Conspicuous circumpolar greening in the end of growing season over the Arctic region

This study has investigated the long-term changes in monthly vegetation greenness during the growing season (May-October) over the Arctic region (>60°N) using the satellite-retrieved normalized difference vegetation index (NDVI), and surface-measured temperature and precipitation for the period 1982-2008. While monthly NDVI shows obvious monotone increasing trend in October, its values in other months are increasing from 1982 to late 1990s followed by decreasing trend in the 2000s. These different monthly changes between May-September and October (i.e., multi-decadal variations vs monotone increase, respectively) are found to be primarily attributed to the alterations in dependence of NDVI to temperature and precipitation as shown in twofold. Firstly, in May-September, monthly NDVI is significantly and positively correlated with temperature for 1982-1999, but such correlations diminished afterward. By contrast, NDVI has a strong positive correlation with precipitation in the 2000s. Further analysis suggested that obvious arctic warming leads to increase in NDVI (i.e., greening) at a rate of 0.02 decade-1 for 1982-1999, and then decrease in precipitation reduces NDVI (i.e., browning) at a rate of -0.04 decade-1 in the 2000s. Secondly, in October, the NDVI is significantly increased following the warming and moistening during the analysis period. Specifically, the October NDVI increases at a rate of 0.05 decade-1 for 1982-1999. The increasing trend has dramatically enhanced at a rate of 0.09 decade-1 for 2000-2008, following the amplified warming. Comparing NDVI changes between seasonal mean (e.g., May-October) and each month revealed that although the magnitude of vegetation greenness has been diminished during the averaged growing season, significant circumpolar greening in October further implicates a lengthening vegetation growing season in the recent decades over the Arctic region.