The question is: What fraction of the observed increase in atmospheric CO2 since 1750 is due to the burning of fossil fuels? Is it close to 1.0 as the IPCC and the climate policy makers would have us believe by saying that all, or nearly all, of the increase is due to the burning of fossil fuels i.e., the climate changes we are seeing are caused by humans. This paper is all about this fraction. I use networks of boxes and arrows that visualize the carbon accounting. I incorporate the photosynthesis and the temperature dependent non-fossil emission. The networks are balanced by solving simultaneous, linear equations, one for each box. Based on the law of the conservation of mass, in addition to elementary linear algebra, I present and apply a simple, universal, method for global carbon accounting. Assuming that the atmosphere behaves like a physical system with constant residence time 4.1 years, I apply the method and calculate the fraction to be 0.25. In addition, I find that the fraction is linked to the greening of the Earth, quantified as the increase in the photosynthesis on land and in the ocean. Finally, I render it probable that the increase in the non- fossil emission is caused by the temperature increase, the warming of the globe.