Conspicuous greening in the end of growing season over Arctic region

This study examines long-term changes in monthly vegetation greenness during a growing season (May–October) over arctic region (> 60°N) by analyzing satellite-derived normalized difference vegetation index (NDVI),temperature, and precipitation for the period 1982-2008. Detailed examinations of long-term changes in NDVI by each month from May to October reveal that distinguished changes in NDVI in October: that is, multi-decadal changes in from May to September, but continuous increase with time in October. These different monthly variations between the two periods (May-September versus October) are found to be mainly attributed to changes in dependence of NDVI to temperature and precipitation. In May-September, NDVI and temperature have significant positive correlation for the period 1982–1999, but become insignificant after 1999. It is noted that positive correlation between NDVI and precipitation is newly found in the 2000s. Summing up, obvious arctic warming leads to increase in NDVI (i.e., greening) at a rate of 0.02 per decade for 1982–1999, while decrease in precipitation reduces NDVI (i.e., browning) at a rate of 0.04 per decade thereafter. In contrast to NDVI changes in May-September, its October value is significantly increased associated with warming and moistening for the whole period. Specifically, in the period 1982–1999, the NDVI increased at a rate of 0.005 per decade, whereas in the period 2000–2008, the NDVI dramatically amplified at a rate of 0.09 per decade&. This intensified greening in October further implicates a lengthening of the duration of vegetation growth in the recent decades. The present results indicates that, in addition to warm temperature, moisture availability is an important factor for vegetation growth over arctic region and importance of sub-seasonal variation in growing season related to the changes in dependence of climatic variables.