The snow season has become shorter and the growing season longer over the past 20 years in Alaska's Arctic National Parks. The NPS Arctic Inventory and Monitoring Network (ARCN) uses MODIS (Moderate Resolution Imaging Spectroradiometer) satellite images and remote automated cameras to monitor the timing of the growing season and the snow cover in five large national parks in northern Alaska. To monitor the snow season, researchers use the National Snow and Ice Data Center's MODIS daily snow cover product, with additional processing by cooperators at the University of Alaska to fill cloud-related gaps. On-the-ground camera observations generally show good agreement with the MODIS satellite results. The trend across the study area over the 20 years of MODIS satellite observations was toward earlier snow-off and later snow-on. Linear models suggest average snow-off in the study area became earlier by about 1 week and snow-on later by about 5 days across the 20-year period, with about a month of year-to-year variation at any location. Start and end of the green season were located using curves fit to plots of Normalized Difference Vegetation Index (NDVI) vs. date. NDVI was calculated from all available MODIS satellite observations, extracted for sample points using the AppEEARS (Application for Extracting and Exploring Analysis Ready Samples) data extraction tool. Much of the annual variation in NDVI was due to gain and loss of snow and not vegetation growth. Researchers located the NDVI level of snow-free, senesced vegetation from multi-year data for the fall and used it to locate the true start and end of the green season on annual NDVI curves. Comparison of the satellite green season dates with those obtained from on-the-ground cameras showed good agreement in the spring but weak agreement in the fall. The study area-wide trend over 20 years of satellite observations was toward earlier start of green season (SOG) and later end of green season (EOG). Linear models suggest that average SOG became about 11 days earlier across the 20 years of observations, and EOG became later by a similar amount, with about a month of year-to-year variation between years in both SOG and EOG.
https://home.nps.gov/im/arcn/landscapedynamics.htm
Trend in start of the green season in Alaska's Arctic National Parks, 2000–2020. At nearly all of the sample points (marked with circles) the trend was negative, meaning the start of the growing season has become earlier. Most of these trends were highly statistically significant (large red circles indicate p < 0.05, Mann-Kendall test).