Fully modeling the global CO2 cycle requires understanding of not only the atmosphere, but of the carbon reservoirs that freely exchange carbon with the atmosphere. The biosphere is the most obvious of those reservoirs. Atmospheric CO2 data show a strong seasonal cycle that reflects the growing season in the Northern Hemisphere. The seasonal cycle is a prominent feature on charts of both bulk atmospheric CO2 and carbon isotope data. As discussed in previous posts, the seasonal cycle is strongest in high latitudes of the Northern Hemisphere, and very weak, with opposite polarity, in the Southern Hemisphere.
Curves are color-coded by latitude; cool colors represent the Northern Hemisphere, and warm colors represent the Southern Hemisphere. A key map is given in figure 3.
Plants preferentially take up C12 during the growing season, which changes the ratio of C13 to C12, and causes the seasonal signal in carbon isotopes.
By using a one-year rolling average, the seasonal cycle can be removed from the data, yielding the long-term trend of atmospheric CO2 (figure 3).
After removing the long-term trend from global CO2 observations, the remainder is the seasonal cycle. A casual look at the chart reveals increasing amplitude of the cycle over the period of observations, particularly at high latitudes in the Northern Hemisphere. The amplitude can be extracted from the data, as shown in Figure 5.
Amplitude of the seasonal CO2 cycle is growing. If we add human population to the chart, we see a clear correlation between amplitude at high latitudes and human population.
We can make a linear proportion between the amplitude of the seasonal cycle and population. We can simply divide change in amplitude by the change in population, over the period of observations. From that, we extrapolate that ratio to the point of zero population. This gives us the portion of the seasonal cycle that is the result of human activities, and the remainder, which is the naturally occurring cycle..
The following table shows the fraction of the cycle amplitude which is due to human activities, and the fraction that is due to nature.
Fraction of Seasonal Cycle Amplitude
Alert, Canada 30.2% 69.8%
Barrrow, Alaska 37.8% 62.2%
La Jolla, California 74.0% 26.0%
Baha, Mexico 30.8% 69.2%
Mauna Loa, Hawaii 37.0% 63.0%
Kumukahi, Hawaii 34.5% 65.5%
The La Jolla data appears widely different from the other data, and was dropped from further calculations. The average of the consistent observations shows that 34% of the 2009 seasonal cycle is the result of human activities, and 66% is the result of natural causes.
Made-Made CO2 Flux; Calculating Agriculture vs. Fossil Fuel Influence
Global agriculture produced 140 gigatonnes of biomass in 2009, removing about 96 gigatonnes of CO2 from the atmosphere. Allocating agriculture by hemisphere by population gives a seasonal flux of 84.5 gigatonnes of CO2 in the Northern Hemisphere.
By contrast, total fossil fuel combustion produced 28.9 gigatonnes of CO2, substantially less than the seasonal CO2 flux due to agriculture. And because fossil fuel use through the year is relatively consistent, data shows that there is only a 3.1 gigatonne seasonal flux of CO2 from fossil fuels in the Northern Hemisphere. These figures show that 90 percent of the man-made portion of the seasonal CO2 cycle is due to agriculture.
Calculating the Size of the Northern Hemisphere Biosphere
The relation established between amplitude of the seasonal cycle and volume of agricultural biomass, allows us to estimate the volume of natural biomass. Ninety percent of the man-made amplitude change, or 30.6%, is due to agriculture, representing 140 gigatonnes of biomass, annually. Natural forests, grasslands, and other seasonal growth in the Northern Hemisphere are responsible of 66% of the seasonal amplitude. According to the volumetric relationship between seasonal amplitude and biomass established by agriculture, we can estimate the volume of plant growth in the temperate (seasonal) Northern Hemisphere. The volume of natural, seasonal plants in the Northern Hemisphere is a little more the double the volume of agriculture, or 302 gigatonnes of biomass. After corrections for carbon content, the natural seasonal volume in the Northern Hemisphere is 68 gigatonnes of carbon.
This estimate is based on the seasonal CO2 cycle of the Northern Hemisphere, and necessarily can only represent seasonal plant growth of the Northern Hemisphere. All Southern Hemisphere biomass, equatorial oceans and rainforests, and non-seasonal biomass are excluded from the estimate. But this finding may still be useful in modeling global the global CO2 cycle.
It is worth a quick comparison with other published estimates of the total global plant biomass. Three respected sources include the U.S. Global Change Research Program, the U.S. Carbon Dioxide Information Analysis Center, and David McKay’s book, “Sustainable Energy Without the Hot Air”.
The US Global Change estimate for global vegetation biomass is 650 gigatonnes; the CDIAC estimate is 550 gigatonnes; and McKay’s cited estimate is 700 gigatonnes.
Our estimate of 300 gigatonnes for natural Northern Hemisphere seasonal biomass, plus 140 gigatonnes for agriculture yields 99 gigatonnes of carbon for seasonal growth in the Northern Hemisphere. This appears to be reasonable, considering the range of estimates for global vegetation given above, including equitorial (non-seasonal) regions, the Southern Hemisphere, and the oceans. If all estimates are correct, Northern Hemisphere seasonal growth represents between 1/5 and 1/7 of total vegetation in the world
And the BP Statistical Review of World Energy:
Global population figures from 1970 - 2010 were taken from Wikipedia.
The estimate for annual global agricultural biomass, circa 2009 was taken from a UN report:
Monthly data for US fossil fuel consumption were taken from the EIA website:
Estimates for global vegetation biomass were found on sites for the CDIAC, US Global Change Research Program, and David McKay's "Without the Hot Air".
This link is broken, but is the source for data used in this blog post. It was a publication of the US Global Change Research Progam.