Spatial and Temporal Variations of Net Primary Productivity at Century Scale in Earth System Models and Its Relationship with Climate

The spatial and temporal changes in net primary productivity (NPP) during 1901-2005 were studied using six earth system models. The relationship between NPP changes and the climatic factors of air temperature and precipitation was analyzed. The results show that: (1) In the past 100 years, the global NPP has shown an upward trend. The trend coefficient of the ensemble model average is 0.88, which passes the 99.9% confidence test. The trend in the Northern Hemisphere is more pronounced than in the Southern Hemisphere. (2) In the past 100 years, high NPP values of 800 g(C) m^-2 a^-1 or higher are mainly distributed in tropical rainforest areas in the equatorial regions of South America, Equatorial Africa, the Indochina Peninsula, and Indonesia. Low NPP values are mainly distributed in the high latitudes of the Northern Hemisphere, Northern Africa, the arid and semi-arid regions of the Asian continent, and the northwestern Tibetan Plateau. (3) The global NPP had a positive correlation with temperature in most of the regions during this century period. This correlation only becomes negative for South America, Africa, and India near the equator, mainly because radiation in these areas is a limiting factor of NPP. The 100-year changes in the global NPP and precipitation are also primarily positively correlated in most regions, but are negatively correlated in the arid and semi-arid regions of Northern Africa and Western Asia. (4) The six earth system models yield relatively consistent NPP and temperature/precipitation changes in most of the 21 regions of the world. In West Africa, the pattern changes are inconsistent where the uncertainty of the NPP simulation is greater, followed by the Mediterranean region. (5) In particular, the evolution of the NPP in the East Asia region and climate is synchronized and highly correlated, which reflects the strong process of atmospheric interaction with vegetation.