Trends in Global Vegetative Drought From Long-Term Satellite Remote Sensing Data

In this article, the trends in global vegetative drought were investigated using MODIS- and AVHRR-based NDVI products. A set of selected methods were employed to perform trend analysis including trend test, trend location detection, and trend estimates. Accounting for the effect of the global geographical heterogeneity on trend analysis, the NDVI dataset was aggregated on designated divisions in view of latitude ranges and climate zones. From the results, it was concluded that: AVHRR has longer-term records that provide a critical historical perspective on vegetation activities necessary for global change research, and continuity and correctness is achievable from AVHRR VCI given the systematic offset between the NDVI values derived from the two sensors and the characteristics of the VCI algorithm. From a pixel level global trend analysis map, the proportion of pixels with rising trends is 54.7% in the world, 67.6% and 47.5% in the northern hemisphere and the southern hemisphere, respectively, which means there is an overall rising trends in the global VCI values, especially in the northern hemisphere. The North Temperate and the South Tropical have overall increases in the VCI values while all climate zones have overall increases in the VCI values. The piecewise trends basically adhere to the results of overall trend identification although there are some local variations. There are obvious rising trends during the latest years for all the climate zones. Dominant down trends were identified in A, B, Cw, Ds, and E while the piecewise trends for both Cs and Df are dominantly rising before 2000. An average of about four breakpoints were detected from both the climate zone- and latitude range-aggregated divisions Thus, the mean duration for a piecewise trend is 7-9 years.