A land surface phenology assessment of the northern polar regions using MODIS reflectance time series

The study of changes in phenology and, in particular, land surface phenology (LSP) provides an important approach to detecting responses to climate change in terrestrial ecosystems. LSP has been studied primarily through analysis of time series of vegetation indices retrieved from passive optical sensors, such as the series of AVHRRs on polar-orbiting satellites and the pair of MODIS sensors on the Terra and Aqua platforms that provide higher spatial, spectral, and radiometric resolution. Most broad-scale vegetation studies use normalized difference vegetation index (NDVI) data. Here, we provide an overview of the LSP of the northern polar and high-latitude regions (>= 60 degrees N) based on MODIS data at climate modeling grid (0.05 degrees) resolution. We demonstrate the relationship between three onset-of-greening measures and snow cover and accumulated growing degree-days. We show that the Arctic Oscillation index is significantly correlated with the peak timing of the growing seasons since 2000 for a range of ecoregions, and we demonstrate that there were more than three times as many negative NDVI changes since 2000 as positive changes (25.3% versus 7.3%) based on all land area above 60 degrees N. We reveal that these changes are predominantly driven by minimum temperature changes.