Reanalysis of global terrestrial vegetation trends from MODIS products: Browning or greening?

Accurately monitoring global vegetation dynamics with modern remote sensing is critical for understanding the functions and processes of the biosphere and its interactions with the planetary climate. The MODerate resolution Imaging Spectroradiometer (MODIS) vegetation index (VI) product has been a primary data source for this purpose. To date, the MODIS team had released several versions of VI products that have widely used in global change studies and practical applications. In this study, we re-examined the global vegetation activity by comparing the recent MODIS Collection 6 (C6) VIs with Collection 5 (C5) VIs including Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI) from Terra (2001-2015) and Aqua Satellites (2003-2015). We found substantial differences in global vegetation trends betweenTerra-C5 and -C6 VIs, especially EVI. From 2001 to 2015, global vegetation showed a remarkable greening trend in annual EVI from the Terra-C6 (0.28% year− 1; P < 0.001), in contrast to the decreasing EVI trend from the Terra-C5 (− 0.14% year− 1, P < 0.01). Spatially, large portions of the browning areas in tropical regions identified by Terra-C5 VIs were not evident in Terra-C6 VIs. In contrast, the widespread greening areas in Terra-C6 VIs were consistent with Aqua-C6 VIs and GIMMS3g NDVI. Our finding of a greening Earth supports the recent studies suggesting an enhanced land carbon sink. Our study suggests that most of the vegetation browning trends detected by MODIS Terra-C5 VIs were likely caused by sensor degradation, particularly for the period after 2007. Therefore, previous studies of temporal vegetation trends based solely on Terra-C5 VIs may need to be reevaluated. Our new analysis offers the most updated understanding of the global vegetation dynamics during the past 15 years and contributes to accurately understanding the role of vegetation played in the Earth's biogeochemical and climatic systems.