## 1 Introduction  
Vegetation over terrestrial ecosystems is a major component of the global carbon cycle since it regulates climate through the exchange of energy, water vapor, and momentum between the land surface and the atmosphere, controls CO\({}_{2}\) and currently absorbs about one-third of anthropogenic fossil fuel emissions to the atmosphere [1; 2; 3]. Terrestrial net primary production (NPP), known as the difference between carbon gain via gross primary productivity (GPP) and carbon loss through plant respiration, integrates these and other climatic, ecological, geochemical, and human influences on the biosphere [4; 5; 6]. Several studies have developed a global and regional understanding of NPP variations though methods, time periods used and results have varied. [PERSON] _et al._[4] has reported a global increase in NPP by 3.4 petagrams of carbon between 1982 and 1999, based on estimates modeled using AVHRR data inputs and climate variables, with 80% of the increase being attributable to ecosystems in tropical regions (mainly rainforests) and those in the high latitudes of the North Hemisphere (NH). Studies have demonstrated that terrestrial NPP has increased in the middle and high latitudes in the NH [3; 4; 5; 6; 7; 8; 9] during the 1980-1999 time period. However, [PERSON] and [PERSON] [10] concluded that NPP reduced by 0.55 petagrams of carbon globally from 2000 to 2009, when studied using Moderate-resolution Imaging Spectroradiometer (MODIS) derived input data in association with climate. As such, over the longer time frame of 1980 through 2010 there is still no agreement on global GPP or NPP change [11] and there is still much uncertainty even with the availability of satellite observations for the past three decades. From the studies to date it is clear that location is key, with many northern hemisphere higher latitude regions showing initial (1982-1999) increases in biomass [4; 5], but in the latter period of 2000-2010, declines also becoming obvious especially in southern hemisphere and tropical locations [5]. Given the global importance of these trends, in terms of the potential decline in the rate of carbon sequestration in vegetated systems, understanding the multi-decadal patterns of NPP trends is of paramount importance [5]. Atmospheric CO\({}_{2}\) is an essential component in photosynthesis, and thus influence vegetation production. Higher concentrations may improve plant growth known as the \"fertilization effect\", which is limited by nutrients, air pollution, temperature, and precipitation [12]. Also there is general agreement among scientists that the climate system is changing as a result of increasing atmospheric concentrations of CO\({}_{2}\), although the degree to which temperature and precipitation patterns will change is still uncertain. Temperature, radiation, and water interact to impose complex limitations on vegetation activity in different parts of the world, and ultimately determine the spatial and temporal NPP patterns [4]. However, these factors tend to be co-limiting, and differ in their contribution in different ecosystems. In the middle and high latitudes, in the NH, where temperature is limited, higher temperatures have increased NPP by enhancing vegetation growth [4; 6; 8; 13]. In the tropics (23.5\({}^{\circ}\)S to 23.5\({}^{\circ}\)N) the NPP variation is largely constrained by solar radiation due to severe cloudiness [10; 14]. In drier and colder ecosystems NPP increases linearly with the increase in mean annual precipitation and mean annual temperature [15]. In the arid and semi-arid ecosystems and seasonally dry tropical climates NPP is primarily controlled by water availability as precipitation can trigger the emergence of green leaves and controls growth duration [16,17]. Savanna ecosystems, which are characterized by the coexistence of trees and grasses, are more sensitive to climate variability, especially precipitation changes.