Research

Research

The gross carbon uptake of terrestrial vegetation through photosynthesis is a crucial parameter in climate change research. A global, observation-based characterisation of ecosystem gross primary production (equivalent to ecosystem gross photosynthesis) can only be performed with satellite measurements. However, the traditional description of vegetation productivity from space is based on the so-called spectral vegetation indices, which are better indicators of the potential plant photosynthesis than of the actual photosynthetic efficiency driving carbon assimilation. This results in an inherent limitation of existing satellite products to provide an accurate description of ecosystem functioning.

By contrast, ongoing developments in instrument design and modelling approaches have facilitated the retrieval of vegetation sun-induced chlorophyll fluorescence from space measurements. Chlorophyll fluorescence is an electromagnetic signal emitted by the chlorophyll-a of green leaves: part of the solar energy absorbed by chlorophyll is not used for photosynthesis, but emitted at longer wavelengths as a two-peak spectrum roughly covering the 650–850 nm spectral range. A vast number of laboratory and field experiments have demonstrated that chlorophyll fluorescence is a direct proxy to vegetation light use efficiency which can therefore enable a much more accurate description of gross primary production.

GlobFluo is focused on the global monitoring and interpretation of chlorophyll fluorescence from existing and upcoming Earth Observation missions. This is implemented as a multi-disciplinary approach involving the development of a variety of atmospheric-surface radiative transfer modelling approaches, data processing and information retrieval techniques and ecosystem modelling tools, with the ultimate objective of developing a new approach to the observation of carbon assimilation by vegetation from space. Our work is organized in three major tasks as follows.

→ Global retrievals of fluorescence from space

→ Interpretation and exploitation of global fluorescence data

→ Process-based modeling of chlorophyll fluorescence and photosynthesis

Spectrum of the radiation reflected by a green vegetation canopy and measured at the satellite level together with a spectrum of sun-induced chlorophyll fluorescence. The retrieval of chlorophyll fluorescence from space consists in the decoupling of the reflected radiance spectrum from the much less intense fluorescence signals. Global fluorescence retrievals have recently been achieved thanks to the high spectral and radiometric sensitivity of spaceborne atmospheric spectrometers such as GOSAT, SCIAMACHY and GOME-2 (figure by Luis Guanter)
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