Seasonal variations in CO₂ concentrations

Global mean variations of temperatures and those of the northern and southern hemispheres are known from satellite and terrestrial records. If it is assumed that no other factors influence the concentration of CO₂ the equilibrium values can be calculated from programmes such as the one supplied by the Brookhaven National Laboratory. Results from such calculations are shown in Figures 1 and 2.

 

 

Figure 1 The calculated monthly mean equilibrium values of CO₂ concentrations for the globe and the N/S hemispheres

The data yield a mean value for the dependence of CO₂ concentration upon surface temperature of 17.5 ppmv K^-1. This should be regarded as the maximum possible value since equilibrium is never established.

Such concentrations as those shown in Figure 1 are never found in real circumstances because at all times there are the competing processes of photosynthesis and vegetation decay, both of which cause variations of CO₂ concentrations of a similar magnitude to those calculated for the effect of temperature.

Figure 2 shows the actual values for the variation of CO₂ concentration throughout the year and the deviations from the mean values of the CO₂ concentration derived from the data given in Figure 1.

 

 

Figure 2 Plots of the Mauna Loa actual data for the monthly variation of CO₂ concentration, the calculated global variation based upon the effect of temperature on the oceans and the derived variation due to the photosynthetic/vegetation decay cycles. Note the different scales for the calculated values and those from Mauna Loa

Figure 2 also shows the derived variation of CO₂ concentration based upon the actual Mauna Loa values and the calculated values for the mean ocean temperature dependence. Essentially the Photo/decay curve represents the differences between the Mauna Loa data and the calculated values for the CO₂ concentrations that could be expected if the temperature of the oceans were the only causal factor. The conclusion is that the Mauna Loa data, representative of the global situation, is the difference between two much larger varying quantities.

The latest version of the carbon cycle is shown in Figure 3.

Figure 3 The carbon cycle

This shows the two main flows of CO₂ between the atmosphere and the surface as the ~90 Gt C per year exchanging between the atmosphere and the ocean surface and the ~120 Gt C per year exchanging between the atmosphere and the biosphere in the annual photosynthesis/vegetation decay cycle. These in general oppose each others effect on the total amount of carbon in the atmosphere so that the range of amounts as measured at Mauna Loa is only  12.1 ± 1.1 Gt C per year. The flows of carbon into the atmosphere due to changing land use of 1.6 Gt C per year and the 5.5 Gt C per year from fossil fuel burning and cement manufacture are the main cause of the continual year-by-year increase of CO₂ concentration of 3 ± 1.3 Gt C per year, an amount equal to the remainder of the carbon being dispersed in the oceans and in extra growth of vegetation.