GCOS Terrestrial EVC T10
Fraction of Absorbed Photosynthetically Active Radiation (FAPAR)

Introduction: The Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) is a non-dimensional value that measures the fraction of the incoming solar radiation at the top of the vegetation canopy that contributes to the photosynthetic activity of plants, and thus indicates the presence and productivity of live green vegetation. Spatially-detailed descriptions of FAPAR provide information about the strength and location of terrestrial carbon sinks and can be of value in verifying the effectiveness of the Kyoto Protocol’s flexible-implementation mechanisms.
 
FAPAR is not directly measurable, but is inferred from models describing the transfer of solar radiation in plant canopies, using remote-sensing observations as constraints. Space agencies, including NASA and ESA, and research institutions such as the JRC have been generating FAPAR products on a regular basis, using a variety of sensors. These efforts, including archiving and distribution, remain funded by research budgets and are in need of more systematic support to ensure the continuous, long-term operational availability of this product. Daily recovery of FAPAR is possible in principle, but cloud and thick aerosols often obscure the surface and result in incomplete maps. This issue is normally addressed by generating time-composited FAPAR maps that aim to convey information on central tendencies (statistical first moments of the distribution) while providing as complete a spatial coverage as possible. To detect trends in the presence of inter-annual variability requires long time series. To that end, existing archives of satellite data from instruments such as SeaWiFS have been reprocessed to generate long time series that are coherent and consistent with the most recent sensors (e.g., MERIS). This is best achieved with sensors that include a blue channel, which is particularly sensitive to atmospheric aerosols. This effort may need to be periodically repeated in the future to take advantage of further advances in algorithm design and to improve the compatibility and reliability of the products, or to extend the records further back in time using even more sophisticated approaches to compensate for the absence of blue band measurements. GTOS and GCOS encourage the space agencies and other entities to continue to generate and disseminate weekly to 10-day global FAPAR products at spatial resolutions of 2 km or better (modern sensors offer the possibility of systematically generating global products at a spatial resolution of about 300 m).
 
FAPAR is recovered from a range of sensors by various algorithms using the visible and infrared parts of the spectrum, and the accuracy and reliability of these products is not always properly documented. Currently available products have been shown to exhibit significant differences, which detract from their usefulness in downstream applications. The CEOS WGCV, in collaboration with GCOS and GTOS, should lead the comparison and evaluation of these FAPAR products as well as the benchmarking of the algorithms used to generate them. Reference sites making in situ observations should be fully engaged in this process, and it would be desirable if these sites were collocated with the terrestrial reference sites proposed in Action T3, provided that these sites offer a reasonable degree of spatial homogeneity over spatial scales comparable to the resolution of the sensors. WGCV is identifying a core set of sites and measurement campaigns, which should be supported by the CEOS agencies and by national research budgets. 
 
Relatively homogeneous sites, at scales comparable to the typical spatial resolution of modern sensors, are preferable, but a detailed characterisation of the spatial variability of those sites is required.

Changes in land cover are important aspects of global environmental change, with implications for ecosystems, biogeochemical fluxes and global climate. Land cover change affects climate through a range of factors from albedo to emissions of greenhouse gases from the burning of biomass. Deforestation inter alia increases the amount of carbon dioxide (CO₂) and other trace gases in the atmosphere. When a forest is cut and burned to establish cropland and pastures, the stored carbon joins with oxygen and is released into the atmosphere as CO₂. The IPCC notes that about three-quarters of the anthropogenic emissions of CO₂ to the atmosphere during the past 20 years were due to fossil fuel burning. The rest was predominantly due to land use change, especially deforestation. In 2005, a number of developing countries proposed to incorporate deforestation prevention into the Kyoto Protocol, in part through an emissions trading system. The initiative, known as REDD, (Reducing Emissions from Deforestation in Developing countries) would allow developing countries to sell emissions savings from forest conservation. Developed countries would buy the savings to credit against their own emissions targets. IGOS has set up an Integrated Global Carbon Observation (IGCO) Theme (report available from www.igospartners.org ) to develop a flexible, robust strategy for international global carbon observations over the next decade. A key component of IGCO is terrestrial carbon observations aimed at the determination of terrestrial carbon sources and sinks with increasing accuracy and spatial resolution. The IPCC has highlighted an improved understanding of carbon dynamics as vital in tackling one of the biggest environmental problems facing humanity. The IGCO work will be an essential input to the implementation of the United Nations Framework Convention on Climate Change (UNFCCC), particularly on the role of natural sinks in meeting targets under the UNFCCC Kyoto Protocol. Satellite observations allow scientists to map land cover and the dynamics of fire disturbance, and track two key elements of Earth’s vegetation – the ‘Leaf Area Index’ (LAI) and the FAPAR. LAI is defined as the one-sided green leaf area per unit ground area in broadleaf canopies, or as the projected needle leaf area per ground unit in needle canopies. FAPAR is the fraction of photosynthetically active radiation absorbed by vegetation canopies. Both LAI and FAPAR are data necessary for understanding how Sunlight interacts with the Earth’s vegetated surfaces.

(Source: WMO/IOC Implementation Plan for the Global Observing System for Climate in Support of the UNFCCC (2010 Update) GCOS-138/GOOS-184/GTOS-76/WMO-TD/No. 1523)

Satellite Observations: Multiple types of satellite observations are used in agricultural applications. Space imagery provides information which can be used to monitor quotas and to examine and assess crop characteristics and planting practice. Information on crop condition, for example, may also be used for irrigation management. In addition, data may be used to generate yield forecasts, which in turn may be used to optimise the planning of storage, transport and processing facilities. Classification and seasonal monitoring of vegetation types on a global basis allow the modelling of primary production – the growth of vegetation that is the base of the food chain – which is of great value in monitoring global food security. A number of radiometers provide measurements of vegetation cover, including the ATSR series, AVHRR/3, MODIS, MERIS, SEVIRI and Vegetation. These instruments are helping production of global maps of surface vegetation for modelling of the exchange of trace gases, water and energy between vegetation and the atmosphere. Multi-directional and polarimetric instruments (such as MISR and POLDER) will provide more insights into corrections of land surface images for atmospheric scattering and absorption, as well as Sun-sensor geometry, allowing better calculation of vegetation properties. Synthetic aperture radars (SARs) are used extensively to monitor deforestation and surface hydrological states and processes. The ability of SARs to penetrate cloud cover and dense plant canopies makes them particularly valuable in rainforest and high-latitude boreal forest studies. Instruments such as ASAR, SAR (RADARSAT), and PALSAR provide data for such applications as agriculture, forestry, land cover classification, hydrology and cartography. CEOS and GCOS have concluded that many of the Essential Climate Variables (ECV) related to vegetation and supported from space will require reprocessing of the moderate resolution historical record (in particular AVHRR) to be of greater value for climate purposes, and appropriate actions have been defined, including the development of enhanced calibration and validation schemes which guarantee long-term stability and consistency over different temporal and spatial scales. Research topics like scaling, and the development of ‘community radiative transfer models’ integrated into sophisticated assimilation schemes, are of paramount importance for an integrated approach. (Satellite Missions) (Source: CEOS EO Handbook - Earth Observations Plans by Measurement)
 
References:

• Implementation Plan for the Global Observing System for Climate in Support of the UNFCCC (2010 Update) GCOS-138/GOOS-184/GTOS-76/WMO-TD/No. 1523
• The Second Report on the Adequacy of the Global Observing Systems for Climate in Support of the UNFCCC - April 2003 (GCOS-82/WMO/TD Noc 1143)
• ECV T10 FAPAR: Fraction of Absorbed Photosynthetically Active Radiation - Assessment of the Status of the Development of the Standards for the Terrestial Essential Climate Variables (ECV), Version 8, 11 May 2009 (GTOS-65)
• CEOS EO Handbook - Earth Observations Plans by Measurement
• CEOS EO Handbook - Measurement Categories (Updated October 2010)

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