The GIMMS NDVI used here are 10-day maximum value
composites that minimize the effects of cloud contamination.
Remaining clouds have been filled in using average values
from adjacent periods.
Because NDVI is a ratio of difference between two close bands,
it is largely insensitive to errors. Preprocessing of the GIMMS NDVI data
includes corrections for sensor degradation, for satellite orbital drift
and for atmospheric volcanic aerosols from eruptions in 1982 and 1991.
