Land Surface Phenology in Global Change Studies

As of this writing, there are two global phenology products in the twentyfirst century at 500 m spatial resolution. Here we present a comparative analysis of the mid-green-up phenometric from these standard VIIRS and MODIS products for the year 2022. We evaluate the results by ecoregion and biome and report linear regression results to compare on the phenometrics timings. Next, we performed temporal analysis by calculating non-parametric Kendall correlations between midgreen- up timings and year to identify whether the previously identified trends toward earlier springs are consistent and comparable between the two datasets. We also performed similar analysis using indices for two major climate oscillations: NAO and SOI. The comparisons between VIIRS and MODIS mid-green-up metrics reveal significant discrepancies, particularly in ecoregions with attenuated or irregular seasonality, as well as in ecoregions with close proximity to water, such as mangroves. The phenometrics for VIIRS and MODIS align well for ecoregions in temperate and colder biomes, but not in wetter and drier biomes. Stability and robustness are observed mainly in temperate regions dominated by unimodal seasonality. The lack of robustness across biomes influences the inferences we draw from the products. For example, we find fewer significant temporal changes and fewer correlations with climate oscillations for biomes with poorer correspondence between the two datasets. The lack of change and correlation may be due to nonsignificant influences of the selected climate oscillations in these regions or to heightened uncertainty in the LSP metrics.