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MODIS Data Product Non-Technical Description - MOD 17

Terrestrial biological productivity, MODIS’ Net Primary Production (NPP) product, is important both practically and theoretically. With increasing global concentration of CO2 and increasing temperatures, scientists are working to model the whole carbon cycle, which is the process by which carbon is cycled through air, life, water and soil. These models must determine where and how much carbon is likely to be stored in carbon sinks, for example, within vegetation or in the sediments at the bottom of the ocean. The models must also determine how much carbon will be given off by sources such as fossil fuel and vegetation burning. Carbon cycle models are closely tied to global climate models, and regular measurements of gross and net primary production (GPP/NPP) are essential for both.

Gross Primary Production is the foundation for calculations of the Net Primary Production (NPP) product, which is an annual measure of the biological production activity or growth of land-based vegetation. NPP takes into account how much carbon dioxide (CO2) is taken in by vegetation during photosynthesis (GPP) and how much CO2 is given off during respiration, which is the process by which organisms use food to produce energy.

NPP is designed to monitor global ecological conditions, such as large-scale climate shifts, deforestation, desertification, pollution damage, crop conditions, glacial retreats, flooding, wildfires, and urbanization. Understanding the planet’s processes and global ecological system is essential to managing Earth’s resources wisely and ensuring that sufficient resources are available well into the future. These conditions are signs that the global ecological system is changing. Studying those signs and gaining an understanding of how, why, and when they occur will help us to avoid, or at least anticipate and prepare for them in the future.

On a more immediate level, GPP/NPP provides information on several vegetation characteristics that have important social and economic impacts, including crop yields, range land forage, and forest production. Knowledge about crop yields is vital information for food producers and resource managers because market prices and emergency food stocks depend very much on accurate information. Rangeland forage data is essential to owners of large-scale herds of grazing animals that forage for their food. If herd owners looked at this data and saw that rangeland forage was declining, they would know that they needed to find other food sources for their herds. As for forest production, this data is important to both those who wish to harvest forest products as well as those who wish to monitor and preserve them. Forests are a shrinking commodity, and knowing at what rate forests are replenishing themselves compared to the rate at which they are diminishing could benefit preservationists, oversight agencies, and those with commercial interests.

MODIS GPP is based on the type of vegetation in a 1-km2 area and how much of the photosynthetically useful and available sunlight is absorbed. MODIS GPP is calculated globally on a daily basis. With respect to carbon cycling and vegetation, however, GPP is only the beginning of the story. Vegetation takes in carbon via photosynthesis, but it also burns stored fuels in order to maintain itself and to grow. This burning of fuels, called respiration, releases CO2 back into the atmosphere. This release of carbon, which many people do not associate with vegetation, must be subtracted from GPP to calculate net primary production, or NPP.

Whether one is trying to predict the immediate or future consequences of the changing primary productivity, accurate data and knowledge are essential. For more information on carbon storage and cycling, please read http://earthobservatory.nasa.gov/Library/CarbonCycle/carbon_cycle.html.

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