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Sea level rise (SLR) is a long-lasting consequence of climate change because global anthropogenic warming takes centuries to millennia to equilibrate for the deep ocean and ice sheets. SLR projections based on climate models support policy analysis, risk assessment and adaptation planning today, despite their large uncertainties. The central range of the SLR distribution is estimated by process-based models. However, risk-averse practitioners often require information about plausible future conditions that lie in the tails of the SLR distribution, which are poorly defined by existing models. Here, a community effort combining scientists and practitioners builds on a framework of discussing physical evidence to quantify high-end global SLR for practitioners. The approach is complementary to the IPCC AR6 report and provides further physically plausible high-end scenarios. High-end estimates for the different SLR components are developed for two climate scenarios at two timescales. For global warming of +2°C in 2100 (RCP2.6/SSP1-2.6) relative to pre-industrial values our high-end global SLR estimates are up to 0.9 m in 2100 and 2.5 m in 2300. Similarly, for a (RCP8.5/SSP5-8.5), we estimate up to 1.6 m in 2100 and up to 10.4 m in 2300. The large and growing differences between the scenarios beyond 2100 emphasize the long-term benefits of mitigation. However, even a modest 2°C warming may cause multi-meter SLR on centennial time scales with profound consequences for coastal areas. Earlier high-end assessments focused on instability mechanisms in Antarctica, while here we emphasize the importance of the timing of ice shelf collapse around Antarctica. This is highly uncertain due to low understanding of the driving processes. Hence both process understanding and emission scenario control high-end SLR. publishedVersion

Decadal climate predictions fill the gap between seasonal climate forecasts and long-term climate projections; they support private and public sectors in developing sustainable strategies for the future. MiKlip (German for mid-term climate prediction) was a national research project funded by the Federal Ministry of Education and Research to investigate the scientific basis for climate predictability and to develop a climate prediction system for a time horizon of one year up to ten years. Here we present how during the project a scientific concept was transformed into the prototype of an operational forecasting system, which scientific questions were addressed, and which climate information was provided to public and private users.

Dekadische Klimavorhersagen füllen die Lücke zwischen saisonalen Klimavorhersagen und langzeiti'gen Klimaprojektionen und ermöglichen damit der Wirtschaft und dem öffentlichen Sektor die Entwick-lung zukunftsfähiger Handlungsstrategien. MiKlip (Mittelfristige Klimaprognosen) war ein nationales,vom Bundesministerium für Bildung und Forschung gefördertes Projekt, das die wissenschaftlichenGrundlagen der Klimavorhersagbarkeit erforschte und ein Klimavorhersagesystem mit einem Zeitho'rizont von einem Jahr bis zu zehn Jahren entwickelte. Hier stellen wir dar, wie während des Projektesaus einem wissenschaftlichen Konzept der Prototyp eines operationellen Vorhersagesystems entstand,welche wissenschaftlichen Fragen untersucht und welche Klimainformationen für private wie öffentlicheNutzer bereitgestellt wurden. Decadal climate predictions fill the gap between seasonal climate forecasts and long-term climate projec-tions; they support private and public sectors in developing sustainable strategies for the future. MiKlip(German for mid-term climate prediction) was a national research project funded by the Federal Ministryof Education and Research to investigate the scientific basis for climate predictability and to develop aclimate prediction system for a time horizon of one year up to ten years. Here we present how duringthe project a scientiflc concept was transformed into the prototype of an operational forecasting system,which scientific questions were addressed, and which climate information was provided to public and private users.

International audience; SKIM is an ESA Earth Explorer-9 candidate mission designed to measure directly and simultaneously, for the first time, directional ocean surface current vector (OSCV) and ocean wave spectra. SKIM builds on satellite altimetry, including a nadir altimeter beam, and the first ocean wave spectrometer SWIM on CFOSAT. Using rotating beams across a 330 km swath, SKIM will explore beyond the 200 km - 15 days resolution of ocean currents that can be diagnosed today from sea level at mid latitudes. In particular SKIM will fill two important blind spots: in the tropics and in the marginal ice zone, and expand the effective space and time resolution of the altimeter constellation by a factor 2 or more. The novel direct measurement of surface currents in the top two meters will produce the first maps of the equatorial upwellings that are critical for understanding and forecasting the heat budget at the equator with far-reaching weather and climate consequences, for example on the African monsoon. OSCV maps will also allow the first monitoring of the highly dynamic currents at the ice edge. Adding this new and fundamental variable to Earth Observation capability together with high fidelity measurements of wave spectra will allow scientists to address a wide range of questions, including: How OSCV and waves influence upper ocean mixing and large scale circulation? How do OSCV and waves influence the dynamics of the ice edge in the Arctic and Antarctic?What are the roles of eddies, wind-driven flows and waves in setting the surface concentration of marine litter and shaping marine ecosystems?This presentation will explain how SKIM measurements will be used to address these scientific challenges using examples from mission and instrument simulator outputs.

An approach to analyze high‐end sea level rise is presented to provide a conceptual framework for high‐end estimates as a function of time scale, thereby linking robust sea level science with stakeholder needs. Instead of developing and agreeing on a set of high‐end sea level rise numbers or using an expert consultation, our effort is focused on the essential task of providing a generic conceptual framework for such discussions and demonstrating its feasibility to address this problem. In contrast, information about high‐end sea level rise projections was derived previously either from a likely range emerging from the highest view of emissions in the Intergovernmental Panel on Climate Change assessment (currently the Representative Concentration Pathway 8.5 scenario) or from independent ad hoc studies and expert solicitations. Ideally, users need high‐end sea level information representing the upper tail of a single joint sea level frequency distribution, which considers all plausible yet unknown emission scenarios as well as involved physical mechanisms and natural variability of sea level, but this is not possible. In the absence of such information we propose a framework that would infer the required information from explicit conditional statements (lines of evidence) in combination with upper (plausible) physical bounds. This approach acknowledges the growing uncertainty in respective estimates with increasing time scale. It also allows consideration of the various levels of risk aversion of the diverse stakeholders who make coastal policy and adaptation decisions, while maintaining scientific rigor. Ministry of Education (MOE) Published version This paper is a contribution to the Grand Challenge of Regional Sea level change and coastal impacts of the World Climate Research Programme. The content emerged from a workshop on high‐end sea level scenarios and held in Hamburg during September 2017 with partial support of the DFG‐funded Excellence Cluster CliSAP of the University Hamburg and of the DFG SPP 1889 on Sea Level Change and Society. R. W. acknowledges the ALW‐NPP program of NWO for financial support. J. C. was partially supported by the Centre for Southern Hemisphere Oceans Research, a joint research center between QNLM and CSIRO. B. P. H. acknowledges the funding from Singapore Ministry of Education Academic Research Fund Tier 2 MOE218‐T2‐1‐030. G. L. C. acknowledges ERA4CS support (grant 690462).

C-band microwave sea surface radar backscatter observations from the FINO-2 tower in the western Baltic are analyzed with respect to their relevance for air-sea CO2 transfer velocity parameterizations. The scatterometer measurements observed from a height of 25 m above the sea surface using a multifrequency scatterometer instrument of the University of Hamburg were obtained quasi-simultaneously with eddy covariance CO2 flux measurements. Both data sets are merged here to derive a gas transfer velocity parameterization based on radar backscatter measurements. At the location of the FINO-2 tower, the resulting time-averaged gas transfer velocity amounts to 26.95 cm/hr. In combination with ΔPCO2 measurements available from the vicinity of the FINO-2 platform, a time-mean CO2 flux of 0.23 μmol·m−2·s−1 into the Baltic was estimated. Applied to monthly mean satellite-based C-band ASCAT scatterometer data, the newly derived gas transfer velocity parameterization provides estimates of seasonal and annual mean global maps of air-sea transfer velocities. The new results agree in their general pattern with previous estimates using wind speed parameterization. However, the backscatter-based transfer velocities appear smaller at higher latitudes. Globally averaged air-sea CO2 fluxes would thereby be reduced by 20%. To what extent this is a robust result, or if it depends on the fact that the training data set did not represent conditions, has to be investigated in the future.