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The stability and resilience of the Earth system and human well-being are inseparably linked 1-3 , yet their interdependencies are generally under-recognized; consequently, they are often treated independently 4,5 . Here, we use modelling and literature assessment to quantify safe and just Earth system boundaries (ESBs) for climate, the biosphere, water and nutrient cycles, and aerosols at global and subglobal scales. We propose ESBs for maintaining the resilience and stability of the Earth system (safe ESBs) and minimizing exposure to significant harm to humans from Earth system change (a necessary but not sufficient condition for justice) 4 . The stricter of the safe or just boundaries sets the integrated safe and just ESB. Our findings show that justice considerations constrain the integrated ESBs more than safety considerations for climate and atmospheric aerosol loading. Seven of eight globally quantified safe and just ESBs and at least two regional safe and just ESBs in over half of global land area are already exceeded. We propose that our assessment provides a quantitative foundation for safeguarding the global commons for all people now and into the future., (© 2023. The Author(s).)

[Display omitted] • Arguments for the importance of diversity within scenario sets are developed. • The diversity of six global scenario sets is assessed. • Results show relatively good performance with regards to scenario set diversity. • There is a trade-off between covering the scenario space and redundancy. Scenario development and analysis is an important methodological approach in the assessment of global environmental change. Scenarios are used to assess climate change and its impacts on societies, economies, and ecosystems, and identify, test, and improve policy responses to manage the negative consequences of a changing environment. Evaluation of scenarios, especially global scenarios widely used in the climate research and policy community, is therefore urgently needed. While the quality and usefulness of individual scenarios have been extensively examined through a wide array of criteria, standards for systematic evaluation of scenario sets are only emerging. Scenario diversity (i.e., diversity between individual scenarios in a set) has been advanced as a measure to assess the quality of global scenario sets. In this paper, we use scenario diversity analysis, a systematic and transparent quantitative method, to examine six major global scenario sets with regards to scenario set diversity. Results show that the assessed scenario sets show relatively good performance with regards to scenario set diversity. However, the use of classic methods for building scenario architectures and the number of scenarios included in a set raise important questions about the potential trade-offs between covering a bigger space of futures possibilities and adding redundancy to the scenario set. [ABSTRACT FROM AUTHOR]

• Scenario planning for climate adaptation is increasingly popular but has not been evaluated. • We present reflections from practitioners at the European Climate Change Adaptation conference. • Participatory processes focussed locally over short (<20 year) timeframes were most effective. • Scenario planning did not sufficiently engage in decision-making processes, limiting outcomes. • Reflexive 'transient scenarios' embedded in iterative adaptation pathways were recommended. Scenario planning is a popular decision-support method that is increasingly being applied to climate change adaptation. However, evaluation of scenario planning for adaptation is lacking. In this paper we summarise a science-policy session held at the European Climate Change Adaptation Conference in May 2019, where practitioners explored the strengths and weaknesses of scenario planning for climate adaptation and identified modifications to enhance the method's utility. Eight case studies spanning three scenario planning types (problem-focused, actor-focused and reflexive-interventionist) from varied socio-cultural contexts were presented by the authors, followed by discussion amongst the 40 participants. Strengths focussed on opportunities provided by scenario planning for stakeholder participation, and raising their awareness about future risks, vulnerability and uncertainty. Participatory scenario planning was most useful for building stakeholder consensus at the local scale (e.g. communities, neighbourhoods) over shorter timeframes (e.g. 20 years). Weaknesses centred on the inability of scenarios to generate quantitative predictions and concrete adaptation solutions. This was partly attributed to practitioners' limited understanding of stakeholder politics and power dynamics, and the resulting lack of integration of scenario exercises within decision-making processes. Scenarios were also limited by being static, and participatory processes were resource-intensive. Suggested modifications were to develop iterative scenario planning embedded within decision-making cycles. Such 'transient scenarios' could absorb system feedbacks and updated information to prioritise adaptation responses, thus actively contributing to ongoing adaptation pathways. Applying monitoring, evaluating and learning would enable reflexive refinement of the method, adapting it to become an agile approach applicable to varied socio-cultural and political contexts. [ABSTRACT FROM AUTHOR]

The complex interactions of drivers represented in scenarios and climate change impacts across scales have led to the development of multiscale scenarios. Since the recent development of global shared socioeconomic pathways (SSPs), which have started being downscaled to lower scales, the potential of scenarios to be relevant for decision making and facilitate appreciation and inclusion of different perspectives has been increasing, compared with a single-scale global scenario set. However, in practice, quantitative downscaling of global scenarios results in narratives that are compressed from the global level to fit the local context to enhance consistency between global and local scales. We brought forward the concept of scenario archetypes to analyze multiscale SSP scenario narratives and highlight important diverging assumptions within the same archetype. Our methodology applied scenario archetypes both as typologies, to allocate specific cases of scenarios into existing scenario archetypes, and building blocks, conceptualized with worldviews from cultural theory. Although global SSPs generally match existing archetypes and tend to be well defined, the socially unequal SSPs at subglobal scales are more nuanced, and dominant worldviews are much less straighforward to interpret than in global scenarios. The closest match was the great transition--sustainability (SSP1) archetype, whereas the most divergent was the market forces--fossil fuel development (SSP5) archetype. Overall, our results highlight the need to improve uptake of bottom-up approaches in global scenarios to improve appreciation of different perspectives as sought after in multiscale scenarios. [ABSTRACT FROM AUTHOR]

The Paris Agreement established the 1.5 and 2 °C targets based on the recognition "that this would significantly reduce the risks and impacts of climate change". We tested this assertion by comparing impacts at the regional scale between low-end (< 2 °C; RCP2.6) and high-end (> 4 °C; RCP8.5) climate change scenarios accounting for interactions across six sectors (agriculture, forestry, biodiversity, water, coasts and urban) using an integrated assessment model. Results show that there are only minor differences in most impact indicators for the 2020s time slice, but impacts are considerably greater under high-end than low-end climate change in the 2050s and 2080s. For example, for the 2080s, mitigation consistent with the Paris Agreement would reduce aggregate Europe-wide impacts on the area of intensive agriculture by 21% (on average across climate models), on the area of managed forests by 34%, on water stress by 14%, on people flooded by 10% and on biodiversity vulnerability by 16%. Including socio-economic scenarios (SSPs 1, 3, 4, 5) results in considerably greater variation in the magnitude, range and direction of change of the majority of impact indicators than climate change alone. In particular, socio-economic factors much more strongly drive changes in land use and food production than changes in climate, sometimes overriding the differences due to low-end and high-end climate change. Such impacts pose significant challenges for adaptation and highlight the importance of searching for synergies between adaptation and mitigation and linking them to sustainable development goals. [ABSTRACT FROM AUTHOR]

Uncertainties in our understanding of current and future climate change projections, impacts and vulnerabilities are structured by scientists using scenarios, which are generally in qualitative (narrative) and quantitative (numerical) forms. Although conceptually strong, qualitative and quantitative scenarios have limited complementarity due to the lack of a fundamental bridge between two different concepts of uncertainty: linguistic and epistemic. Epistemic uncertainty is represented by the range of scenarios and linguistic variables within them, while linguistic uncertainty is represented by the translation of those linguistic variables via the fuzzy set approach. Both are therefore incorporated in the models that utilise the final quantifications. The application of this method is demonstrated in a stakeholder-led development of socioeconomic scenarios. The socioeconomic scenarios include several vague elements due to heterogeneous linguistic interpretations of future change on the part of stakeholders. We apply the so-called 'Centre of Gravity' (CoG) operator to defuzzify the quantifications of linguistic values provided by stakeholders. The results suggest that, in these cases, uniform distributions provide a close fit to the membership functions derived from ranges of values provided by stakeholders. As a result, the 90 or 95% intervals of the probability density functions are similar to the 0.1 or 0.05 degrees of membership of the linguistic values of linguistic variables. By bridging different uncertainty concepts (linguistic and epistemic uncertainties), this study offers a substantial step towards linking qualitative and quantitative scenarios. [ABSTRACT FROM AUTHOR]

To provide fundamental decision support information for climate risk assessment in Hungary, an urban spatial development model of land cover change and population age structure dynamics was developed and applied to local integrated scenarios of climate change and stakeholder-derived socio-economic change. The four integrated scenarios for Hungary produced contrasting projections for urban patterns to 2100, but peri-urbanisation around Budapest was estimated to occur under all scenarios, together with a decline in working age population in the centres of the capital and major towns. This suggests that future urban planning needs to take into consideration the potential for underutilised urban infrastructure in the centre of the capital and pressures for social service provisioning in its outskirt. The integrated scenarios and model developed can be used in future studies to test the effectiveness of inter-sectoral policy responses in adapting urban planning to multiple climate and socio-economic challenges. [ABSTRACT FROM AUTHOR]

Abstract The majority of assessments of future heat-related health risk are based on projections of heat hazards superimposed solely on current socioeconomic conditions, thus neglecting the potential contribution of drivers of heat stress risk other than climate change. Partly to address this drawback, the climate change research community has developed a new scenario framework, made up of distinct sets of climate and socioeconomic scenarios. The few assessments of future heat-related health risk that have employed this new framework have focused on changes in population exposure but have often not accounted for future populations' vulnerability. In this paper, we combine European Shared Socioeconomic Pathways with Representative Concentration Pathways to provide spatially explicit European projections of heat-related health risk that account for multiple changes in both socioeconomic and climatic conditions. In doing so, we also address the challenge of accounting for projections of determinants of vulnerability under varying levels of socioeconomic development. Results reveal that the proportion of the European population at very high risk of heat stress will show a steady increase – from 0.4% currently to 20.3%, 32.6%, or 48.4% in 2050 depending on the scenario combination – unless substantial political changes occur rapidly and steadily shift the current socioeconomic development pathway towards sustainability. Ambitious mitigation policies associated with rapid technological progress to enhance human capital could also moderate future heat-related health challenges. Such challenges are unevenly spread across Europe, with the Mediterranean region and Scandinavia being respectively the most and the least impacted regions. Future heat-related health challenges are substantially influenced by varying levels of socioeconomic development, primarily through changes in vulnerability – changes in population exposure being only of secondary importance. The former may even have a more significant impact on future heat stress risk than climate change, particularly in the British Isles and in the Iberian Peninsula. Thus, there is an undeniable necessity to consider the future state of vulnerability – and its uncertainties under varying socioeconomic scenarios – when assessing future heat-related health challenges and designing health adaptation strategies. [ABSTRACT FROM AUTHOR]