Carbon, as carbon dioxide, enters and leaves the atmosphere by various processes, known as the CARBON CYCLE, depicted below.
From these carbon flows, it can be seen that although there are many CO2 sources and sinks, there are only two main sinks: the atmosphere and the ocean. Quantities are in petagrams (1015 grams, or billion tonnes of carbon per year.
The plot below shows that from 2009 to 2020 41.2% of CO2 emitted from fossil fuels is absorbed, primarily, by the ocean:
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 increases, CO2 in atmosphere in atmosphere increases (oceans emit dissolved CO2), and vise-versa (oceans absorb CO2), per Henry’s Law.
HENRY’S LAW: The dependence of the absorption of a gas by a liquid as a function of partial pressure of gas and liquid temperature.
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)
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).
Notice that proportionality constant k is the same as ko in the set of dissociation equations:
dissolved CO2 as [H2CO3] = ko*PCO2
[HCO3–] = k1*[H2CO3]/[H+]
[CO3=] = k2*[HCO3–]/[H+]
Where bracketed species are concentration, typically in moles per kg of solvent. These equations also show the dependency on pH.
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: