Comments on ?Variations in northern vegetation activity inferred from satellite data of vegetation index during 1981 to 1999

Introduction. In their paper, Zhou et al. [2001] analyzed advanced very high resolution radiometer-normalized difference vegetation index (AVHRR-NDVI) satellite data relative to the vegetated areas of the Northern Hemisphere for the period July 1981 to December 1999, at 8 km resolution. They concluded that an extensive and persistent increase in NDVI for the growing season is affecting most of the investigated areas, especially in the Eurasian continent. This increase was ascribed to changes in vegetation biological activity driven by global warming. At present, we are using AVHRR-NDVI for the investigation of land degradation phenomena in southern Italy for the period 1985–1999, at full sensor resolution (1.1 km) [Cuomo et al., 2001; Lanfredi et al., 2003]. Our local analysis, carried out on NOAA 9, NOAA 11, and NOAA 14 data, does not exclude the actual occurrence of greening phenomena, since we found positive NDVI trends in many forested and cultivated areas of southern Italy. Nevertheless, our results indicated that high statistical significance temporal trends are rather rare over the territory. Because of the absence of NOAA 7 data, such results cannot be directly compared with the results of Zhou et al. [2001], who observed instead high significance trends in most of the investigated areas......