Variations in northern vegetation activity inferred from satellite data ofvegetation index during 1981 to 1999

The northern high latitudes have warmed by about 0.8 degreesC since the ear ly 1970s, but not all areas have warmed uniformly [Hansen et al., 1999]. Th ere. is warming in most of Eurasia, but the warming rate in the United Stat es is smaller than in most of the world, and a slight cooling is observed i n the eastern United States over the past 50 years. These changes beg the q uestion, can we detect the biotic response to temperature changes? Here we present results from analyses of a recently developed satellite-sensed norm alized difference vegetation index (NDVI) data set for the period July 1981 to December 1999: (1) About 61% of the total vegetated area between 40 deg reesN and 70 degreesN in Eurasia shows a persistent increase in growing sea son NDVI over a broad contiguous swath of land from central Europe through Siberia to the Aldan plateau, where almost 58% (7.3 x 10(6) km(2)) is fores ts and woodlands; North America, in comparison, shows a fragmented pattern of change in smaller areas notable only in the forests of the southeast and grasslands of the upper Midwest, (2) A larger increase in growing season N DVI magnitude (12% versus 8%) and a longer active growing season (18 versus 12 days) brought about by an early spring and delayed autumn are observed in Eurasia relative to North America, (3) NDVI decreases are observed in pa rts of Alaska, boreal Canada, and northeastern Asia, possibly due to temper ature-induced drought as these regions experienced pronounced warming witho ut a concurrent increase in rainfall [Barber et al., 2000]. We argue that t hese changes in NDVI reflect changes in biological activity. Statistical an alyses indicate that there is a statistically meaningful relation between c hanges in NDVI and land surface temperature for vegetated areas between 40 degreesN and 70 degreesN. That is, the temporal changes and continental dif ferences in NDVI are consistent with ground-based measurements of temperatu re, an important determinant of biological activity. Together, these result s suggest a photosynthetically vigorous Eurasia. relative to North America during the past 2 decades, possibly driven by temperature and precipitation patterns. Our results are in broad agreement with a recent comparative ana lysis of 1980s and 1990s boreal and temperate forest inventory data [United Nations, 2000].