Understanding global spatio-temporal trends and the relationship between vegetation greenness and climate factors by land cover during 1982–2014

Analysis of the correlation between vegetation greenness and climate variable trends is important in the study of vegetation greenness. Our study used Normalized Difference Vegetation Index-3rd generation data from the Advanced Very High-Resolution Radiometer - Global Inventory Modeling and Mapping Studies (AVHRR-GIMMS NDVI3g), land cover data from the Climate Change Initiative (CCI-LC), and climate data from the Climatic Research Unit global time series (CRU TS) of climate variables (temperature and precipitation, solar radiation) over the past 33 years. First, we estimated the overall trends for vegetation greenness and climate variables over five time periods. Second, we subjected the data to correlation, regression, and residual analyses to detect correlations between vegetation greenness and different climate variables. Third, we extracted trends and correlation results by primary land cover types for each climate zone. Our study was focused at the global scale, and findings indicate that the largest decreasing trend of vegetation greenness and grasslands occurred in the mid-latitude regions of the Northern Hemisphere and in parts of South America, Africa, Saudi Arabia, and south and northeast Asia. In particular, the cold climatic zones of forest (36.6%), cropland (36.6%), and grassland (14.1%) suffered significant decline in vegetation greenness. Anthropogenic activities are mainly responsible for declining vegetation greenness particularly in northern Africa, central and western Asia. However, residual analysis shows an increase in vegetation greenness in some parts of western Europe, southern Australia, and the northern part of South America. The study also identified temperature and precipitation as the main factors responsible for controlling vegetation growth. Hot-spot areas with the largest temperature increases were found in the Amazon, Central America, southern Greenland, east Africa, south-east Asia, and other areas. However, temperatures decreased in the western part of South America, Angola, the Philippines, Indonesia, and Papua New Guinea. Precipitation decreased the most from March to May over most parts of the world with high correlation (r = 0.88) in Russia Canada, northeast Asia, and central Africa. In general, climate factors were the principal drivers of the variation in vegetation greenness globally in recent years.