What does this mean?

Normalized Difference Vegetation Index (NDVI) is a measure of the concentration of green vegetation for a given area of the land surface. The maps and bar graphs show how NDVI in Arctic land areas for different years and months compares to the long-term average for the period 1982 to 2010.  On the map, areas with higher NDVI than the average for the selected month are indicated in greens (positive anomalies), and areas with lower than average NDVI are shown in browns (negative anomalies). 

The bar graph indicates the NDVI anomaly for the selected month and year for the entire Arctic land area (the land area north of 70 N).  Continue reading...