Daily multispectral observations from Envisat's MERIS sensorare being combined with a sophisticated processing algorithm andpowerful Grid computing to reveal global photosynthesis activity onland. This permits researchers to trace the state of health ofterrestrial plant cover, identifying areas under stress and assessingdamage from drought or fires.
An algorithm developed by theEuropean Commission's Joint Research Centre (EC-JRC) in Ispra, Italy isthe basis for global monthly photosynthesis maps derived from MERISimagery. Their production represents a demanding data-processing taskonly made possible on a routine basis through the Earth ScienceGrid-On-Demand service available from ESRIN, the European Centre forEarth Observation, in Frascati.
Maps of anomalies inphotosynthesis levels over European countries have then been producedby scientists at the Institute for Environment and Sustainability ofEC-JRC, based on observations gathered from 1998 to 2002.
Grid-processedresults shown here indicate a decrease in vegetation activity againstthe average for Spain and Portugal, due to unusually cold and dryconditions at the beginning of the year, leading to plant stress. Inthe past the effects of the 2003 European drought were detected in asimilar way, but in the absence of any long-term continental-scaleperspective on vegetation growth.
Forget fossil fuels and nuclearplants: on a global scale solar power is the dominant source of energy.Across land and sea, our world's plant life uses the process calledphotosynthesis to convert incoming sunlight into chemical energy, alongthe way producing all the oxygen we breathe.
The fraction ofincoming solar radiation useful for photosynthesis that is actuallyabsorbed by vegetation – a value known as the Fraction of AbsorbedPhotosynthetically Active Radiation (FAPAR) – is well-recognised as anindicator of its condition and an influence on its capacity for growth,or 'productivity'. FAPAR has been recognised as a fundamental surfaceparameter by international organisations including the Global ClimateObserving System (GCOS), charged with providing data on the Earth'sclimate system.
Vegetation plays a critical role on Earth as allanimals and humans ultimately depend on it for food. Plants oftenprovide a local habitat for other species to live in, and constitute amajor source of raw materials for human activities. Wood in particularconstitutes a critical source of energy storage for a large fraction ofhumanity which relies on it for heating and cooking. It is especiallysignificant as a 'sink' of carbon that might otherwise enter theatmosphere as carbon dioxide and hasten global warming.
Envisat'sMedium Resolution Imaging Spectrometer (MERIS) acquires multispectralimagery of the Earth, and is used to monitor the state and evolution ofthe planetary vegetation cover. In particular the instrument's standardLevel-2 product on land, known as the MERIS Global Vegetation Index(MGVI) provides FAPAR values based on reflected radiation from theEarth's land surface. EC-JRC is responsible for the processingalgorithm that yields the MGVI product.
"It is a complex task toextract the required information from the reflected light MERISmeasures," explains Nadine Gobron of EC-JRC. "It requires thedevelopment of mathematical tools to minimise the effects due toscattering from atmospheric particles, the presence of soils and thechanging geometry of illumination and observation."
Thevalidation of MGVI is in fact an ongoing activity, along with that fora family of related FAPAR algorithms developed by JRC for othersatellite sensors, including NASA's MODIS and SeaWiFS, intended toensure the availability of a long time series of global products. Thelatter are essential to assess environmental trends, guide policymaking and support sustainable development.
"Considerable effortsare made to evaluate the quality of the MGVI and comparable products,"explains Gobron. "This takes place both through inter-comparisonsbetween the products generated by various instruments and by directcomparisons with field measurements. These ongoing efforts involve notonly ESA Cal/Val groups but similar international groups including CEOSas well as close collaborations with major research programmes, such asCarboEurope funded by the European Union."
The monthly FAPARanomaly products are an example of close cooperation between ESRIN andEC-JRC. The products are basically mosaics created from multiple imagesacquired by MERIS over the course of the month. Handling and processingthis much data is extremely demanding in processing power, but ESRIN'sGrid-On-Demand service makes it feasible.
Grid computing takes itsname and organising principle from the electricity grid: anything onecomputer can do, a pool of computers can do faster and better. Thesemachines do not have to be in the same building, country or evencontinent – they simply have to be linked together. A Grid provides itsuser with massive amounts of memory and processing power in order totackle very complex tasks.
ESRIN has taken up Grid computing as ameans of supporting its primary task: to archive and distribute an evergrowing volume of Earth Observation data. Since the start of this yearthe Earth Science Grid-On-Demand service has been available toregistered users. It allows access to a large amount of data and theswift performance of a wide variety of data processing.
Envisat'sMERIS sensor acquires optical multispectral imagery of Earth's oceans,land and atmosphere. It and a fellow Envisat instrument are the subjectof next week's MERIS and AATSR Workshop. More than 200 PrincipalInvestigators from 30 countries will gather at ESRIN, the EuropeanCentre for Earth Observation in Frascati near Rome, for the five-dayevent beginning Monday 26 September.
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