FIG. 3b. Geographic distribution of the change from 1982 to 1990 in normalized difference vegetation index (NDVI) of land areas north of 27.75° N, expressed as the average over the northern active growing season of May through September. (b), Nine year average NDVI over northern active growing season of May through September determined by averaging the monthly NDVI of 0.25 x 0.25 degree pixels from 1982 to 1990.