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Vegetation all over the planet’s surface provides us
more than food and shade – vegetation is also a major
player in the global climate. Because the MODIS mission is
to gather data that will improve our understanding of global
dynamics and processes, MODIS gathers data that help us understand
how the worldwide presence and density of vegetation affects
global warming, clean air, and a host of other concerns.
In a plant canopy, the leaves determine the productivity of
the biomass (usually wood) below. This is because leaves absorb
solar radiation, and in the process of photosynthesis, convert
that energy to woody biomass and the other products that plants
produce, like fruit. It is for this reason that plants are
sometimes thought of as solar energy traps – the higher
the amount of solar energy they capture, the greater their
capacity for photosynthesis.
Vegetation indicator maps are widely used by biologists, natural
resource managers, and climate modelers, among others. With
these maps, scientists can track and study natural and man-made
fluctuations in vegetation, such as seasonal changes and deforestation.
These vegetation indicator products can also monitor changes
in vegetation that are caused by climate changes--expanding
deserts, receding forests, and longer or shorter growing seasons.
To understand these phenomena, MODIS scientists produce a
number of vegetation products. One set of products is the
MOD 13 suite, the Normalized Difference Vegetation Index (NDVI)
and the Enhanced Vegetation Index (EVI), which are common
vegetation index products that describe vegetation density.
(For more information on these indices, please read the MOD
13 Non-Technical Product Description.)
Because NDVI and EVI maps cannot quantitatively determine
how much vegetation there is in a given spot, Researchers
have to observe NDVI over a long period of time and compare
different regions to one another to determine in general what
the normal vegetation density is for a given region. But there
is an easier, and timelier, way to do so – by using
the Leaf Area Index (LAI).
LAI describes a key characteristic of a solar energy trap:
the area of leaves that cover a given unit of ground area.
In essence, this property tells scientists how many layers
of leafy vegetation between the ground and the top of the
canopy are available to absorb and convert solar energy. But
for climate and carbon modeling, scientists need to know not
only how many layers of vegetation there are, but also, how
much photosynthetically useful light the plants are absorbing.
MODIS provides these observations in the form of a product
called "FPAR," which stands for Fraction of Photosynthetically
Active Radiation. FPAR measures how much of the photosynthetically
active wavelengths of radiation a canopy absorbs. Knowing
exactly how to model and predict energy exchange between the
Earth’s land surfaces and atmosphere is vital to understanding
how the global climate works naturally and how much we are
affecting it.
Both LAI and FPAR have been used extensively for calculation
of photosynthesis, evaporation and transpiration of water,
and net primary productivity (NPP, which estimates how much
carbon is taken in by vegetation). Because plants are so prevalent
on the planet’s land surfaces, it is a given that they
have a considerable impact on the climate. Besides being able
to affect cloud cover, surface temperatures, and rain on a
regional scale, globally plants can lower the amount of carbon
dioxide (one of the most abundant greenhouse gasses) and potentially
cool the atmosphere.
The MODIS vegetation products will help answer critical climate
questions: How will vegetation respond to increasing global
temperatures? Will growing seasons lengthen? Could increased
plant growth and carbon dioxide uptake offset global warming?
Will increased evaporation in a warmer climate decrease soil
moisture and harm plant growth? The ability of the MODIS products
to reduce the uncertainty of this unfolding drama is what
makes them so important.
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