Spaceborne Observations of Sun-Induced Vegetation Fluorescence: A 2007-2015 global time series from GOME-2A

Abstract

The increasing CO2 levels in the atmosphere over the past two centuries give rise to a global greenhouse effect and so interest in measuring, modeling and predicting the global carbon cycle is growing. We have a good understanding of the global carbon cycle, but large uncertainties still remain, particularly concerning the role of the terrestrial biosphere. Fixation of carbon through photosynthesis is the largest terrestrial sink for atmospheric CO2. One way of gaining further insights into carbon fixation is to quantify the biosphere photosynthetic activity on a global scale. During photosynthesis, solar light is absorbed by chlorophyll and this provides the energy for photosynthesis. Any energy that is not used for photosynthesis is dissipated as heat or re-emitted as fluorescence photons at longer wavelengths. This means that photosynthetic activity in turn could be quantified by measuring sun-induced fluorescence (SiF). Although these SiF emissions are small (about 1% of the total absorbed radiation energy [Porcar-Castell et al., 2014]), they can can be measured from space. This study uses the GOME-2 spectrometer (on board the MetOp-A platform) to retrieve the SiF signal from radiance spectra. We adopt a retrieval methodology from Joiner et al. [2013] and Guanter et al. [2014], to implement a retrieval setup that enables near real-time processing. A data set is constructed providing a time series of SiF on a global scale spanning the entirety of the GOME-2A mission (January 2007 to present). Furthermore, the study investigates the relation between in situ (flux tower) derived GPP measurements and the remotely sensed SiF retrieval. This study focuses on the United States Cornbelt which provides an excellent region of research, as its homogeneous distribution of vegetation enables good comparison with the coarse GOME-2A footprint. In this area, the correlation between Cornbelt-averaged SiF and flux tower derived GPP is high (R=0.88) and this new SiF retrieval method shows promises for better future understanding of carbon dynamics in the terrestrial biosphere.