1) The Keeling Curve
In this post, I generate a model for the global CO2 record from 1971 - 2009. Inputs to the model include agricultural biomass, fossil fuel emissions, absorption of excess CO2 by carbon sinks, and atmospheric mixing between the Northern and Southern Hemispheres.
The ease with which the model was created, and the lack of any reasonable, quantifiable alternatives, indicate that changes in atmospheric CO2 are primarily the result of human activity.
This model considers separately the CO2 flux in the Northern Hemisphere and the Southern Hemisphere, and matches observations for the rate of mixing between the hemispheres. A more complex model could be built, perhaps at increments of 5 or 10 degrees of latitude, and more closely identify the location of agriculture and fossil-fuel emissions, and that might be useful to addressing deeper questions. But I believe a model should be simple in essence; sufficiently complex to answer the question at hand, and not any more complex. This model is intended to answer the question of human influences on global CO2, and the division of the globe into two hemispheres is sufficient to answer that question.
- 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.
I constructed a model for annual CO2 uptake through photosynthesis, beginning with the volume of biomass generated through agriculture. Agriculture generates about 140 gigatonnes of biomass every year.
Adjustments for moisture content (50%), carbon content (45%), and conversion to CO2 (3.67x) results in about 96 gigatonnes of CO2 removed from the Northern Hemisphere atmosphere annually. Keep in mind that this is only half of the air on the planet. Thus, during the growing season, CO2 in the Northern Hemisphere falls sharply.
In the model, I 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.
I assigned the 96 gigatonnes of agricultural CO2 intake in the summer growing months, as shown in the following graph. For the oxidation part of the cycle, we can observe a very sharp rebound in CO2 in the data during the fall months. It is possible that some of the rebound is from CO2 sinks, seeking equilibrium after the change during the growing season. However, isotope data shows an equally sharp rebound. (http://dougrobbins.blogspot.com/2012/03/seasonal-carbon-isotope-cycles.html) It appears to me that vegetation is giving back to the atmosphere the very same CO2 that was absorbed during the summer.
I adopted an oxidation/respiration model to return the CO2 to the atmosphere as a zero-sum annual exchange. I tried an exponential decline for the oxidation part of the cycle, then tweaked it to match the annual CO2 cycles of the Northern Hemisphere high latitudes (with the long-term trend removed).
This model produced a surprisingly easy fit to the high latitude data of the Northern Hemisphere (see below).
Note that the long-term rising CO2 trend has been removed from the real world data, and there are no CO2 emissions from fossil fuels in the model at this point.
Annual cycles from intermediate latitudes have lower amplitude than cycles from the high northern latitudes. This was the topic of an earlier post: http://dougrobbins.blogspot.com/2012/03/keeling-curve-and-seasonal-carbon.html. The Northern Hemisphere, with its large CO2 fluctuations, dominates global CO2 cycles. CO2 cycles from low latitudes in the Southern Hemisphere (pink) follow the seasonal pattern of the Northern Hemisphere, showing the range and influence of atmospheric mixing between Northern and Southern Hemispheres.
The model shows that the long-term trend of rising CO2 is attributable to fossil-fuel emissions. Fossil fuel emissions account quantitatively for the rise in CO2 over the last 38 years, and fit the data with regard to differences in concentration in the Northern and Southern Hemispheres.
The model also shows that the annual cyclicity of the biologic cycle is strongly influenced by agriculture. Agricultural biomass alone can be used to model and match observed data for seasonal CO2 cyclicity.
And finally, the uptake of CO2 through agriculture clearly outpaces emissions of CO2 from fossil fuels, at least on a seasonal basis. As a tool for the management of CO2 concentrations, policy-makers should consider banning the burning of agricultural waste, and consider options for disposal of agricultural waste as a means of sequestering significant volumes of carbon.
Global CO2 concentration data in this report is credited to C. Keeling and others at the Scripps Institute of Oceanography, also Gaudry et al, Ciattaglia et al, Columbo and Santaguida, and Manning et al. The data can be found on the Carbon Dioxide Information Analysis Center.
Data for CO2 released by fossil fuels is available from EIA CO2 Emissions from Fuel Consumption,
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: