I think their approach of seeking CO2-concentrations via tree-ring growth over time is fundamentally flawed for a variety of reasons. There is so much intrinsic uncertainty in paleo-climate reconstructions, especially tree-rings, which vary according to an indeterminate number environmental factors. Even so, assuming what you say is correct, the current δ13C value is -8.3 and this gives us a direct measure of how much human CO2 is in the atmosphere today. The natural atmospheric reservoir has a δ13C of -7 when in equilibrium with dissolved CO2 in the oceans and anthropogenic CO2 has a δ13C value of around -29. Hence at a δ13C value of -8.3 the total portion of anthropogenic CO2 residing in the atmosphere today amounts to ~6% or around 18ppmv (i.e. 6% of -29 and 94% of -7). In other words 94% of CO2 in the atmosphere is not anthropogenically derived. This is far from being "meaningless" as you say. It is unequivocal and means that atmospheric δ13C measurements cannot prove humans have increased the atmopsheric CO2 level by 120ppmv because essentially all human CO2 has already been absorbed by sinks and is no longer in the atmosphere.
Okay Joel,
First off, this is not a thread that concerns itself about whether the current rise in atmospheric CO2 is due to anthropogenic sources or not. In fact, that discussion is off topic for the thread. If you wish to continue beating that horse I suggest you do so in one of he many therads where that discussion is on topic. I have no problem with getting a moderator involved with respect to staying on topic. Also, I would appreciate it if other posters would not respond to off topic posts.
Second, the fact that you are a new member to the CF with only 4 posts to your credit, three of them being specifically to this topic, is just a bit more than curious.
Third, the topic of this thread is about how paleoclimatologists are able investigate and determine paleo-atmospheres, in particular with this thread through ice cores. Certainly ice cores are not the only means by which paleotemperatures and atmospheres can be determined, but it is the one where direct observation of "fossil atmospheres" is available, as to proxys with paleosols, MOI's, or speleothems, to mention a few.
Fourth, this is the last time I will address the claims you presented by the Segalstad et al, source. It is far outside mainstream paleoclimatology with many claims that can clearly and easily be shown to be incorrect and unfounded.
The numbers presented by Segalstad et al, are not at all representative of the actual ratio change (decrease) of the 13C/12C, rather a misdirection of what it actually measures. Once again, during the past 10,000 years those ratios were relatively stable, which has been measured through several different means, until the beginning of the industrial revolution to present day. The decrease in the 13C/12C ratio is 5 times that of the natural variability. That is more than significant.
Pertaining to the 5 year residence time of atmospheric CO2, that is correct, but specific to the molecular level, and not indicative of the volume increase. The natural carbon cycle is not an exact exchange ratio but it is very close. What the 5 year molecular residence time does not take into account with respect to anthropogenic CO2 is the atmospheric increase of overall CO2, which is more than obvious due to fossil fuel emissions. Without the fossil fuel emissions the atmospheric CO2 concentration would remain at less than 280 ppmv as it has over the past 800,000 years. So yes, the residence time is only 4 to 5 years but the carbon cycle also returns it and we know the increase and atmospheric concentration, besides being physical observed, is due to fossil fuels from the observed decrease in 13C/12C ratios, which again, is 5 fold that of pre-industrial times.
... Did you see this, Rick?
