Establishing SI in space for climate action

Abstract

The Earth is unequivocally warming with consensus mankind is the major cause. The potential for societal catastrophe is second to none!. The ‘Paris Agreement’ (2015) seeks to contain the rise in mean Earth temperature to <2°C above pre-industrial levels with a target of <1.5°C and led to the establishment of ‘NetZero’ emissions targets for the 2050s. Even if NetZero goals are met, the world still faces significant consequences from embedded climate change. There is thus an urgency to establish a robust integrated (satellite/local) global climate observing system to monitor the Essential Climate Variables (ECV’s) of GCOS, especially the anthropogenic forcing agents e.g. Green House Gases and results of their mitigation. However, understanding decadal: climate signatures, feedbacks and potential tipping points together with complexities and interdependencies of Earth-system-cycles is also critical to ensure mitigation and adaptation strategies are timely and fit-for-purpose. The magnitude of the signals/trends that need to be detected are at the level of a few tenths %/decade, commensurate with observational noise from natural variability and current instrument performance. Minimising the time to unequivocal detection of trends requires improved sensitivity/uncertainty and be considered, ‘throwing a gauntlet’. to the observational community from climate modellers and policy makers. This challenge, ultimately rests with the metrology community to tailor and/or enhance the principles and infrastructure of the SI, that has served society for 150yrs, to facilitate trust in long-time-base data-sets of bio/geo-physical variables, derived from multiple observing systems. Remote sensing of Earth from space, is the primary means to obtain the global data/knowledge needed to inform our understanding of climate sensitivity. Two thirds of the 55 ECVs require observations from space. The worlds space-agencies have long understood the need for robust calibration and validation, particularly post-launch, seeking to correct degradation caused by the harsh environments of launch and space. Difficult to quantify biases and uncertainties to SI, limit confidence in climate understanding and hesitancy in action. To address this challenge space-agencies have embraced metrology and the SI. Embedding key principles: evidence of SI-traceability, comprehensive uncertainty budgets, comparisons, documented methods, within an international framework to facilitate independent assessment of satellite data quality against, ‘independent Fiducial Reference Measurements’ (FRMs). In addition to trust/confidence it allows harmonisation of different data-sets. The ultimate FRM needs to be established in space and this paper will also describe the ESA satellite mission, TRUTHS, under development for launch in 2030. TRUTHS will, for the first time, establish a ‘metrology institute in space’. Flying a primary standard of the SI, the satellite will not only establish direct observations of Earth and Sun at uncertainties needed for climate (~10X improvement on current instruments) but will also serve as a ‘gold standard’ reference to provide in-flight calibrations of instruments on-board other satellites, improving their performance and robustly anchoring them to SI. This symbiotic partnership of metrology and space-agencies firmly extends the reach of SI into space addressing the challenge placed upon the observational community to deliver trustworthy data with the fidelity needed by policy makers to undertake ‘fit-for-purpose' climate action.