NOTE: This post is the second in the series from Dr. Roy Spencer of the National Space Science and Technology Center at University of Alabama, Huntsville. The first, made last Friday, was called Atmospheric CO2 Increases: Could the Ocean, Rather Than Mankind, Be the Reason?
Due to the high interest and debate his first post has generated, Dr. Spencer asked me to make this second one, and I’m happy to oblige.
Here is part2 of Dr. Spencer’s essay on CO2 without any editing or commentary on my part.
(Side note: Previously, I erroneously reported that Dr. Spencer was out of the country. Not so. That was my mistake and a confusion with an email autoresponse from another person named “Roy”. Hence this new update.)
More CO2 Peculiarities: The C13/C12 Isotope Ratio
Roy W. Spencer
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.
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!
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??
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Could you post a link or a citation to the data you used in your analysis?
The monthly C13/C12 ratio data from Mauna Loa (1990-2005) are available here:
ftp://ftp.cmdl.noaa.gov/ccg/co2c13/flask/month/mlo_01D0_mm.co2c13
The monthly Mauna Loa CO2 data (1958-2007) are contained in the 5th file listed here:
ftp://ftp.cmdl.noaa.gov/ccg/co2/trends
[…] CO2 Peculiarities: The C13/C12 Isotope Ratio Spencer Part2: More CO2 Peculiarities – The C13/C12 Isotope Ratio « Watts Up With That? […]
Fossil coal is buried wood. So why would you expect a difference in the C12/C13 relationship between fossil and living wood in the first place?
The annual vegetation amplitude is approx 8GtC, but it’s a cycle, what goes up comes down, whereas the fossil signal is cumulative one way.
http://home.casema.nl/errenwijlens/co2/vegetation2003.gif
This would make good sense if the increase in C-13 is owed to increased temp.; increasing CO2 increasing biomass; biomass increasing C-13. Plants would respire C-13 preferentially, wouldn’t they?
Roy: can you say what would be the C13 ratio of outgassing ocean CO2 resulting from ocean warming?
I was saying that the exchanges other than industry were negative (more atmospheric CO2 absorbed than exuded).
Therefore, even if the absorption “eased off” if under “less pressure” (which in turn means you’d get diminishing benefits from CO2 cuts. What reinforces in one direction must, perforece, “un-reinforce” if headed in the other direction), any CO2 accumulation must come from man.
But Spencer seems to be challenging the basic premise. He’s saying that the evidence implies that the exchange between atmospher and ocean is NOT negative, but positive.
OTOH, Ferdinand (IIRC) or someone(s) else was saying that ony an average of 10 ppmv comes out of the ocean for every degree C, and that this matches the 100 ppm ice core measurements for the 10C swing from here to the Geological Ice Ages caused by eccentricity, wobble, and/or tilt.
But Ferdinand then indicates that CO2 emissions are estimated by ice cores to be what sees to be a SMALLER percentage drop than the historical records than the precipitous drop in industrial production and fuel consumption would seem to indicate during the Great Depression.
So I am calling the accuracy of ice-core measurement as CO2 proxy into question, not from a scientific viewpoint, but from the historical angle.
I therefore ask, do MODERN ice core measures INDEPENDENTLY match modern air-measured CO2 records? And if they do, is it possible that some of that CO2 somehow bleeds out of the archaic ice?
I want this all to add up.
“Fossil coal is buried wood. So why would you expect a difference in the C12/C13 relationship between fossil and living wood in the first place?”
Well, it must be different for C14 or carbon dating wouldn’t work. So why not for C12/13?
Come to think of it, all fossil fuel is dang old. Should maybe they should also be looking at C14 levels to determine man’s ‘contribution”?
I don’t understand the relevance of the wood issue. The natural CO2 changes appear to be related to ocean temperatures (we are just starting to look at what regions are most responsible).
The point is that the C13/C12 ratio is the SAME for the long terms TRENDS (supposedly manmade) and the NATURAL interannual variability in SSTs. So, the C13/C12 ratio does not appear to be a discriminator of an Anthropogenic source.
Also, very old ice core measurements come from highly compressed layers. How much diffusion of CO2 has there been across these thin layers of ice over thousands of years? Anything like what we have measured at Mauna Loa over the last 50 years would be smoothed out, giving the appearance of stable CO2 concentrations over centuries or millenia.
Evan Jones,
To answer your last remark first, fossil fuels still have the same d13C composition as when they were formed, many millions of years ago. 12C and 13C are stable isotopes. 14C is a radioactive carbon isotope (made by the collision of cosmic rays with nitrogen in the high stratosphere), and has a half-life time of 5730 years, see: http://en.wikipedia.org/wiki/Radiocarbon_dating.
Radiocarbon dating works for objects up to about 60,000 years. But fossil fuel is completely depleted of 14C (much too old), while current wood shows 14C/12C ratios more or less equal to the current atmospheric level.
That fossil fuel is completely depleted of 14C was observed and radiocarbon dating need a correction after about 1870, due to fossil fuel burning, up to the 1950’s, when the atomic bomb testing made radiocarbon dating impossible.
C-14 dating ‘works’ because of radioactive decay. The half-life of 5362 (I didn’t look it up) means after that period half the C-14 has decayed. A simple differential equation is used to estimate the elapsed time. The error has been reported at <5% but some stunningly bad dates have been put forward from time to time which can happen simply from water leaching away carbon or because the original C-14 proportion is poorly estimated.
For the C-13 model age is not a first order concern, radioactivity is not involved in any way.
Sorry, I brought up C-14 on the other thread before Dr. Spencer’s reply on the C-13/C-12 ratio because the originator, Suess, had estimated the appropriate slope using his C-14/C-12 study. I thought it important to mention that we now know the rate of C-14 creation was diminishing over his study period.
Dear Dr. Spencer,
A few remarks on the d13C changes…
To begin with, there is no practical difference in d13C of fossil fuel and vegetation decay, both are in average around -25 per mil d13C. As the seasonal changes of CO2 levels in the NH are governed by vegetation uptake and decay, it is no wonder that you can find (near) exact the same change as for the general trend.
The interesting point of d13C ratio’s is that there are only two known sources of low d13C, that are fossil fuels and decaying vegetation. All other known sources (volcanic degassing, deep oceans, ocean surface, carbonate rocks,…) have slighlty negative to slightly positive d13C values.
Thus (deep) ocean (0-4 per mil d13C) degassing can not be resposible for the decreasing trend in d13C values.
But vegetation decay can be responsible. That depends of which is prevailing: more vegetation decay than growth or the opposite.
Lucky, we have another, independent, measurement to know which one is prevailing: oxygen use or production. Since about 1990 we have oxygen measurements (at the edge of analytical possibilities), which are accurate enough to see the small difference between oxygen use from fossil fuel burning and the oxygen use/production of vegetation decay/growth.
This revealed that (at least) since 1990, somewhat less oxygen was used than calculated from fossil fuel burning. Thus vegetation produces more oxygen than it uses. And as vegetation growth prefers 12C over 13C, more 13C is left in the atmosphere. Vegetation thus is not the cause of the d13C decline…
As fossil fuel burning is the only known source of 13C depletion in the atmosphere left, it probably is entirely responsible for the whole d13C decrease…
For a more in-depth analyses of the d13C/O2 analyses and the resulting partitioning of CO2 sinks between oceans and vegetation, see Battle ea.:
http://www.sciencemag.org/cgi/content/abstract/287/5462/2467
Thanks, Ferdinand, for the very informative post.
It now looks like it is the warm ocean areas of the west Pacific and Indian Ocean that are highly correlated with the interannual variations in CO2 at Mauna Loa. So, this sounds more like some sort of temperature-related outgassing, doesn’t it? (The upwelling zones show little or no correlation. There is also a large area of high correlations over the entire eastern North Atlantic.)
This paper on “The Global CO2 Survey” shows that the ocean-atmosphere exchange is a strong function of wind speed…so that could be involved, too:
http://www.pmel.noaa.gov/pubs/outstand/feel2331/feel2331.shtml
Ferdinand:
You said: “To begin with, there is no practical difference in d13C of fossil fuel and vegetation decay, both are in average around -25 per mil d13C. As the seasonal changes of CO2 levels in the NH are governed by vegetation uptake and decay, it is no wonder that you can find (near) exact the same change as for the general trend.”
But it’s not the seasonal signal I’m measuring. It’s interannual variability (running 12-month averaging removes the seasonal signal). And since that signal correlates much better with SST than it does N.H. land temperatures, I’m assuming that the CO2 in question is coming from, and going into, the ocean
You said: “As fossil fuel burning is the only known source of 13C depletion in the atmosphere left, it probably is entirely responsible for the whole d13C decrease”
I’me beginning to wonder whether you read my post, Ferdinand. I already showed that the interannual variability has exactly the same ratio of C13/C12 as does the trend. So, how is it that mankind is responsible for decreasing dC13 in the long term, but natural variability (which has exactly the same C13/C12 relationship) can be ruled out for the long-term trend?
FYI, here’s a couple of NESDIS sea surface temp images for comparison
1998: Big El Nino
2008: Big La Nina
There’s a lot of energy flux going on there. It’s hard to argue against temperature/solubility CO2 outgassing effects with that kind of energy change involved over such a broad area. I would think that outgassing occurs faster than uptake due to CO2 partial pressure.
What is the physical process in which C12 moves into plants faster than C13?
http://home.casema.nl/errenwijlens/co2/co2lt_2007.gif
Roy you are referring to the CO2 thermometer that Jarl Ahlbeck discovered, and for which he demonstrated that the UAH sat temperature is a better metric than the surface temperature (!).
The South Pole has an even more distinctive ENSO signature in the CO2 growth:
(NOAA acknowledged herewith)
ftp://ftp.cmdl.noaa.gov/ccg/co2/in-situ/spo/spo_01C0_mm.co2
monthly dCO2/dy graph here:
http://home.casema.nl/errenwijlens/co2/dCO2_sp.gif
But that’s not the point is it? What we see is not a temperature dependent source, but a temperature dependent sink flux! Or in electronic equivalent: we see a temperature dependent power source, and a temperature dependent resistance:
I= V(T)/R(T) +constants
But the V(T) part is only 10 ppm/K, whereas the R(T) part is 4 ppm/y/K. So what you see in Mauna Loa (and South Pole) is that the flux resistance is temperature modulated, and not the gradient itself, that is only visible in ice cores.
Also the sink flux is in first order proportional to the excess gradient as is observed, and is dictated by Fick’s law (diffusion) or Ohm’s law (electricity). In warm years CO2 accumulates in the atmosphere, only increasing the gradient, which will flow out even more rapid in colder cycle years. So I also don’t see a reason why CO2 should remain in the atmosphere “forever”, as some alarmists want us to believe.
As the sink flux is also dCO2/dt=KCO2 we immediately see the solution to the differential equation that governs the CO2 sink flux: an e-folding decay function for a spike input, a Peter Dietze demonstrated 55 years.
Dear Dr. Spencer,
I think that we need to take into account the different mechanisms which govern d13C and total CO2 changes vs. temperature. And between seasonal variations and interannual variations in temperature.
– If the temperature increases, this has opposite effects for vegetation and oceans on CO2 flows: more CO2 release in the tropics, less absorption near the poles for oceans. But more CO2 uptake by plants (especially in the mid-latitudes). This leads to more CO2 uptake in summer and more decay over the whole year.
The CO2 uptake by plants wins in the case of seasonal changes (lower CO2 levels) for the NH (more vegetation), but there is little variation in the SH.
The CO2 release/less uptake by oceans wins in the case of interannual temperature changes, that is what we see for the ppmv/°C changes.
– If the temperature increases, this has similar effects for oceans and vegetation: more degassing of 13C rich CO2 (0-4 d13C) vs. the atmosphere (-8 per mil d13C), more 12C uptake by vegetation, leaving relative more 13C in the atmosphere. Both increase the d13C level.
– But we see that in the atmosphere (ice cores – firn – atmosphere), as well as in the upper oceans (coralline sponges), d13C levels decline, starting around 1850. See:
http://www.ferdinand-engelbeen.be/klimaat/klim_img/sponges.gif
Thus the long term trend has had little influence from interannual natural variations, neither from increasing temperatures…
Dear Dr. Spencer,
My previous post was sent before I had read your reaction…
Here some more clarification:
In general, we can say that over a relative short time span we have three influences with increasing SST:
– the influence of the emissions (in general twice the CO2 variation, but near equal at El Niño episodes) on CO2 levels: increasing, on d13C levels: decreasing, not influenced by temperature.
– the influence of vegetation on CO2 levels: decreasing, 13C levels: increasing.
– the influence of the oceans on CO2 levels: increasing, 13 C levels: increasing.
The total influence on CO2 is (based on a few guesses):
+7 GtC with -24 per mil d13C from the emissions
-2 GtC wich acts as +24 per mil d13C from vegetation uptake
+2.5 GtC with 0 per mil d13C from warmer oceans.
Total increase (observed in the atmosphere): +7.5 GtC
Effect on atmospheric d13C (observed in the atmosphere): – 0.05 per mil d13C
The latter can be calculated, including the 90 GtC seasonal ocean exchanges (which dilute the emissions fingerprint with about 20%), but it may be clear from this figures that the 7 GtC from the emissions with very low d13C minus 2 GtC with very high d13C by far outweigh the 2.5 GtC with near zero d13C from the oceans. This only temporarely reduces the decrease speed, not the decrease itself.
One need about 4 GtC from the (deep) oceans to compensate 1 GtC from the emissions with a neutral effect on atmospheric d13C levels…
Thus even during natural variations, the d13C decrease from the emissions by far dominates the change in ratio’s, that is why you don’t see a difference between short term and long term variations in d13C.
Forgot to add: the figures used were for the 1998 El Niño year…
Ferdinand:
Thanks for the detailed explanation. It will take me awhile to digest all of the various numbers and compensating influences in the carbon budget you have listed.
I was under the impression that we didn’t understand the details of the carbon cycle…but obviously, I was mistaken. 😉
-Roy
Dr. Spencer makes some convincing arguements.
I would hasten to add, however, that much of the “ice core average global
temperature” is based on the Oxygen 16 to Oxygen 18 ratios.
The problem with this “proxy”, which I have yet to illuminate a good rational for, is that the geo-physics types have used this number to trace water flows from various areas as it has been established that WARM WEATHER thunderstorms in costal areas push the O18 up (significantly, as in a 30% higher isotope concentration).
As such, the O18 to O16 ratio, as far as I’m concerned, gives perhaps an “atmopheric energy” indication (i.e., a number reflecting the number of warm weather T.S.’s versus cold weather, continental ones), but does NOT give a reasonable proxy for “global temperature”.
I read a less rigorous computation, years ago..making the fundamental mistake Dr. Spencer has pointed out…and one of my first thoughts was,
“Is there some other ‘mechanism’ which could shift isotope ratios around?”.
Thus, I’m totally CAUTIOUS about drawing “yea” or “ney” conclusions on the isotope ratios.
Hans Erren wrote(05:34:29) :
Fossil coal is buried wood. So why would you expect a difference in the C12/C13 relationship between fossil and living wood in the first place?
The annual vegetation amplitude is approx 8GtC, but it’s a cycle, what goes up comes down, whereas the fossil signal is cumulative one way.
http://home.casema.nl/errenwijlens/co2/vegetation2003.gif
Plants have preference for the “lighter” C12. If you would look at the total Carbon cycle, you would see that geological CO2 emissions have higher concentrations of C13. If you would attribute all increases in CO2 to the anthropogenic i.e. fossil origin C. The balance should change to C12 side. What it means: oceans and earth plants are able to deal with all anthropogenic CO2. What we have is geological outgassing that changing concentration of CO2 in the atmosphere.
Very interesting.
Hard to follow, but very interesting. (If I didn’t have a handle on the 5th-grade version of the exchange rates I wouldn’t be anywhere close.)
Hmmm.
To let man off the hook:
It’s the ocean because the C12 ratio is not increasing. The stuff man is putting out is absorbed. Man’s contibution to ocean sink is lost in the crowd (6.3-to-38,000 BMTC). PDO/AMO or whatever causes the increase in upper ocean temperatures and C13 exudes. (Which I think RS is saying.)
To blame it on man:
Is it possible that the fossil-fuelC12 is being absorbed by the ocean but this is creating upper ocean saturation and therefore forcing increased C13 outgassing? The C12 incease being lost in the sink, masking the anthropogenic fingerprint? (Which I think FE is saying.)
Is this a correct 5th-grade understanding of the argument?