One of the key "fingerprints" of human influence in the rise of atmospheric carbon dioxide is the declination in d13C, the ratio between 13C/12C.
Fossil fuels are supposed to have more of the carbon-12 isotope in comparison to the carbon-13 isotope compared to what's "average" in the atmosphere prior to burning. Hence burning fossil fuels should decrease the 13C/12C ratio. And this is what's found.
But while human emissions of CO2 from fossil fuel combustion are only increasing with time (you should see what China's doing)...
....the decline in the ratio of 13C/12C is failing to keep pace, following a basically linear path.
Update 6 Dec 2015: Not only are human emissions increasing exponentially but so is atmospheric carbon dioxide. You can get the data here. MLO = Mauna Loa, Hawaii.
|Mauna Loa CO2. Black line by me for reference to linear trend|
Data for 13C/12C ratio can be obtained here:
You can look up the data at individual sites here.
I haven't found a suitable global average yet, so I picked 8 locations at random and plotted the d13C (blue line) and added a moving average in excel (red line).
I also put a straight line (black) freehand on the d13C graphs to show that the trend of the data is close to linear instead of slightly exponential as you'd expect given the rise of fossil fuel combustion.
The graphs with a three letter place name abbreviation, e.g. "MLO", show the d13C time series.
I then tried to make a rough graph of the trend in d13C by subtracting a group of about 8 cells from the previous 8 cells (my worksheet). These graphs have the same three letters plus "diff" in the title, short for "difference". E.g. "MLO diff". I then put a linear trend line on the diff graphs (amber line).
A declining trend in the amber line means d13C is falling slower with time. If CO2 rise was from fossil fuel burning as per the diagram above, the rate should be increasing not decreasing.
There's a declining trend in 7 of the 8 sites I picked. The one site that does have an increasing trend in -d13C has some weird data around 2014 that may have skewed the trend (amber line).
If the CO2 rise really was caused by fossil fuel combustion, the rate of d13C decline should also increase but it isn't keeping pace.
This could mean that other sources with a more 'neutral' value for d13C, such as the CO2 dissolved in the ocean, could be taking over from fossil fuel combustion somewhat in causing the increase. The ocean is after all supposed to be taking up heat, and this would cause CO2 to be less soluble, hence lead to outgassing.
It should also be noted that rises in atmospheric CO2 accompanied by a decline in d13C – a so-called fingerprint – can also be caused by deforestation. So even the declination of d13C is not an exclusive fingerprint of fossil fuel combustion as is often portrayed by Team Consensus™ [1, 2]. More on this in an upcoming post.
The y-axis in the following graphs is the declination of 13C/12C ratio to arbitrary standard (δ13C) .
KEY = Key Biscayne, Florida, United States
KEY seems to have some weird data around 2014 that may skew the difference.
MLO = Mauna Loa, Hawaii, United States
S05 = Pacific Ocean (5 S)
SMO = Tutuila, American Samoa
SPO = South Pole, Antarctica
UUM = Ulaan Uul, Mongolia
WLG = Mt. Waliguan, Peoples Republic of China
ZEP = Ny-Alesund, Svalbard, Norway and Sweden
Unfortunately the foregoing only has data from about 1990 onward. You can get some longer running data here. Some graphs go back to the 80's.
Here is an example, and it too doesn't show the acceleration in C13 decline you'd expect if the CO2 rise was all from fossil fuel combustion.