1. Introduction
Throughout human history, the CO2 concentration in the air has significantly increased due to the anthropogenic combustion of fossil fuels, but the concentration of CO2 is not as high as the amount that has been emitted. Why? The active pools of the carbon cycle that can take up CO2 include the atmosphere, terrestrial biosphere, recoverable fossil fuels, and oceans. Beside these uptakes, there is a global, unidentified carbon sink of approximately 4000 gigatons that remains a mystery (Smith and Smith, 2009).