Metrology to establish an SI-traceable climate observing system: September 2020 to August 2023 Earth’s climate is changing. The potential for societal catastrophe is unprecedented. Remote sensing from space is the major means of obtaining data for climate change research and guidance for policy-makers on appropriate mitigation and adaptation strategies. However, changes in relevant measurands are only at a level of a few tenths of a percent per decade, meaning the required accuracies are only realisable in metrology laboratories. Building on previous EMRP projects MetEOC and MetEOC-2 and EMPIR project MetEOC-3, this project is develop ing metrological tools and frameworks capable of underpinning a globalised climate observing system. The objectives of Meteoc-4 are to: Develop metrological infrastructures and methods, traceable to the SI, for new highly accurate, cost-effective, sensors for calibrating and validating instruments used for space-based climate observations. These new sensors will be suitable for pre- and in-flight measurements, including for the current mission studies TRUTHS and FORUM. Develop SI traceable measurement methods with associated uncertainties for bio-geophysical parameters, that account for scene-specific characteristics including provision of means to optimally parameterise, validate and assess the uncertainties of retrieval algorithms. These methods will consider harmonisation of sampling methods including optical and synthetic-aperture radar (SAR) based techniques. Develop satellite derived SI traceable measurement methods (including uncertainty assessment, associated validation and interoperability) for greenhouse gases emissions and natural carbon sinks, including robust monitoring of implemented policies intended to reduce anthropogenic carbon emissions. Develop instrumentation and standards for traceable climate quality measurements, including temperature of the Mesopause and thermal infrared sky radiance, from surface-based networks such as those operated under the WMO and UN. For detailed objectives and expected impacts see: Publishable Summary Metrology to establish an SI-traceable climate observing system (19ENV07) Publications Trim S.A., (2021). Spectroradiometer spectral calibration, ISRF shapes, and related uncertainties. The Optical society Vol 60, iss 18, Article Number 5405. https://doi.org/10.1364/AO.425676 Partners Internally Funded Partners: NPL, United Kingdom Aalto University, Finland CMI, Czech Republic PTB, Germany SFI Davos, Switzerland Externally Funded Partners: FZ-Juelich, Germany JRC, European Commission NLS, Finland Rayference, Belgium University of Reading, United Kingdom University of Surrey, United Kingdom University of Zurich Switzerland Unfunded Partners: Bergische Universität Wuppertal, Germany Hukseflux, Netherlands Karlsruher Institut für Technologie (KIT), Germany Universiteit Gent, Belgium Funding EMPIR projects are co-funded by the European Union’s Horizon 2020 research and innovation programme and the EMPIR Participating States. EMRP joint research projects are part of EURAMET’s European Metrology Research Programme. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.