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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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