This is a brief discussion of the evidence that shows that the burning of fossil fuels since the Industrial Revolution has been responsible for almost all the observed increase in the atmospheric concentration of carbon dioxide; from 285 ppmv in pre-industrial times to over 385 ppmv today. As with most topics concerned with humans, there is controversy about whether we are to blame for this effect on the atmosphere
The graph shows the ever-increasing concentration of CO2 in the atmosphere (green) since 1900 and the remarkably parallel graph of estimates of the emissions of carbon into the atmosphere from the burning of fossil fuels (black). The correlation is clearly very strong and the value of the correlation coefficient between the two sets of data is 0.999, near to perfection. As ever, correlation does not prove causation and other evidence is required other than straightforward statistics.
The evidence for a dominating human role in the CO2 increase is extremely strong. The 36% increase (in 2006) in atmospheric CO2 observed since pre-industrial times cannot be explained in terms of natural changes. CO2 concentrations have varied naturally throughout Earth's history.
Furthermore, the observed progressive depletion in carbon-13 (see below) shows that the source of the CO2 is either fossil fuels or deforestation because they produce CO2 depleted in carbon-13. The atmospheric CO2 increase cannot have come from the oceans because that would not have caused any depletion of carbon-13. In fact, carbon in the oceans has itself become gradually depleted, with the greatest depletion at the surface. That implies that the signal is imposed from the atmosphere. The next piece of evidence is that we also observe a depletion of radioactive carbon-14 in the atmosphere and oceans, with the strongest signal in the atmosphere suggesting it is the place where the depletion originates. Fossil fuels contain no carbon-14, and their combustion produces CO2 without carbon-14. Deforestation does not cause a change in atmospheric carbon-14. The observed carbon-14 depletion in the atmosphere, oceans and land ecosystems is in fact quantitatively consistent with the amount of fossil fuels burned until now. Finally, annual mean CO2 concentrations in the northern hemisphere are higher than in the southern hemisphere, and more so in recent years compared to the early years of atmospheric CO2 measurements. This suggests a growing source of CO2 in the northern hemisphere, which is in fact where most of the fossil fuel burning takes place.
Natural carbon contains two stable [i.e., non-radioactive] isotopes. Mass 12 carbon represents 98.9% of the element and the remainder is mass 13 carbon. The ratio of the abundances of the two isotopes in standard limestone, 13C/12C is 0.01122. Reduction in the ratio caused by fossil fuel burning has produced a value of about 0.01113, 0.8% lower than the standard value. Carbon-14 is present in the atmosphere to a very small extent and is produced when cosmic rays react with the nuclei of atoms in the upper atmosphere to produce neutrons. These interact with nitrogen atoms to give carbon-14 which has a half-life of 5715 years and is useful in dating carbon-based materials.
The science is independent of whether recent CO2 increases are 100% anthropogenic or 100% natural. So, why is there so much reluctance to address the clear evidence that some of it could, in fact, be natural? In the 20th century, for the increase in the CO2 concentration to be entirely due to an increase in temperature that should have been around 5oC. That has not occurred and leads to the inevitable conclusion that the recent increases in CO2 concentrations are because of the burning of fossil fuels. The answer comes from a study of the temperature dependence of the Henry's law coefficient for the solubility of CO2 in seawater. This has been very carefully quantified by the Brookhaven National Lab and can be relied upon. That the 20th century saw an increase of temperature of 0.6 +/- 0.2 C implies that an increase in CO2 atmospheric concentration should have resulted of around 10 +/- 3 ppmv. That much more than this increase occurred seems to lead to the conclusion that there is another source. All the evidence we have in the literature points towards the burning of fossil fuels. So, at most, about 10 ppmv from a slight temperature increase and the rest is down to us.
Fates of Anthropogenic Emissions of CO2
Recent yearly emission means [2000-2005] were 7.2 +/- 0.3 Gt C of which 0.9 +/- 0.6 Gt C entered the biosphere and 2.2 +/- 0.5 Gt C were absorbed into the oceans. The atmosphere retained 4.1 Gt C and the CO2 concentration increased by ~1.9 ppmv per annum. These changes are shown in the following diagram.
Although the general turnover of carbon between surface and atmosphere is about 210/820 Gt C per year, i.e., 25.6% per year, this does not seem to happen to anthropogenic carbon. It might be expected from such turnover figures that ~74.4% of the anthropogenic carbon should remain in the atmosphere after one year but this is not so. The mean values [2000-2005] indicate that ~57% of any year's injection remains in the system, much less than might be expected if there was full mixing. The 57% figure is in agreement with the 13C/12C data so there must be a real difference between the kinetics of the interaction of the anthropogenic CO2 and its sinks than those of the general bulk atmosphere. Of course CO2 is CO2 is CO2..., but there is a large difference between the sources of anthropogenic CO2 and the naturally cycled gas.
The diagram below is based on the known distribution of land and oceans in the two hemispheres and on data from the latest BP Statistical Review.
Anthropogenic CO2 is produced mainly over the land and about 93% [BP Statistical Review 2008] is produced in the northern hemisphere where it initially has the first opportunity to interact at close hand with the land biosphere. In the next three months or so it mixes with the bulk of the NH and after another year is pretty well mixed with the SH. Of the ~43% absorbed by the sinks in one year the biosphere accounts for ~12.5% and the remainder [~30.6%] is taken up by the oceans. If there is indeed a difference in interaction with the sinks between the anthropogenic and general CO2 this needs to be sorted out.