Source: EO DAAC Study: Perspective on Plants.
With the launch of the Terra spacecraft in December 1999 … the Moderate Resolution Imaging Spectroradiometer (MODIS) monitors the Earth’s atmosphere, oceans, and land, measuring both visible and invisible light at high resolution, and producing better quality data and images than earlier instruments. A crucial part of MODIS Land Science is vegetation monitoring, and NTSG uses MODIS data to develop plant productivity maps. These maps include croplands, temperate and tropical forests, deserts, and tundra.
Why study vegetation so closely? "Since vegetation is the cornerstone of all biospheric development — all animals eat plants directly or indirectly — it’s the fundamental measure of the Earth’s habitability. So the most important thing our products can do is quantify whether global vegetation is declining in magnitude or vigor."
NTSG uses infrared and red channels on the MODIS sensor to calculate the proportion of ground surface covered by vegetation.
To determine how much sunlight is being absorbed, NTSG adds daily surface weather data from NASA’s Data Assimilation Office to its own data. By isolating the cloud-free pixels from each orbit, NTSG algorithms piece together an artificially cloudless image, or at least an image as free from clouds as possible. NTSG uses this data to calculate a refined measurement of photosynthesis. This, combined with a measurement of how much carbon dioxide is absorbed from the atmosphere, is used to calculate plant growth, or NPP.
NTSG does not yet have enough data to predict long-term trends.