The average CO2 concentration in the Southern Hemisphere lags the Northern Hemisphere by about 2.7 ppm. Looking at it another way, rising CO2 in the Southern Hemisphere lags the Northern Hemisphere by about 21 months.
We see some interesting features when we look at the seasonal cycle closely. Northern Hemisphere cycles are high amplitude, while the Southern Hemisphere is very low amplitude. The expected polarity reversal only occurs in high Southern latitudes (near the pole). Readings from latitudes less than 30 degrees south (near the equator; Kermadec Islands and American Samoa) share the polarity of the Northern Hemisphere.
Global cyclicity is dominated by seasons in the Northern Hemisphere. Polarity of the cycle in low latitudes (near the equator) of the Southern Hemisphere follows the seasonal patterns of the Northern Hemisphere. Polarity of the seasonal cycle is reversed near the pole in the Southern Hemisphere.
This long-term observed trend of isotopically lighter CO2 is consistent with an increasing contribution of fossil fuels to atmospheric CO2. A simple calculation combining the isotopic composition of fossil fuels and the atmosphere would predict an even larger decline in atmospheric dC13/C12. The modest decline observed in the data shows involvement of other carbon sinks in the environment, exchanging carbon with the atmosphere and moderating the dC13/C12 decline in the atmosphere.
The Northern Hemisphere, with much greater fertile land area than the Southern Hemisphere, removes a significant volume of light carbon from the atmosphere during the growing season. The isotope cycle shows an asymmetry similar to the asymmetry of the CO2 cycle. The isotopic composition of the atmosphere in the Northern Hemisphere rises sharply in the summer, and then declines gradually as a result of atmospheric mixing and oxidation of the biomass following the growing season.
*C14, an unstable radioactive isotope, occurs in trace amounts in nature. The radioactive isotope is important for age-dating anything containing carbon, within about 10 half-lives of the isotope, or about 60,000 years before present. C14 was also produced by nuclear weapons but has been rapidly decreasing in the environment since the cessation of above-ground nuclear testing.
Modeling Atmospheric CO2
- CO2 taken up by Plants during the growing season
- Oxidation of carbon in plants following the growing season
- CO2 emissions from Fossil Fuels
- Absorption of CO2 by carbon sinks (e.g. oceans)
- Exchange of CO2 between Northern and Southern Hemispheres.
The CO2 seasonal cycle is dominated by the Northern Hemisphere, representing 67% of the earth's landmass, 90% of the human population (agriculture), and 83% of the industrial activity (GDP). Modeling the cycle required consideration of fossil fuels and the photosynthesis/oxidation cycle.
Upon seeing the seasonal CO2 cycle in the Northern Hemisphere, my initial thought was that seasonal burning of fossil fuels accounted for much of the fluctuation. Monthly consumption of oil, coal, and natural gas does show a seasonal fluctuation, with the correct polarity for the observed CO2 cycle. However, fossil fuel consumption in the Northern Hemisphere produces only a 0.5 ppm seasonal cycle in atmospheric CO2, as compared to the 17 ppm cycle observed in actual data. The following chart of seasonal CO2 emissions (with long-term growth of CO2 removed) was calculated from 2009-10 data from EIA and the BP Statistical Review of World Energy.
The model for seasonal CO2 uptake through photosynthesis was constructed beginning with the volume of biomass generated through agriculture. Agriculture generates about 140 gigatonnes of biomass every year.
After adjustments for the portion in the Northern Hemisphere (83%), moisture content (50%), carbon content (45%), and conversion to CO2 (3.67x) we can calculate about 96 gigatonnes of CO2 removed from the Northern Hemisphere atmosphere during the summer growing season. Thus, during the growing season, CO2 in the Northern Hemisphere falls sharply.
The model distributed agricultural carbon and fossil fuel use according to economic output by hemisphere. The Northern Hemisphere represents 83% of global economic output, and the Southern Hemisphere represents 17% of global economic output.
The following chart shows the monthly scheduling of photosynthesis, oxidation and fossil fuel consumption in the model for the Northern Hemisphere.
In modeling the Southern Hemisphere, I found that 17% of global agricultural biomass produced CO2 fluctuations that were far too large to match the data in the Southern Hemisphere. I found a good match by using only 5% of global agricultural biomass. The chart below shows the model parameters.
Here is the modeled match to high-latitude Southern Hemisphere CO2, using 5% of global agricultural biomass. It seems likely to me that the Southern Hemisphere photosynthesis/oxidation cycle is overwhelmed by CO2 mixing from the Northern Hemisphere, thus requiring a smaller volume to match the data.
Although the fossil fuel input is much too small to account for the seasonal fluctuation in CO2, the long term effect is significant. Carbon dioxide from fossil fuel emissions was added according to estimates from IEA and the BP statistical review of world energy. Annual figures in these reports were scheduled on a monthly basis by analogy to US monthly consumption of coal, natural gas, and oil. The volume of fossil fuel CO2 emissions was reduced by 40% to reflect the volume of CO2 absorbed by carbon sinks.
As previously noted, rising CO2 in the Southern Hemisphere lags CO2 in the Northern Hemisphere by a period of about 22 months. The model was constructed to transfer half of the fossil fuel CO2 of the Northern Hemisphere to the Southern Hemisphere, using a lag of 22 months to represent the necessary mixing time.
Despite the general simplicity of the model, the resulting CO2 curve shows a reasonable correlation to actual data recorded across the global range of latitudes, and after 38 years of CO2 addition and subtraction, the model concludes at the appropriate concentrations of CO2 across the globe.
The most important conclusion from modeling global CO2 is that both long-term and seasonal change in atmospheric CO2 can be easily modeled using only inputs from human activities.
Other conclusions are as follows:
6) Calculations comparing isotope changes from fossil fuel emissions to the observed carbon isotope record suggest that the total reservoir of environmental carbon (including the atmosphere) is about three times the volume of carbon in the atmosphere.
This post is a summary of previous posts about atmospheric CO2. Additional details about the work can be found in these posts.
And the BP Statistical Review of World Energy:
Monthly data for US fossil fuel consumption were taken from the EIA website:
Global population figures from 1970 - 2010 were taken from Wikipedia.
The estimate for annual global biomass, circa 2009 was taken from a UN report: