SPACECO2VEG · Space-based analysis of the relationship between vegetation functioning and atmospheric CO2 and CH4 greenhouse gases

Space-based analysis of the relationship between vegetation functioning and atmospheric CO2 and CH4 greenhouse gases

Project context and objectives The main scientific objective of this project is the monitoring of vegetation functioning from space, and in particular the photosynthesis and subsequent CO2 uptake processes. Sun-induced chlorophyll fluorescence (Fs) is used as a proxy to photosynthesis. The terrestrial Fs signal is emitted by the chlorophyll molecules of assimilating leaves: part of the energy absorbed by chlorophyll is not used for carbon fixation, but re-emitted at longer wavelengths in the 650-800~nm spectral region. The Fs signal originates at the core of the photosynthetic machinery and responds instantaneously to perturbations in the environmental conditions such as light and water stress. This makes it a more direct proxy to the plant photosynthetic activity than the reflectance-based vegetation indices traditionally used in vegetation remote-sensing applications. Project work The research carried out during this project has been focused on:-1) the implementation of a method for the retrieval of Fs from high spectral resolution measurements by the FTS instrument on-board the GOSAT satellite; -2) the exploitation of the Fs maps produced for the global assessment of vegetation photosynthesis and gross primary production (GPP, the CO2 assimilation rate by vegetation). These two core research activities have been complemented with a third one dealing with the measurement and modelling of Fs at the ground level using high-resolution measurements to simulate GOSAT-FTS observations. The method for Fs retrieval from GOSAT-FTS measurements designed and implemented during the project is based on a linear forward model derived by a singular vector decomposition technique, which enables a fast and robust inversion of GOSAT-FTS top-of-atmosphere radiance spectra. Retrievals are performed in two spectral micro-windows (~2-3 nm width) containing several strong solar Fraunhofer lines. The statistical nature of this approach enables the avoidance of potential retrieval errors associated with the modelling of the instrument line shape or with a given extraterrestrial solar irradiance data set. It can be argued that the method improves the existing methods published by Frankenberg et al. and Joiner et al. Project results Important new findings in the field of Fs modelling derived from this GOSAT-based Fs data set are the demonstration of the linear relationship between Fs and GPP on a monthly basis, the biome-dependent linear scaling between Fs and GPP, the existence of important directional effects in the Fs signal, and the capability of the Fs signal to track vegetation water-stress conditions. It can be claimed that very innovative research has been performed within this project in the field of carbon monitoring from space. The timing of this fellowship has been optimum so that this project could benefit from the first developments in the field and later become part of the state of the art. The results achieved in this project enable confidence to be reinforced in the feasibility of Fs retrievals with GOSAT-FTS and several openings to be made for future research in this emerging field. Project outcomes This IEF research project carried out at the Atmospheric, Oceanic and Planetary Physics (AOPP) sub-department of the University of Oxford (UK) over 14 months has resulted in a contribution from the researcher to four peer-reviewed papers directly linked to the remote sensing of chlorophyll fluorescence, and to three other papers on related earth observation disciplines. The researcher has also contributed to several conferences in the field of remote sensing and carbon modelling, and has participated in steering panels and grant evaluation committees. The fellow has established a wide network of collaborations with European and non-European scientists, including AOPP-Oxford, NASA-JPL, the University College London, the Japanese Space Agency, the University of Twente and the University of Milano. To complete his training activities, he has taken an active part in the academic activities at the host institution, with involvement in undergraduate and graduate teaching. Taking advantage of the training provided by this IEF, this research line is currently being further developed by the researcher through the implementation of a junior research group on chlorophyll fluorescence at the Free University of Berlin funded by the German Research Foundation (DFG).

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