Global Vegetation Greenness Interannual Variability And Its Evolvement

Terrestrial ecosystems play an important role in the Earth’s climate system.Changes in global vegetation greenness profoundly affect the carbon cycle,water cycle,and energy balance of the Earth system,and reflect the changes in land cover and ecosystem services.Satellite remote sensing is indispensable for monitoring the dynamics of global vegetation greenness.Many studies have been conducted using satellite remote sensing to study global vegetation greenness phenology changes and long-time series greenness trends.With more frequent extreme climatic events（i.e.,droughts,heat waves,fires,and storms）and the increasing global climate variability,how does the interannual variability（IAV）of the Earth’s vegetation greenness change?A comprehensive and systematic answer to this question is still lacking.The IAV of global vegetation greenness and its evolution are related to the stability of the climate,conservation of biodiversity,global food security,and sustainable ecosystem services.This study investigated the spatial feature and temporal evolvement of global vegetation greenness IAV using satellite remote sensing data from 1982 to 2015 and explored the climatic,environmental,and anthropogenic factors driving these changes.In particular,the long-term trends of vegetation greenness changes and the differences in greenness among biome types are considered in the calculation of vegetation greenness IAV.This thesis also provides a focused analysis of vegetation greenness IAV in some hotspot regions and discusses the uncertainty of global vegetation greenness IAV assessed by remote sensing.The main findings of this research are as follows:（1）The vegetation greenness IAV was significantly greater in northern high latitudes and in arid and semi-arid regions than in other parts of the globe.In terms of vegetation type,the greenness IAV in herbaceous and shrub vegetation zones was greater than that in tall woody biome types（7.9%vs.2.9%）.This indicates that short vegetation zones are affected by climate variability more than tall woody vegetation zones.As the frequency of global extreme climate events increases,the greenness IAV in short vegetation zones will further be greater than that in tall woody vegetation zone.（2）More than half of the global vegetated lands（53.5%）experienced increasing IAV of vegetation greenness from 1982 to 2015.Regions with significant increases include the northern high latitudes,Australia and Central Asia,and the Three-North Shelter Forests Program of China.In addition,the Eastern European croplands also experience increasing trends of vegetation greenness IAV.However,agricultural lands in China and India showed decreasing trends of vegetation greenness IAV（–0.037%/year for China and–0.004%/year for India）.The trends of vegetation greenness IAV for croplands have implications for food security in the corresponding regions.This study also found decreasing trends of vegetation greenness IAV in tropical forests.（3）The evolution of global vegetation greenness IAV suggests changes in climate,environment,and human activities that affect vegetation dynamics.The increasing trend of vegetation greenness IAV in northern high latitudes may be explained by the succession of vegetation types there in the era of climate change.The increasing trend of vegetation greenness IAV in arid and semi-arid regions（e.g.,Central Asia and Australia）is mainly explained by the atmospheric CO₂ fertilization effect.The evolution of vegetation greenness IAV in agricultural lands is closely related to agricultural management practices:due to the improvement of agricultural management practices in China and India,there were decreasing trends of cropland greenness IAV in both countries;while the abandonment of croplands in Eastern Europe leads to increasing trends of croplands greenness IAV.The increase in vegetation greenness IAV in the Three-North Shelter Forests Program of China may be due to the few types of trees planted in the region.（4）The spatial feature and temporal evolvement of global vegetation greenness IAV assessed by remote sensing have uncertainty.With little to moderate cloud and atmospheric aerosol contamination,the vegetation greenness IAV assessed by remote sensing in Evergreen Needleleaf Forests,Deciduous Broadleaf Forests,Mixed Forests,Open Shrublands,and Croplands has less uncertainty compared to other biome types.This study analyzed the spatial feature and temporal evolvement of global vegetation greenness IAV from 1982 to 2015.To better understand the terrestrial ecosystem dynamics in the context of climate change,updated remote sensing data with a longer time span and finer spatial resolution can be used to study the global vegetation greenness IAV.