This section continues the discussion of the carbon cycle and looks in detail at the annual cycle observed in the Mauna Loa carbon dioxide analyses
This diagram shows the changes that occurred during the year 2007 to the monthly mean concentration of CO2. The changes are typical of all the years for which there is data and there are two aspects to the results. One is the regularity of the changes; the maximum is observed in May and the minimum in September of each year. The other notable change is the overall increase each year which is more obvious in the diagram on the previous page.
The changes in carbon dioxide concentration in the atmosphere for the last nine years and the current one are shown below.
The seasonal changes are consistent with the opposing changes of fluxes to and from the atmosphere from the biosphere and the oceans. The fluxes are very large compared to the seasonal changes which are around 5-6 Gt C per annum. The seasonal change must represent the resultant change between the two major flows. May is the height of the growing season in the northern hemisphere and also the time when the northern oceans are warming and those in the south are cooling. The process of photosynthesis causes a reduction in the CO2 concentration together with the reduction due to the cooling of the southern oceans and this change is opposed by the increase due to the warming of the northern oceans. The annual amplitude - the change observed between May and September of each year - does vary but is not connected to the annual increases.
That the disconnection is apparent from the above plots. The changes in the ever-increasing CO2 concentrations [also observable from the Mauna Loa graph of the previous page] are shown in red and have very little connection with the seasonal amplitudes shown in green. The correlation coefficient between the two sets of data is only 0.2, an indication of a fairly small statistical significance. This is an important conclusion since there are some attempts to associate the annual increases to the seasonal inflows and outflows in the carbon cycle.