The term “biosphere” is commonly used to describe all of the living creatures on earth; and the term hydrosphere is used to describe all of the water at the surface of the earth. In the same sense, I would like to propose a new term: “carbonsphere”, to describe the sum of carbon reservoirs freely exchanging carbon with the atmosphere. For the purpose of modeling CO2 in the atmosphere, and understanding interactions of the atmosphere, biosphere and oceans, it is important to answer the question: “How big is the carbonsphere”?
Figure 2, for reference, shows the bulk CO2 concentration, commonly called the "Keeling Curve". We can compare the rising bulk CO2 curve to the falling carbon isotope curve shown below.
Finding Niño -- Correlation CO2 Carbon Isotopes in the Atmosphere with the El Niño Cycle