Carbon, as carbon dioxide, enters and leaves the atmosphere by various processes, known as the CARBON CYCLE, depicted below.
However, actual data disagrees with the close balance depicted above, the oceans dominating. My argument:
From 1981 to 2010 the amount of carbon emitted to earth’s atmosphere by burning fossil fuels was 200,909 million tonnes. Over that period, the increase in atmospheric carbon is 109,116 million tonnes. That leaves 91,893 tonnes of carbon unaccounted for, or about 1/2 of total carbon emissions.
Most of the unaccounted for carbon is absorbed by the oceans as CO2. The oceans cover about 70% of the earth’s surface. I consider the seas as a “big bottle of pop”, that is, when temperature is increases, CO2 in atmosphere in atmosphere increases (oceans emit CO2), and vise-versa (oceans absorb CO2), per Henry’s Law.
In the graph below, notice the coincidence of the peaks and valleys of the red line (predicted by Henry’s Law), the blue line (changes in atmospheric CO2) and the green line (ocean temperature variation).
Henry’s Law states that at a given temperature, the partial pressure of a compound in the gas phase is proportional to the concentration of that compound in liquid solution, or
P = k C
The proportionality constant k is dependent upon temperature, and for CO2 that relationship is
k2 = k1 exp (2400* (1/T1 – 1/T2)
When water absorbs CO2, its pH is lowered. In the case of the oceans, they are made less alkaline but never become acidic (pH never becomes < 7). See analysis of the effect of absorbed CO2 on oceanic pH for calculations and results.
In the figure below, the blue line follows the red, showing that Henry’s Law is in effect:
Data for fossil fuel combustion carbon is from the Dept. of Energy’s Oak Ridge National Laboratory. Data for atmospheric carbon is from NOAA’s Mauna Loa Observatory. A tonne is one metric ton, or 2200 pounds.