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This  paper  reports  on  the  analysis  of  Pathfinder AVHRR land (PAL) data set that spans the period July 1981 to September 1994. The time series of normalized difference vegetation index (NDVI) data for land areas north of 45^oN assembled by  correcting  the  PAL  data  with  spectral  methods  confirms  the northerly greening trend and extension of the photosynthetically active  growing  season.  Analysis  of  the  channel  reflectance  data indicates  that  the  interannual  changes  in  red  and   near-infrared reflectances  are  similar  to  seasonal  changes  in  the  spring  time period when green leaf area increases and photosynthetic activity ramps up. Model calculations and theoretical analysis of the sensitivity of NDVI to background reflectance variations confirm the hypothesis that warming driven reductions in snow cover extent  and  earlier  onset  of  greening  are  responsible  for  the  observed changes in spectral reflectances over vegetated land areas