Hamburg. Therefore, a flyer was created that is understandable to non-climate scientists, visually attractive and generates interest. It contains up-to-date climate information, readily understandable texts and several graphical visualisations. The team has been working intensively on analysing and processing further the consequences of a 1.5 ∘C global warming for the Hamburg metropolitan region. While the team's natural scientists elaborated the impacts on specific climate indices, other team members focused on the visualisation and communication of the results.

Germany 2050: For the first time Germany reached a balance between its sources of anthropogenic CO2 to the atmosphere and newly created anthropogenic sinks. This backcasting study presents a fictional future in which this goal was achieved by avoiding (similar to 645 Mt CO2), reducing (similar to 50 Mt CO2) and removing (similar to 60 Mt CO2) carbon emissions. This meant substantial transformation of the energy system, increasing energy efficiency, sector coupling, and electrification, energy storage solutions including synthetic energy carriers, sector-specific solutions for industry, transport, and agriculture, as well as natural-sink enhancement and technological carbon dioxide options. All of the above was necessary to achieve a net-zero CO2 system for Germany by 2050.Plain Language Summary Here a net-zero-2050 Germany is envisioned by combining analysis from an energy-system model with insights into approaches that allow for a higher carbon circularity in the German system, and first results from assessments of national carbon dioxide removal potentials. A back-casting perspective is applied on how net-zero Germany could look like in 2050. We are looking back from 2050, and analyzing how Germany for the first time reached a balance between its sources of CO2 to the atmosphere and the anthropogenic sinks created. This would consider full decarbonization in the entire energy sector and being entirely emission-free by 2050 within three priorities identified as being the most useful strategies for achieving net-zero: (a) Avoiding- (b) Reducing- (c) Removing emissions. This work is a collaboration of interdisciplinary scientists with the Net-Zero-2050 cluster of the Helmholtz Climate Initiative HI-CAM.

As cities are sensitive to climate change, they must be prepared for its consequences. While high-performance urban climate models are the basis for prospective planning decisions, as of today, practice-oriented models are rare. Within the framework of the Urban Climate Under ChangeUC2 research programme, the new urban climate model PALM‑4U will be developed to simulate climate and air-pollution problems in urban areas. The joint projects KliMoPrax and UseUClim deal with both the model\'s practicability and usability, and, therefore, are to be conducted together with partners from German municipalities and industry, with user requirements identified and summarised for implementation into the new model. Furthermore, case studies on current and future municipal issues will be performed using the PALM‑4U model. At the conclusion of the project, the new model\'s practicability and usability are evaluated, with the results to be summarised in an evaluation report including recommendations on further model development for both current and future everyday municipal planning.

Human-induced climate change is one of the most pressing challenges of our time. The Helmholtz Association is making essential contributions to curbing the causes of climate change and finding ways to adapt. With the Helmholtz Climate Initiative, research is concentrated on the two focal points, "reduction of net emissions" and "adaptation to climate impacts". In Net-Zero-2050, Cluster I of the Helmholtz Climate Initiative, strategies and ways to reduce carbon emissions are scientifically investigated and evaluated. Furthermore, two digital communication formats are being developed to comprehensively show the research's complex results. Firstly, the web-based National Net-Zero-2050 Atlas informs the user about different methods and technologies for CO2 reduction and possible reduction paths. Secondly, the Soil Carbon App enables actors of the agricultural sector to assess climate mitigation potentials that arise from using different land management methods. A land surface model is used to simulate future scenarios presented in the app via cloud-based, model-driven workflows.Both formats aim to support users in making decisions and developing strategies. The work on the products follows the principles of comprehensibility, transparency and appropriate information presentation. During the work on the products, we identified challenges such as:How to deal with multi-dimensional data sets and uncertainties? How to present scientific results in a user-friendly manner for varied target groups? How to guide the users of varied target groups through the communication formats? With the aid of a critical internal reflection, approaches to overcome these challenges were developed and applied. For example, the atlas introduces different complexity levels to enable users to gain understanding, despite very diverse backgrounds, prior knowledge and information needs. The app, for its part, offers two main sectors that address the users’ different demands and prior knowledge: (1) it features options to choose from the data and subsequently delivers graphical analyses, and (2) it provides respective interpretation, texts, and web links.The article presents the two dissemination products as well as the challenges and solutions from the development work.

Der durch den Menschen verursachte Klimawandel ist eine der drängendsten Herausforderungen der Gegenwart. Die Helmholtz-Gemeinschaft leistet wichtige Beiträge um die Ursachen der klimatischen Veränderung einzudämmen und Anpassungsmöglichkeiten zu finden. Mit der Helmholtz-Klima-Initiative konzentriert sich die Forschung auf die beiden Schwerpunkte „Vermeidung von Emissionen“ und „Anpassung an Klimafolgen“. In Netto-Null-2050, Cluster I der Helmholtz-Klima-Initiative, werden Strategien und Wege zur Minderung von Kohlenstoff Emissionen wissenschaftlich untersucht und bewertet. Im Clusters I werden zwei Formate entwickelt, mit denen die komplexen Ergebnisse der Forschung verständlich gezeigt werden können. Mit dem web-basierten Netto-Null-Atlas wird der Nutzer zu verschiedenen Methoden und Technologien zur CO2-Verminderung, sowie zu möglichen Reduzierungspfaden informiert. Die native Boden-Kohlenstoff-App ermöglicht Akteuren in der Landwirtschaft, Potentiale für Klimaschutz durch landwirtschaftliche Managementmethoden abzuschätzen. Mit dem Landoberflächenmodell CLM werden Zukunftsszenarien simuliert, die über cloud-basierte, modell-gesteuerte Workflows in der App dargestellt werden. Beide Formate haben das Ziel die Nutzer in Entscheidungen und bei der Entwicklung von Strategien zu unterstützen. Wichtige Ziele während der Arbeit an den Produkten sind u.a. eine gute Verständlichkeit, Transparenz und eine angemessene Informationsaufbereitung. Dabei werden beispielsweise im Atlas verschiedene Abstraktionslevel eingeführt, die den Nutzern mit sehr unterschiedlichem Vorwissen und Informationsbedarf Erkenntnisgewinne ermöglichen. Daneben ergibt sich unter dem Aspekt der Transparenz z.B. die Frage, wie in der App mit der Thematik „Unsicherheiten“ umgegangen werden kann. Der Beitrag stellt die beiden Disseminations-Produkte sowie die Herausforderungen und Lösungsansätze aus der Entwicklungsarbeit vor.

Numerical urban climate models have the potential to be commonly used tools in urban development processes in practice. With integrated modules for building and envelope simulation these complex models allow the assessment of measures in the building sector on the urban climate, e.g. for mitigation of urban heat island effects or preparing for the effects of climate change. However, the currently existing models do not fulfil the requirements that arise in the field of urban planning, as they lack for example functionality, user-friendliness, are hard to integrate in the municipalities’ technical equipment or are not freely available. The German research and development project Urban Climate under Change (2016-2019) developed and validated a new innovative urban climate model called PALM-4U. Aim of the research was to create a model that meets the requirements of users in science as well as practitioners in engineering offices and urban administrations. Therefore, technical features and operational functionalities which the model has to meet to support users in their daily work have been assessed in a first project phase. In total more than 200 requirements were collected which are summed up in the so called “User and Requirements Catalogue”. They served as the basis for testing and evaluating the model’s real-world applicability. To ensure that these complex requirements are met, the whole project follows a transdisciplinary approach integrating science (model development and data assimilation) and practice (user requirements, testing and evaluation) applying a living lab approach: Stakeholders from participating cities and companies took part in on-site workshops, introducing the model with practical use-cases. Afterwards, participants were given tasks covering different features of the model’s applications, which they tested in personal use. The model fulfils the majority of the tested requirements and the users appreciated the model’s concept and functionality. But further development is necessary to provide the practitioners a tool that is applicable in their daily work: Preparation of input data, a user-friendly graphical user-interface, enhanced interfaces to other software and planning tools, use cases that were prepared from experts as well as guidelines and tools for result assessment and interpretation were main suggestions for improvement..

Climate change is among the most important societal challenges of our time. The IPCC special report “Global Warming of 1.5 °C” (IPCC 2018) has shown that even a warming of 1.5 °C compared to pre-industrial levels will have a significant impact on society. Climate change-related risks and impacts are increasing steadily. Many of these risks and impacts will hit urban areas in particular, regardless of their size, location and economic and social circumstances. Key issues include rising temperatures (especially the urban heat island effect) and related issues such as heat stress and increased air pollution, longer dry periods in combination with rising water stress, extreme weather events such as heavy rains, river flooding, and, in coastal areas, sea-level rise. The combined effects of these events are already causing health problems and even death, as well as serious economic damage (Smith et al., 2014). It is expected that these challenges will become even more severe in the coming decades and will require additional efforts.

Cities and urban agglomerations are particularly vulnerable to the expected impacts caused by climate change. A building-resolving urban climate model as a tool for urban planning can contribute to prepare to these issues. However, currently available urban climate models do not correspond to the level of expertise of the various stakeholders in planning practice and their respective computing infrastructures. The German research program Urban Climate Under Change [UC]2 aims at the development, validation and application of a high-resolution and user-friendly urban climate model (PALM-4U) for entire cities. UseUClim, one of four joint research projects, reviews PALM-'s user-friendliness and practicality by applying the living lab approach. Living labs are platforms that gather all relevant stakeholders and, hereby, support a systematic user-science interaction. Both stakeholders from urban planning practice and model developers provide crucial input to the successful co-development of a scientifically-based and practicable urban climate model. Consequently, it is important that both contribute on an equal footing. To ensure this equality, UseUClim acts as a neutral intermediary between the two. Throughout the project it became apparent, that key success factors are clear and transparent communication flows as well as a common understanding of the project goals.