Research paper on potential interactions between NPP, precipitation, temperature, & atmospheric CO2

Need to give a descriptive answers on the given the trend in net primary productivity (NPP) illustrated above, illustrate potential interactions between NPP, precipitation, temperature, & atmospheric CO2. Present your answer as a graphical model and use arrows to indicate the direction of each potential interaction.
These 3 rubrics to be in your work:
1. Variables
2. Interactions
3. Presentation