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Carbon Isotopes in Natural and Fossil Fuel CO2

People want to be sure their emissions are not building up time bombs for the future. The quantities of all three carbon isotopes can yield evidence about the proportion of manmade CO2 and natural CO2. I suspect that the manmade CO2 hardly enters into the equation, but as a newcomer I still find this science difficult to be sure I have grasped, and I've heard competent scientists hold different opinions. I have a lot of respect for Segalstad and Glassman - but they may be wrong and as yet it is beyond my capacity to be sure.

from Segalstad

10. The breakdown of the dogma - carbon isotopes

Suess (1955) estimated for 1953, based on the carbon-14 "Suess Effect" (dilution of the atmospheric CO2 with CO2 from burning of fossil fuel, void of carbon-14), "that the worldwide contamination of the Earth's atmosphere with artificial CO2 probably amounts to less than 1 percent". Revelle & Suess (1957) calculated on the basis of new carbon-14 data that the amount of atmospheric "CO2 derived from industrial fuel combustion" would be 1.73% for an atmospheric CO2 lifetime of 7 years, and 1.2% for a CO2 lifetime of 5 years.

This is in conflict with IPCC researchers, who assume that 21% of our present-day (as of December 1988) atmospheric CO2, the assumed rise in CO2 level since the industrial revolution, has been contributed from Man's burning of fossil fuel (Houghton et al., 1990).

This large contradiction between the carbon-14 measurements and the dogma, has worried many researchers. In order to make Suess' measurements fit the dogma, it would be necessary to mix the atmospheric fossil-fuel CO2 with CO2 from a different carbon reservoir five times larger than the atmosphere alone (Broecker et al., 1979). It was alternatively proposed that the carbon-14-labelled CO2 would act completely differently than the "ordinary" CO2: "However, the system's responses are not the same for the CO2 concentration and for isotopic ratios" (Oeschger & Siegenthaler, 1978). The explanation is given that the CO2 levels will be governed by the constructed evasion "buffer" correction factor, while on the other hand (strangely enough) the isotope ratios of the atoms in the very same CO2 molecules would be unaffected by the evasion "buffer" factor, and further: "would be equal in both reservoirs [the atmosphere and the ocean's mixed layer] at equilibrium. This explains why the relative atmospheric CO2 increase is larger than the Suess effect" (Oeschger & Siegenthaler, 1978). This cannot be accepted, when all chemical and isotopic experiments indicate that equilibrium between CO2 and water is obtained within a few hours (see Section 5 above).

Ratios between the carbon-13 and carbon-12 stable isotopes are commonly expressed in permil by a so-called delta-13-C notation being the standard-normalized difference from the standard, multiplied by 1000. The international standard for stable carbon isotopes is the Pee Dee Belemnite (PDB) calcium carbonate.

CO2 from combustion of fossil fuel and from biospheric materials have delta-13-C values near -26 permil.
"Natural" CO2 has delta-13-C values of -7 permil in equilibrium with CO2 dissolved in the hydrosphere and in marine calcium carbonate.
Mixing these two atmospheric CO2 components: IPCC's 21% CO2 from fossil fuel burning + 79% "natural" CO2 should give a delta-13-C of the present atmospheric CO2 of approximately -11 permil, calculated by isotopic mass balance (Segalstad, 1992; 1996).

This atmospheric CO2 delta-13-C mixing value of -11 permil to be expected from IPCC's model is not found in actual measurements. Keeling et al. (1989) reported a measured atmospheric delta-13-C value of -7.489 permil in December 1978, decreasing to -7.807 permil in December 1988 (the significance of all their digits not justified). These values are close to the value of the natural atmospheric CO2 reservoir, far from the delta-13-C value of -11 permil expected from the IPCC model.

From the measured delta-13-C values in atmospheric CO2 we can by isotopic mass balance also calculate that the amount of fossil-fuel CO2 in the atmosphere is equal to or less than 4%, supporting the carbon-14 "Suess Effect" evidence. Hence the IPCC model is neither supported by radioactive nor stable carbon isotope evidence (Segalstad, 1992; 1993; 1996).

To explain this apparent contradiction versus the IPCC model, the observed delta-13-C value of atmospheric CO2 "must be affected by other heavier [i.e. with high delta-13-C values] carbon sources, such as is derived from the air-sea exchange process" (Inoue & Sugimura, 1985). One way to make this happen, would be if the isotopic exchange from air to sea were different from the isotopic exchange from sea to air; i.e. a gross non-equilibrium situation would be required. Siegenthaler & Münnich (1981) were able to construct such a simple theoretical kinetic, non-equilibrium model: "Diffusion of CO2 into the water, which is rate limiting for mean oceanic conditions, fractionates the carbon isotopes only little. 13-C/12-C fractionations are found to be -1.8 to -2.3 permil for atmosphere-to-ocean transfer, and -9.7 to -10.2 permil for ocean-to-atmosphere transfer."

Inoue & Sugimura (1985) attempted to verify these kinetic isotope fractionations experimentally at three temperatures: 288.2; 296.2; and 303.2 Kelvin, versus their equilibrium values of -8.78; -7.86; and -7.10 permil, respectively, all with uncertainty given as +/- 0.05 permil. Their reported air to sea fractionations at these temperatures were -2 +/- 3; -4 +/- 5; and -5 +/- 7 permil, respectively. Their sea to air fractionations were found to be -10 +/- 4; -13 +/- 6; and -12 +/- 7 permil, respectively. (Reported alpha fractionation factors and uncertainties have here been recalculated to alpha minus one, multiplied by 1000, to get comparable fractionation values). They conclude that the agreement is fairly good with the theoretically deduced values of Siegenthaler & Münnich (1981). Looking at the reported uncertainties, however, the experimental data cannot be grouped in three populations: their air-to-sea and sea-to-air data are not significantly different from their reported air/sea/air equilibrium value at the three different temperatures. Hence the experimental data cannot be used as evidence for the proposed theoretical difference in isotopic fractionation for air/sea versus sea/air CO2 transfer due to differences in kinetic isotope fractionation. I cannot follow this paragraph for certain.

Siegenthaler & Oeschger (1987) touch in their carbon cycle modelling, with carbon isotopes included, on the possibility that the apparent atmospheric CO2 level increase is due to marine degassing instead of accumulation of anthropogenic CO2: "We will also discuss the sensitivity of the model results to uncertainties in the ice core data, to different model assumptions and to the (unlikely) possibility that the non-fossil CO2 was not of biospheric, but rather of marine origin." The word "unlikely" in parentheses is indeed their wording. Their modelling shows ambiguously that: "as expected, the results are similar to those for the fossil-only input". But their modelling shows a discrepancy with the ice core CO2 data, in addition to: "it is somewhat surprising that observations and model agree for 13-C but not for 14-C; this can, however, not be discussed here any further". In their abstract, however, they conclude on the contrary: "Calculated 13-C and 14-C time histories agree well with the observed changes." Humph, once again the conclusion belies the evidence just explained!

The carbon cycle modelling of Siegenthaler & Oeschger (1987) run into several problems making their models fit all the data, leading them to write: "One possibility is that the assumptions underlying our results are not fully correct, i.e., that either the Siple ice core data deviate from the true atmospheric concentration history or that the carbon cycle models used do not yield the correct fluxes. If we dismiss these possibilities, then other carbon sinks than the ocean seem to exist." For the lack of validity of the Siple ice core, see Section 4 above.

Based on this kind of modelling, IPCC states as part of their "evidence that the contemporary carbon dioxide increase is anthropogenic" (their Section 1.2.5; Houghton, 1990): "Third, the observed isotopic trends of 13-C and 14-C agree qualitatively with those expected due to the CO2 emissions from fossil fuels and the biosphere, and they are quantitatively consistent with the results from carbon cycle modelling." Such a correspondence is, however, not evident to the present author.

Segalstad (1992; 1993; 1996) concluded from 13-C/12-C isotope mass balance calculations, in accordance with the 14-C data, that at least 96% of the current atmospheric CO2 is isotopically indistinguishable from non-fossil-fuel sources, i.e. natural marine and juvenile sources from the Earth's interior. Hence, for the atmospheric CO2 budget, marine equilibration and degassing, and juvenile degassing from e.g. volcanic sources, must be much more important; and the sum of burning of fossil-fuel and biogenic releases (4%) much less important, than assumed (21% of atmospheric CO2) by the authors of the IPCC model (Houghton et al., 1990).

The apparent annual atmospheric CO2 level increase, postulated to be anthropogenic, would constitute only some 0.2% of the total annual amount of CO2 exchanged naturally between the atmosphere and the ocean plus other natural sources and sinks (Section 9 above). It is more probable that such a small ripple in the annual natural flow of CO2 is caused by natural fluctuations of geophysical processes. We have no database for disproving this judgment (Trabalka, 1985). Like Brewer (1983) says it: "Nature has vast resources with which to fool us . . .".

Segalstad's mass balance calculations show that IPCC's atmospheric CO2 lifetime of 50-200 years will make the atmosphere too light (50% of its current CO2 mass) to fit its measured 13-C/12-C ratio. This proves why IPCC's wrong model creates its artificial 50% "missing sink" (Segalstad, 1996).I don't understand this paragraph either.

 

 

Updated 23rd October 2008

 

 

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