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MODIS Data Product Non-Technical Description - MOD 15

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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