We investigate the probabilities of triggering climate tipping points under five Shared Socioeconomic Pathways (SSPs) and how they are altered by including the additional carbon emissions that could arise from tipping points within the Earth's carbon cycle. The crossing of a climate tipping point at a threshold level of global mean surface temperature (threshold temperature) would commit the affected subsystem of the Earth to abrupt and largely irreversible changes with negative impacts on human well-being. However, it remains unclear which tipping points would be triggered under the different SSPs due to uncertainties in the climate sensitivity to anthropogenic greenhouse gas emissions, the threshold temperatures and timescales of climate tipping points, and the response of tipping points within the Earth's carbon cycle to global warming. We include those uncertainties in our analysis to derive probabilities of triggering for 16 previously identified climate tipping points within the Earth system. To conduct our analysis, we use the reduced complexity climate model FaIR (Finite amplitude Impulse Response) which is coupled to a conceptual model of the tipping processes within the Amazon rainforest and permafrost, which are the two major tipping points within the Earth's carbon cycle. Uncertainties are propagated by employing a Monte Carlo approach for the construction of large model ensembles. We find that carbon tipping points increase the risk for high-temperature pathways, but on average their warming effect remains small, with its median staying 1 order of magnitude lower than the median anthropogenic warming for all SSPs. Therefore, they have low potential to increase the probability of triggering other tipping points. The maximum triggering probability increase from carbon tipping points among all SSPs occurs under SSP2-4.5, with a 3 percentage point increase averaged over all tipping points. The warming trajectory expected from current policies compares best to SSP2-4.5, which we find to be unsafe with regard to triggering climate tipping points. Our most conservative estimate of triggering probabilities averaged over all tipping points is 62 % under SSP2-4.5, and nine tipping points have a more than 50 % probability of getting triggered. Under SSP1-2.6 and SSP1-1.9, the risk of triggering climate tipping points is reduced significantly compared to SSP2-4.5; however, it also remains less constrained since the behaviour of climate tipping points in the case of a temperature overshoot is still highly uncertain.

Cost risk analysis (CRA) continues emerging as a noticed decision-analytic framework within climate economics. It introduces the construct of a climate target into an expected utility-based structure and thereby avoids the dynamic inconsistencies of cost effectiveness analyses under uncertainty (i.e. of chance constraint programming). We present three innovations on CRA. They might complete the set of assumptions necessary for CRA as a ‘bridging technology’, before cost benefit analyses can more unequivocally be applied, in view of a currently potentially underdetermined global warming impact function. (i) We modify CRA such that risks below the climate target are taken care of, while still reflecting the specific choice of the target’s numerical value. (ii) This enables us to provide a consistent mechanism of including matured impact modelling components into a further extended CRA, such in the long run, CRA might converge towards cost benefit analysis. (iii) We provide an updating mechanism for including revisions of the science base of the climate target. We conclude from a thereby extended CRA, that a potential lowering of the temperature maxima the 2° target was based on, as suggested by latest paleo records (Westerhold et al. 2020), would not significantly change the temperature target. Hence, the joint science and economics base behind the Paris agreement could be argued to be still intact.

Abstract. We investigate the probabilities of triggering climate tipping points under various shared socioeconomic pathways (SSPs), and how they are altered by including the additional carbon emissions that could arise from tipping points within the Earth's carbon cycle. Crossing of a climate tipping point at a threshold level of global mean surface temperature (threshold temperature), would commit the affected subsystem of the Earth to abrupt and largely irreversible changes with negative impacts on human well-being. However, it remains unclear which tipping points would be triggered under the different SSPs, due to uncertainties in the climate sensitivity to anthropogenic greenhouse gas emissions, the threshold temperatures of climate tipping points, and the response of tipping points within the Earth's carbon cycle to global warming. We include those uncertainties in our analysis to derive probabilities of triggering for 16 previously-identified climate tipping points within the Earth system. To conduct our analysis, we use the intermediate complexity climate model FaIR which is coupled to a conceptual model of the tipping processes within the Amazon rainforest and permafrost, which are the two major tipping elements within the Earth's carbon cycle. Uncertainties are propagated by employing a Monte Carlo approach for the construction of large model ensembles. We find that intermediate emission scenarios like SSP2-4.5 are highly unsafe with regard to triggering climate tipping points, with an average probability of triggering until the year 2500 of 65 %. Furthermore, the highest long-term temperature increase among all SSPs caused by carbon emissions from the Amazon and permafrost becomes possible under this scenario with 0.16 °C (0.03–0.91 °C) in 2500, which increases the average probability of triggering tipping points by 3.3 percent points (pp). This is due to the fact that maximum carbon emissions from tipping of the Amazon and permafrost become possible under this scenario, and they cause most warming when cumulative anthropogenic emissions are lower due to the saturating response of radiative forcing to increasing greenhouse gas concentrations. The risk of triggering climate tipping points is reduced significantly under SSP1-2.6 and even more so under SSP1-1.9, with average probabilities of triggering of 38 % and 28 % respectively, which are increased by 2.3 pp and 1.1 pp due to carbon emissions from the Amazon and permafrost.

Zusammenfassung Das Kapitel nimmt Fragen des Umgangs mit Unsicherheiten und Bandbreiten der Klimafolgenprojektionen in den Blick, die vor allem angesichts hoher Schadens-, aber auch Vermeidungskosten eine große Herausforderung für die gesellschaftliche Meinungsfindung und das Handeln von Entscheidungsträgern und -trägerinnen darstellen. Wichtige gesellschaftliche Entscheidungen betreffen üblicherweise Handlungen, deren Ziel es ist, Veränderungen an komplexen Systemen vorzunehmen, um das System noch besser auf die Herausforderungen der Zukunft auszurichten. In der Regel lassen sich jedoch die Folgen solcher Entscheidungen nicht genau vorhersagen. Bei den meisten Entscheidungen, die komplexe Umweltsysteme betreffen, spielt Unsicherheit deshalb eine entscheidende Rolle. Das Kapitel stellt verschiedene Methoden vor, unter Unsicherheit zu entscheiden, und plädiert für Risikominderung durch breite und offene Dialogprozesse.

Werden gesicherte Erkenntnisse sowie Unsicherheiten zum Thema Klimawandel in Schulbüchern adäquat kommuniziert und bilden sie den aktuellen Diskurs ab? Im Rahmen der Studie werden Klimawandelkapitel in Schulbüchern analysiert und Vorschläge zu Gestaltung und Struktur von Schulbüchern entwickelt.Uncertainties are transparently presented and communicated in the Assessment Reports from the Intergovernmental Panel on Climate Change (IPCC). This study examines the extent to which textbooks reflect this discourse, in the sense of the Nature of Science . The aim is to identify the representation of both virtually certain findings and uncertainties on the topic of climate change in textbooks. For this purpose, climate change chapters in lower and upper secondary school geography textbooks were analyzed, using qualitative content analysis with deductive-inductive category formation according to Philipp Mayring. It was discovered that uncertainties are represented predominantly by linguistic means (e. g., subjunctive), while technical terminology (such as that of the IPCC) is hardly used. Similar results can be seen for virtually certain findings.

The carbon budget concept (TCRE; Transient Climate Response to cumulative carbon Emissions) emerged as a major concept in climate research since the late 2000s. Due to its simplicity, it is intensively utilized in the international policy arena. It is based on the claim that one can derive the global mean temperature increase solely from the knowledge of historical cumulative emissions by observing the linear relationship between the two, regardless of the emission pathway that preceded ('pathway independence'). Here, we ask for the maximally possible deviations from the TCRE ideal across emission scenario space. While there has been an extensive focus on quantifying the carbon budget using highly complex climate models, there seems to be a lesser focus on the pathway independence and possibly related deviations from the budget. Furthermore, few analytical examinations have been presented, for highly stylized settings only. This study contributes to filling that gap, utilizing the energy balance model FAIR. FAIR incorporates climate feedbacks and correctly emulates the temperature response to an emission pulse. If the carbon budget approach was perfectly valid, the temperature response to an emitted unit of carbon should be a perfect step function. The actual temperature evolution following the emission pulse is reinterpreted as a Green's function and as such, utilized to calculate the total temperature increase at any given point. The novelty in this work is that the emission pathway is not assumed, but generated by maximizing (minimizing) the temperature output. With the boundary conditions being the fixed total cumulative emissions and the maximal allowed mitigation efforts, two associated pathways are generated with the temperature increase in a given year acting as an objective value. The deviation from the budget is then extracted as a temperature difference between the upper and the lower bound of the optimization process. The results show that the absolute value of the deviation is less than the standard deviation of climate variability, confirming the fundamentals of the carbon budget approach. We also present an analytical upper bound of the deviation from path independence. The result shows that the deviation is a function of the allowed maximum emission slope. The advantage of this method is that it can utilize the impulse response properties already published for highly complex models. The current limitation of the presented approach lies in the assumption that the pulse response is assumed constant even though the climate changes. The implications of a changing pulse remain to be explored. We see our work as a twofold contribution: (i) to predict maximally possible TCRE deviations from already published impulse response experiments, and (ii), to generate analytic understanding for the driving variables.

23 24 This article investigates the genesis and role of the 2° target in international climate policy. 25 We identify a dual role played by temperature targets: (i) a social planner’s option of decision26 making under uncertainty that draws on the precautionary principle, and (ii) a policy instru27 ment to help the social planners’ position become reality. Accordingly, the recent debate over 28 the 2° target as found in the literature is actually a mutual misunderstanding: while the op29 ponents mainly focus on the policy instrument function, the proponents focus on the social 30

Human-driven changes to many features of the Earth system have become so ubiquitous and significant in magnitude that a new era for the planet—the ‘Anthropocene’—has been proposed (Crutzen and Stoermer Crutzen and Stoermer, Glob. Change Newsl. 41:12–13, 2001; Clark et al., Science 293:283–287, 2001). Many of these changes are large in magnitude at the planetary-scale, sometimes even exceeding natural flows in major aspects of biogeochemical cycling. In addition, anthropogenic changes invariably occur at rates that are much larger than those of natural variability, often by an order of magnitude or more.

Side effects of “solar-radiation management” (SRM) might be perceived as an important metric when society decides on implementing SRM as a climate policy option to alleviate anthropogenic global warming. We generalize cost-risk analysis that originally trades off expected welfare loss from climate policy costs and risks from transgressing climate targets to also include risks from applying SRM. In a first step of acknowledging SRM risks, we represent global precipitation mismatch as a prominent side effect of SRM under long-tailed probabilistic knowledge about climate sensitivity. We maximize a social welfare function for the following three scenarios, considering alternative relative weights of risks: temperature-risk-only, precipitation-risk-only, and equally-weighted both-risks. Our analysis shows that in the temperature-risk-only scenario, perfect compliance with the 2 °C-temperature target is attained for all numerically represented climate sensitivities, a unique feature of SRM, but the 2 °C-compatible precipitation corridor is violated. The precipitation-risk-only scenario exhibits an approximate mirror-image of this result. In addition, under the both-risks scenario, almost 90% and perfect compliance can be achieved for the temperature and precipitation targets, respectively. Moreover, in a mitigation-only analysis, the welfare loss from mitigation cost plus residual climate risks, compared to the no-climate-policy option, is approximately 4.3% (in terms of balanced growth equivalent), while being reduced more than 90% under a joint-mitigation-SRM analysis.

The last assessment report by the IPCC (AR5, WGIII, 2014) shows that the 2° target is compatible with continued economic growth and that the globally averaged short-term loss of consumption to finance an energy transition is 1%. Furthermore, initial work on the 1.5°C target shows that the economic expenditures are significantly higher, but are likely to be of the same order of magnitude.This complex of results on the costs of an energy transition was generated with the help of integrated energy and climate economic models, as a result of which hundreds of energy scenarios were evaluated. Most of these scenarios were generated without an explicit representation of uncertainty about essential input parameters such as the learning rates of individual energy technologies or climate sensitivity.This article examines the mechanisms through which explicit consideration of uncertainty has changed or could change policy recommendations. In particular, it is pointed out that the economic paradigm implicitly used in the above, asking for cost-minimal solutions under climate targets, needs to be generalized if one does not want to turn a blind eye to the possibility of future learning about uncertain parameters in today's investment planning.In this context, a separate approach (Held, 2019) is presented and discussed for which class of issues the energy scenarios summarized in the most recent IPCC report are robust under uncertainty and for which qualitatively different policy recommendations would result.ReferenceHeld, Cost Risk Analysis – Dynamically Consistent Decision-Making under Climate Targets, Environmental and Resource Economics, 72 (1), 247-261, DOI 10.1007/s10640-018-0288-y, http://link.springer.com/article/10.1007/s10640-018-0288-y (2019).

So far, scientific analyses have mainly focused on the pros and cons of solar geoengineering or solar radiation management (SRM) as a climate policy option in mere isolation. Here, we put SRM into the context of mitigation by a strictly temperature-target-based approach. As the main innovation, we present a scheme that extends the applicability regime of temperature targets from mitigation-only to SRM-mitigation analyses. We explicitly account for one major category of side effects of SRM while minimizing economic costs for complying with the 2 ∘C temperature target. To do so, we suggest regional precipitation guardrails that are compatible with the 2 ∘C target. Our analysis shows that the value system enshrined in the 2 ∘C target leads to an elimination of most of the SRM from the policy scenario if a transgression of environmental targets is confined to 1/10 of the standard deviation of natural variability. Correspondingly, about half to nearly two-thirds of mitigation costs could be saved, depending on the relaxation of the precipitation criterion. In addition, assuming a climate sensitivity of 3 ∘C or more, in case of a delayed enough policy, a modest admixture of SRM to the policy portfolio might provide debatable trade-offs compared to a mitigation-only future. Also, in our analysis which abstains from a utilization of negative emissions technologies, for climate sensitivities higher than 4 ∘C, SRM will be an unavoidable policy tool to comply with the temperature targets. The economic numbers we present must be interpreted as upper bounds in the sense that cost-lowering effects by including negative emissions technologies are absent. However, with an additional climate policy option such as carbon dioxide removal present, the role of SRM would be even more limited. Hence, our results, pointing to a limited role of SRM in a situation of immediate implementation of a climate policy, are robust in that regard. This limitation would be enhanced if further side effects of SRM are taken into account in a target-based integrated assessment of SRM.

Sustainability in the provision of ecosystem services requires understanding of the vulnerability of socialecological systems (SES) to tipping points (TPs). Assessing SES vulnerability to abrupt ecosystem state changes remains challenging, however, because frameworks do not operationally link ecological, socio-economic and cultural elements of the SES. We conducted a targeted literature review on empirical assessments of SES and TPs in the marine realm and their use in ecosystem-based management. Our results revealed a plurality of terminologies, definitions and concepts that hampers practical operationalisation of these concepts. Furthermore, we found a striking lack of socio-cultural aspects in SES vulnerability assessments, possibly because of a lack of involvement of stakeholders and interest groups. We propose guiding principles for assessing vulnerability to TPs that build on participative approaches and prioritise the connectivity between SES components by accounting for component linkages, cascading effects and feedback processes. (C) 2019 Elsevier B.V. All rights reserved.

Even if surface warming could be kept below 2.0°C or 1.5°C by 2100, global sea-level rise will occur for several centuries or even millennia. One possible interpretation of a successful climate policy for the next few decades could be that it should avoid global-warming induced impacts on climate, ecosystems and human societies not only within this century, but also for the next centuries and beyond. Here, we perform a proof-of-concept study to introduce a constraint on SLR as a new climate target and compare the economic impact to that of a corresponding temperature target. In the 21st yearly session of the Conference of the Parties in Paris in 2015, SLR threats to the Small Island Developing States (SIDS) prompted a commitment to strive for a lower global temperature target goal of limiting surface warming below 1.5°C. However, an SLR target more directly relates to their existential threats. We here substantially augmented the climate model of the optimizing climate-energy-economy model MIND (Model of Investment and Technological Development) from an impulse-response model to a three-layer ocean model with much-improved representation of ocean heat uptake. We introduce a global total SLR model with four components, one due to ocean thermal expansion, one due to Greenland ice-sheet melting, one due to Antarctic ice-sheet melting, and one due to mountain glaciers and ice cap melting. The newly developed integrated-assessment framework has enabled us to investigate, for the first time, a sea-level rise climate target. Our results emphasize a key effect of carbon emissions pathways on the future SLR after the 21st century. The shape of carbon emissions pathways will strongly influence future SLR after the 21st century and generally affect SIDS over centuries. To reduce SLR-induced impacts on SIDS, a target is required that not only keeps surface warming below a certain level but also reduces surface warming substantially thereafter. We find that a global SLR target will provide a more sustainable and a lower-cost solution to limit both short-term and long-term climate changes for stakeholders who primarily care about SLR among all global warming impact categories compared to a temperature target with the same SLR by 2200. We find that the SLR target can provide a temperature overshoot profile through a physical constraint rather than arbitrarily defining an overshoot range of temperature as acceptable. Temperature targets with a limited overshoot have been invoked to make the 2.0° and 1.5°C targets feasible in the context of real-world United Nations climate policy; however, rational constraints on the temperature overshoot have been unclear. SLR targets can be viewed as a reinterpretation of the 2.0° and 1.5°C targets and can provide a rational justification of a certain temperature overshoot for stakeholders who primarily care about SLR. Our present framework with reinterpretation of the widely agreed temperature targets can, in principle, be transferred from SLR targets to impact-related climate targets and can be used to identify a more sustainable path toward meeting the Paris Agreement.

Abstract. So far scientific analyses have mainly focused on the pros and cons of solar geoengineering or solar radiation management (SRM) as a climate policy option in mere isolation. Here we put SRM into the context of mitigation by a strictly temperature-target based approach. As a main innovation, we present a scheme by which the applicability regime of temperature targets is extended from mitigation-only to SRM-mitigation analyses. Hereby we explicitly account for a risk-risk comparison of SRM and global warming, while minimizing economic costs for complying with the 2 °C temperature target. To do so, we suggest precipitation guardrails that are compatible with the 2 °C target. Our analysis shows that the value system enshrined in the 2 °C target would be almost prohibitive for SRM, while still about half to nearly two-third of mitigation costs could be saved, depending on the choice of extra room for precipitation. In addition, assuming a climate sensitivity of 3 °C or more, in case of a delayed enough policy, a modest admixture of SRM to the policy portfolio might provide debatable trade-offs compared to a mitigation-only future. In addition, in our analysis for climate sensitivities higher than 4 °C, SRM will be an unavoidable policy tool to comply with the temperature targets.

The global temperature targets of limiting surface warming to below 2.0°C or even to 1.5°C have been widely accepted through the Paris Agreement. However, limiting surface warming has previously been proven insufficient to control sea level rise (SLR). Here, we explore a sea level target that is closer to coastal planning and associated adaptation measures than a temperature target. We find that a sea level target provides an optimal temperature overshoot profile through a physical constraint of SLR. The allowable temperature overshoot leads to lower mitigation costs and more effective long-term sea level stabilization compared to a temperature target leading to the same SLR by 2200. With the same mitigation cost as the temperature target, a SLR target could bring surface warming back to the targeted temperatures within this century, lead to a reduction of surface warming of the next century, and reduce and slow down SLR in the centuries thereafter.

In the following, we test the validity of a one-box climate model as an emulator for atmosphere–ocean general circulation models (AOGCMs). The one-box climate model is currently employed in the integrated assessment models FUND, MIND, and PAGE, widely used in policy making. Our findings are twofold. Firstly, when directly prescribing AOGCMs' respective equilibrium climate sensitivities (ECSs) and transient climate responses (TCRs) to the one-box model, global mean temperature (GMT) projections are generically too high by 0.5 K at peak temperature for peak-and-decline forcing scenarios, resulting in a maximum global warming of approximately 2 K. Accordingly, corresponding integrated assessment studies might tend to overestimate mitigation needs and costs. We semi-analytically explain this discrepancy as resulting from the information loss resulting from the reduction of complexity. Secondly, the one-box model offers a good emulator of these AOGCMs (accurate to within 0.1 K for Representative Concentration Pathways, RCPs, namely RCP2.6, RCP4.5, and RCP6.0), provided the AOGCM's ECS and TCR values are universally mapped onto effective one-box counterparts and a certain time horizon (on the order of the time to peak radiative forcing) is not exceeded. Results that are based on the one-box model and have already been published are still just as informative as intended by their respective authors; however, they should be reinterpreted as being influenced by a larger climate response to forcing than intended.