"CO2 Peculiarities – The C13/C12 Isotope Ratio", Roy Spencer, Ph.D.

Source:
https://wattsupwiththat.com/2008/01/28/spencer-pt2-more-co2-peculiarities-the-c13c12-isotope-ratio/

"January 28, 2008 

In my previous post, I showed evidence for the possibility that there is a natural component to the rise in concentration of CO2 in the atmosphere. 

Briefly, the inter-annual co-variability in Southern Hemisphere SST and Mauna Loa CO2 was more than large enough to explain the long-term trend in CO2. 

Of course, some portion of the Mauna Loa increase must be anthropogenic, but it is not clear that it is entirely so.

Well, now I’m going to provide what appears to be further evidence that there could be a substantial natural source of the long-term increase in CO2.

BOTTOM LINE:
If the C13/C12 relationship during NATURAL inter-annual variability is the same as that found for the trends, how can people claim that the trend signal is MANMADE?

DETAILS:
One of the purported signatures of anthropogenic CO2 is the carbon isotope ratio, C13/C12.  

The “natural” C13 content of CO2 is just over 1.1%. 

In contrast, the C13 content of the CO2 produced by burning of fossil fuels is claimed to be slightly smaller – just under 1.1%.

The concentration of C13 isn’t reported directly, it is given as “dC13”, which is computed as:

“dC13 = 1000* {([C13/C12]sample / [C13/C12]std ) – 1

The plot of the monthly averages of this index from Mauna Loa is shown in Fig. 1.

Now, as we burn fossil fuels, the ratio of C13 to C12 is going down. 

From what I can find digging around on the Internet, some people think this is the signature of anthropogenic emissions. 

But if you examine the above equation, you will see that the C13 index that is reported can go down not only from decreasing C13 content, but also from an increasing C12 content (the other 98.9% of the CO2).

If we convert the data in Fig. 1 into C13 content, we find that the C13 content of the atmosphere is increasing (Fig. 2).

So, as the CO2 content of the atmosphere has increased, so has the C13 content…

which, of course, makes sense when one realizes that fossil-fuel CO2 has only very slightly less C13 than “natural” CO2 (about 2.6% less in relative terms). 

If you add more CO2, whether from a natural or anthropogenic source, you are going to add more C13.

The question is: how does the rate of increase in C13 compare to the CO2 increase from natural versus anthropogenic sources?

First, lets look at the C13 versus C12 for the linear trend portion of these data (Fig. 3).

The slope of this line (1.0952%) represents the ratio of C13 variability to C12 variability associated with the trend signals. 

When we compare this to what is to be expected from pure fossil CO2 (1.0945%), it is very close indeed: 97.5% of the way from “natural” C13 content (1.12372%) to the fossil content.

At this point, one might say, “There it is!  The anthropogenic signal!”.  But, alas, the story doesn’t end there.

If we remove the trend from the data to look at the inter-annual signals in CO2 and C13, we get the curves shown in Figures 4 and 5.

Note the strong similarity – the C13 variations very closely follow the C12 variations, which again (as in my previous post) are related to SST variations (e.g. the strong signal during the 1997-98 El Nino event).

Now, when we look at the ratio of these inter-annual signals like we did from the trends in Fig. 3, we get the relationship seen in Fig. 6.

Significantly, note that the ratio of C13 variability to CO2 variability is EXACTLY THE SAME as that seen in the trends!"