Summary of atmospheric CO2 sources and sinks (Don Bogard) USofA

SUMMARY OF ATMOSPHERIC CO2 SOURCES AND SINKS Don Bogard April, 2018 1) Human activities (direct and indirect, especially fossil fuel burning), have been the largest cause of growing atmospheric CO2 (~42%) over the past 150 years. The quantity of fossil fuel burned and of CO2 produced is rather well known. Figure 1 shows world, humancaused CO2 emissions from 1850 to 2010, and its tremendous growth from fossil fuel use since 1950.

Earth’s atmosphere currently contains about 7.5 X1011 tons of CO2, and human activities currently add about 4 X1010 tons per year. Figure 2 shows how the relative sources of human-produced CO2 have changed over the past 160 years.

2) Approximately half of the total CO2 human activities added to the atmosphere in 150 years has moved into other C-containing reservoirs, mainly dissolution into the oceans and increased growth of green plants.

3) Atmospheric CO2 movement into other reservoirs has its basis in the simple principle of chemical equilibrium partitioning. When a molecule (here CO2) exists in two reservoirs, then that molecule will partition itself between these two reservoirs in a proportion that depends on characteristics of each reservoir and some environmental parameters (e.g., temperature).

4) A molecule (e.g., CO2) partitioned between two reservoirs will undergo molecular exchange that does not alter the partitioning ratio. That means individual CO2 molecules in the Atmos are continually dissolving in the oceans, and CO2 molecules in the oceans are continually replacing them in the Atmos. The same occurs between CO2 in Atmos and many plants (via photosynthesis and respiration). As the CO2 concentration in the Atmos increases, then the exchange rates will favor moving more CO2 from Atmos to the other reservoir than returns, and the CO2 partitioning ratio between two phases will change until a new equilibrium is restored.

5) With a 42% concentration increase, Atmos CO2 has a strong force driving more into the oceans. However, C in the oceans exists in several phases (e.g., dissolved CO2, bicarbonate, carbonate, carbonic acid), and specific equilibria exists among them all. Only a fraction of ocean C exists as dissolved CO2. As more CO2 dissolves in oceans, the equilibria shift away from CO2.

1

6) As the water temperature increases, simple dissolved gases, including CO2, become less soluble. In the past century, average ocean Temp has increased very little, less than 1 deg-C, so its capacity to hold dissolved CO2 is only slightly decreased. CO2 dissolving into the ocean has been a stronger and more important process than CO2 ocean degassing.

7) Figure 3 shows Earth’s major surface reservoirs of carbon (not just CO2). Also shown are the amounts of C in each reservoir, and the earlier equilibrium amount of C per year (in black) exchanging between each reservoir. The recent increase in Atmos CO2 has caused these normal exchange rates to alter (changes shown in red), such that more C is now moving out of the atmosphere than moving into it.

There is a time constant required for each reservoir’s C partitioning to reach equilibrium. For most green plants this time constant is short, intermediate for the shallow oceans and soil, and very long (centuries) for the deep oceans.

Because Atmos CO2 almost continually increases, none of these reservoirs currently have equilibrium C partitioning. Also shown in red is the approximate yearly rate of C addition due to fossil fuel burning.

If somehow the atmospheric growth of CO2 were to totally cease, most likely Atmos CO2 would decrease so as to bring these reservoirs into real equilibrium C partitioning.

8) Some have attempted to use the carbon isotopic composition of Atmos CO2 to determine its sources. {Carbon has two stable isotopes, 12C and 13C, and radioactive 14C made in the atmosphere by cosmic ray interactions.} However, the 13C/12C ratio varies considerably among different surface C reservoirs (see Figure 4). Because Atmos CO2 likely exchanges with many of these reservoirs, the problem is insufficiently constrained.

9) The fraction of 14C in these C reservoirs depends on how long they have been isolated from the atmospheric source of 14C and with the rate each reservoir exchanges C with other reservoirs. The rate at which 14C is made in the atmosphere is approximately known, and the 14C half-life is 5730 years. Thus, fossil fuels have been isolated for very long times and contain no 14C. Many green plants exchange C with the atmosphere frequently and have 14C concentrations similar to atmospheric C. {The slow decay of 14C when a tree dies forms the basis for 14C age dating.} 2

Knowing the equilibrium production rate of 14C in the atmosphere prior to 150 years ago (when Atmos CO2 began to increase), one might calculate how much addition of fossil fuel C should have lowered Atmos 14C. However, the value so obtained would be too large, because much of the added fossil fuel C has been exchanged into other surface reservoirs, and thus the Atmos 14C content is constantly diluted. Some have estimated that the atmosphere today contains only ~4-5% of the C atoms originally produced by fossil fuel burning.

3