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Quantifying global photosynthesis remains a challenge due to a lack of accurate remote sensing proxies. Solar-induced chlorophyll fluorescence (SIF) has been shown to be a good indicator of photosynthetic activity across various spatial scales. However, a global and spatially challenging estimate of terrestrial gross primary production (GPP) based on satellite SIF remains unresolved due to the confounding effects of species-specific physical and physiological traits and external factors, such as canopy structure or photosynthetic pathway (C3 or C4). Here we analyze an ensemble of far-red SIF data from OCO-2 satellite and ground observations at multiple sites, using the spectral invariant theory to reduce the effects of canopy structure and to retrieve a structure-corrected total canopy SIF emission (SIFtotal). We find that the relationships between observed canopy-leaving SIF and ecosystem GPP vary significantly among biomes. In contrast, the relationships between SIFtotal and GPP converge around two unique models, one for C3 and one for C4 plants. We show that the two single empirical models can be used to globally scale satellite SIF observations to terrestrial GPP. We obtain an independent estimate of global terrestrial GPP of 129.56 ± 6.54 PgC/year for the 2015–2017 period, which is consistent with the state-of-the-art data- and process-oriented models. The new GPP product shows improved sensitivity to previously undetected ‘hotspots’ of productivity, being able to resolve the double-peak in GPP due to rotational cropping systems. We suggest that the direct scheme to estimate GPP presented here, which is based on satellite SIF, may open up new possibilities to resolve the dynamics of global terrestrial GPP across space and time.

Photosynthesis plays a crucial role in regulating the global carbon cycle and mitigating climate change. The diurnal variation in photosynthesis provides key information on the responses of ecosystems to environmental drivers, but there is a critical gap in the large-scale estimation of diurnal photosynthesis. In the last decade, satellite estimates of solar-induced chlorophyll fluorescence (SIF) have been found to mimic the seasonality of photosynthesis. Recently, the deployment of the Orbiting Carbon Observatory-3 (OCO-3) on the International Space Station has provided the opportunity to retrieve SIF at different times of the day. Here we utilized OCO-3 measurements to estimate and analyze diurnal cycles of SIF and gross primary production (GPP) at the global scale. We first mitigated the sun-sensor geometry effects on nadir-mode OCO-3 SIF (SIFnadir) at the sub-diurnal scale (hourly) by deriving the total canopy SIF emission (SIFtotal) using radiative transfer theory. Next, we generated the spatially and temporally continuous hourly SIFnadir and SIFtotal using artificial neural networks under clear-sky conditions, whose extrapolation ability was evaluated using the data from independent years. Compared with SIFnadir, the diurnal relationship between clear-sky SIFtotal and GPP from 38 homogeneous flux sites had smaller variations in the slope (the coefficient of variation was 0.07 vs 0.19). In addition, a correction to account for the bias between clear-sky and overcast conditions was used to estimate all-sky GPP from clear-sky SIFtotal and the resulting GPP was strongly correlated with tower GPP (R2 = 0.75; RMSE = 3.53 μmol/m2/s). Our results demonstrated that the new OCO-3 SIF trained GPP product (GPPSIF) was able to depict the diurnal pattern of photosynthesis globally, capturing also the physiologically hysteresis or afternoon depression of photosynthesis. By doing so, hourly GPPSIF has the potential to improve the modeling of terrestrial photosynthesis and the projection of the global carbon cycle under climate change.

SpecCal software for the spectral calibration of high-resolution spectrometers is presented in this manuscript. The software, written in IDL 7.1, allows estimation of the channel central wavelength and the full width at half maximum of a selected spectrometer at several wavelengths across the VNIR range (350-1050. nm). This is achieved through comparison of the position and width of specific solar and terrestrial absorption features, as observed in the measured data, with those observed in simulated MODTRAN4 irradiance data. SpecCal is operated from a user-friendly graphical user interface that allows semiautomatic application of the spectral calibration algorithm at several wavelengths. The proposed software may be exploited as a useful in situ vicarious spectral calibration tool for field spectrometers operating in the VNIR range, which makes it possible to quickly analyze the spectral characteristics of the instruments and their possible variations with time. © 2011 Elsevier Ltd.

Interest in remote sensing (RS) of solar-induced chlorophyll fluorescence (F) by terrestrial vegetation is motivated by the link of F to photosynthetic efficiency which could be exploited for large scale monitoring of plant status and functioning. Today, passive RS of F is feasible with different prototypes and commercial ground-based, airborne, and even spaceborne instruments under certain conditions. This interest is generating an increasing number of research projects linking F and RS, such as the development of new F remote retrieval techniques, the understanding of the link between the F signal and vegetation physiology and the feasibility of a satellite mission specifically designed for F monitoring. This paper reviews the main issues to be addressed for estimating F from RS observations. Scattered information about F estimation exists in the literature. Here, more than 40 scientific papers dealing with F estimation are reviewed and major differences are found in approaches, instruments and experimental setups. Different approaches are grouped into major categories according to RS data requirements (i.e. radiance or reflectance, multispectral or hyperspectral) and techniques used to extract F from the remote signal. Theoretical assumptions, advantages and drawbacks of each method are outlined and provide perspectives for future research. Finally, applications of the measured F signal at the three scales of observation (ground, aircraft and satellite) are presented and discussed to provide the state of the art in F estimation. © 2009 Elsevier Inc. All rights reserved.