In this section we describe the carbon isotope evidence for the increasing carbon dioxide concentration in the atmosphere being due almost entirely to our fossil fuel and wood burning activities
The graph shows the changes that have taken place in the ratio of 13C to 12C in the atmosphere since January 1980. The data are those from the Mauna Loa site in Hawaii. More general data are plotted further down this page.
The 13C/12C ratio in the vast majority of carbon in the world has the particular value of 0.0112372 but fossil fuels have values that are smaller than this due to the isotopic discrimination that takes place in their formation in which the lighter isotope is favoured. Thus coal has a value of the ratio that is 2.2 % lower than the standard value, expressed as a delta 13C/12C value of -22 per mil [per mil = per cent divided by 10], oil has a value of -30 and natural gas an even lower value of -35. Burning fossil fuel injects CO2 into the atmosphere with a depleted 13C content and that causes the value of the d13C/12C of the atmosphere, when well mixed, to be smaller. The graph shows this continuing depletion which is consistent with the fossil fuel carbon injections. In 1980 the d13C/12C of the atmosphere was around -7.5 per mil and over 20 years [the period shown in the graph] the value decreased to around -8.2 per mil.
The above represents hard evidence for the increase in CO2 concentration in the atmosphere since pre-industrial times being caused by human activities.
The disconnection between the annual increases of CO2 and the seasonal changes is outlines on the previous page. The graph below serves to underline this disconnection and shows how the seasonal changes to d13C/12C of the atmosphere occur.
During the year there are the large exchanges of carbon between the atmosphere and the oceans. These are isotopically neutral since very little happens that depends upon the slightly different masses of 13CO2 and 12CO2. The changes in the value of d13C/12C of the atmosphere during the year are explicable in terms of the photosynthesis/decay cycle. Photosynthesis is a very complex process involving many coupled chemical reactions and isotopic discrimination is to be expected. The lighter isotope is favoured making the vegetation richer in 12C as are the precursors of the fossil fuels. The formation of the latter with many more discriminatory processes involved allows even more depletion of 13C. Vegetation produced by photosynthesis has a d13C/12C value of around -20 per mil, but the effect of this on the atmosphere observed from January to June is effectively reversed when decay of vegetation occurs in the remaining months of the year. So, the changes in d13C/12C of the atmosphere observed during the course of any year are due to natural processes that are cyclic with the seasons and the annual increases in d13C/12C are due to the injections of fossil fuel 13C depleted carbon.
The general distribution of the values of the 13C/12C ratio are shown in the following graph.
This shows the remarkable asymmetry between the northern and southern hemispheres and the seasonal variations that are described for the Mauna Loa data. Globally, there is a general reduction in the isotope ratio that is due to the injection of carbon into the atmosphere that originates in fossil fuels that are know to be considerably depleted in the heavier isotope because of the many chemical changes which have occurred in their production. Each chemical stage discriminates against the heavier isotope's participation in the reaction that is occurring.
It is very significant that the major isotope ratio changes occur in the northern hemisphere where approximately 93% of fossil fuels are burned.