First: You are ignoring the fact that C14 is continually formed in the upper atmosphere by cosmic ray bombardment. The graph shows the excess C14 injected by atmospheric A-bomb tests (the “perturbation”) decaying back to the natural background concentration. The “long tail” is simply the natural background concentration of C14. When you include this in the model, it is fit well by a single lifetime.
Second: The graph demonstrates that there is a real-world process which removes ½ of all radioactive CO2 from the atmosphere every 10-15 years. While it is just possible that the removal process might run at a slightly different rate for C12 than C14, the difference can not be great, so it is reasonable to consider that the lifetime for ALL atmospheric CO2 is ~12 years.
The graph also demonstrates exactly how long it takes a perturbation in CO2 levels to decay – since that is precisely what it is measuring. So, indeed, it does directly address your “question of interest”.
This also means, that half of the the CO2 released this year by Human activity will be gone from the atmosphere in 12 years; as there is no way any physical mechanism can distinguish where a “particular” CO2 molecule comes from.
I know it is fashionable over at RealClimate to yap about “long tails” and “multiple lifetimes”, but there is exactly NO evidence for any such phenomenon. The only thing that might partly save this theoretical House of Cards is the demonstration that the CO2 lifetime is a sensitive function of concentration. AGWers, however, have made NO attempts to make any such measurements – probably because measurements of CO2 lifetimes around 10-15 years (the probably outcome) would be, as I said, a highly “inconvenient fact”.

Oh, boy, that proscription sends shiver’s up my spine. Ooh, I think I’m soiled.

750 Gtons C == 3000 Gtons CO2. Otherwise, the figures I quoted do vary somewhat from the estimates at the link.

The problem with that, Joel, is that all the actual evidence (as opposed to ad hoc theories created to support long lifetimes) shows that CO2 has a relatively short atmospheric lifetime. See, for example, the measured lifetime of radioactive CO2 after the 1964 atmospheric nuclear test ban took effect: http://cdiac.ornl.gov/trends/co2/well-gr.html

The problems with that graph are:

Bob

(1) It doesn’t address the question of interest…which is not how long it takes a particular carbon atom to cycle out of the atmosphere but how long it takes a perturbation in the total atmospheric level of CO2 to decay. They are different things.

(2) It is not obvious from that graph that the lifetime is what you claim it to be. As I noted, the decay of CO2 is simply not described well by a single lifetime…So, some fraction of a perturbation in the CO2 levels disappears quite quickly but there is also a long tail. Whether the long tail is there or not for what you have plotted can’t really be determined from what you showed.

Bruce Cobb says:

Firstly, our contribution of C02 is small, only about 3% or so. Secondly, C02’s warming effect is logarithmic; by far, most of the limited warming effect (small, in comparison to water vapor) has already occurred.

If you don’t want to be taken seriously by any real scientist, I strongly suggest that you continue to repeat the 3% nonsense.

As for its effect being logarithmic, what that means mathematically is that doubling the level from, say 280ppm to 560ppm will produce the same effect as doubling from 140ppm to 280ppm. (Your statement that most of its limited warming effect has already occurred is essentially incomprehensible. If you mean that the rise from 280ppm to 380ppm has already gotten you most of the warming from a doubling from 280ppm to 560ppm, you are demonstrably wrong…even neglecting the issues of the climate not having yet caught up to the current CO2 levels.)

Daublin says:

Joel, you are ignoring the broad point by discussing the details. This magnification you breeze by is what turns a 1-ish degree increase into a 5-ish one or more. Take a moment to really think about this. If a 1 degree increase was all we faced even at 5000 ppm of CO2, then that would take all the wind out of the sales of people saying we face doom if we don’t decrease CO2 emissions. Thus, the positive feedbacks are key. Do you care to address this, or dare I say, concede it?

This is garbled to the point of incomprehensibility. You seem to be (among other things) confusing projections about future temperatures (which depend on how one assumes CO2 levels get) with estimates of the equilibrium climate sensitivity to a doubling of CO2. You are correct that in the absence of positive feedbacks, a doubling of CO2 should produce a bit over a 1 C temperature rise. However, raising CO2 to 5000ppm would be somewhere more than a 16X increase and would thus raise temperatures 4 C in the absence of feedbacks.

And, the IPCC range for the likely equilibrium climate sensitivity is 2 to 4.5 C, not 5 or more. At any rate, I agree that the positive feedbacks are important (at least if we don’t allow the CO2 levels to get too out of hand–if we let them get up to 5000ppm for example, then we are screwed with or without the feedbacks). However, you can’t just wish them away and the current understanding of the paleoclimate seems to require them being there. In the case of the water vapor feedback and the ice-albedo feedback, they also follow from some pretty basic physics…and the water vapor feedback has observational confirmation from the work of Soden et al. among others.

My recollection is that the accepted weight of atmospheric C02 was 3000 Gtons, the surface vegetation 800 Gtons, oceanic dissolved CO2 50,000 Gtons and oceanic precipitate CO2 100,000 Gtons. I’ve seen in comments here recently, someone imply an atmospheric weight comparable to the biogenic weight which I discounted as an implausible typo.

My impression is that the oceanic phytoplankton are tossed in to the oceanic dissolved total.

I make no claim to precision; one source of inaccuracy on my part is to ignore in the daily variance a moderately hydrophylic nature of CO2 carrying it aloft during the day as the temperature rises. I do that because the Mauna Loa daily variance is larger than the AIRS factor and not the inverse.

The central issue is that balanced equations, in the manner of chemical reactions, for the results of CO2 flux computations, are conceptually invalid predictin results. In chemistry we rigidly control inputs, solvent, temperature and pressure. This is not the case in nature in any way, shape, or form. They are valid only as a snap-shot of the present.

Establishing the origin of the CO2 empirically is required for the assertion that it is of antropogenic origin in any genuine expression of fact. This would at present, require use of the 13C/12C fraction. Seuss naively supposed that since the 13C was increasing that this proved anthropogenesis. Unfortunately, he was mistaken, having ignored the temperature dependent oceanic partial-pressure of CO2.

The inference that because total fluences of CO2 in nature are at any moment “in balance” therefore implies that a change composition of one sink or another can be assigned to a fluence of choice is false in principle and false in actuality. Truth and falsehood is not democratically resolved.

Didn’t impolite missionaries end up in the stew pot?

Now that you’ve brought it up, I have noticed a strange similarity between the few “conversations” I’ve had about this subject on less friendly sites than this one and the types of interactions I’ve had with missionaries at my door ;-)

(Except perhaps that missionaries, lacking the cover of anonymity, tend to be more polite.)

However, you talk right past Dan Lee’s much shorter argument.

Dan Lee says:

Finally, some perspective. An atmosphere that has remained stable enough to support the 3+ billion year evolution of life, through all these extremes, isn’t going suddenly spiral out-of-control into a life-baking oven when it hits 400 ppm of CO2.…If you take away atmospheric positive feedback, aren’t we done? End of show? Nothing left for AGW to stand on?

You wrote: “(1) No serious scientist that I know of is claiming that there is a Venus-like instability and the climate is going to “spiral out-of-control into a life-baking oven” once CO2 levels hit 400ppm. The positive feedbacks just magnify the warming due to CO2 alone…They are not strong enough to lead to an instability. (On Venus they could because it is closer to the sun and hence receives more W/m^2 of solar energy.)”

Joel, you are ignoring the broad point by discussing the details. This magnification you breeze by is what turns a 1-ish degree increase into a 5-ish one or more. Take a moment to really think about this. If a 1 degree increase was all we faced even at 5000 ppm of CO2, then that would take all the wind out of the sales of people saying we face doom if we don’t decrease CO2 emissions. Thus, the positive feedbacks are key. Do you care to address this, or dare I say, concede it?

Didn’t you know that Gaia can selectively extract the radioactive CO2 from the atmosphere to help restore her natural balance? And that she is causing her mighty oceans to inject CO2 into the atmosphere just to rid herself of humanity who has spoiled her pristine environment?

Gosh, you must be expecting proven physics and chemistry to rule. ;-)

The problem with that, Joel, is that all the actual evidence (as opposed to ad hoc theories created to support long lifetimes) shows that CO2 has a relatively short atmospheric lifetime. See, for example, the measured lifetime of radioactive CO2 after the 1964 atmospheric nuclear test ban took effect: http://cdiac.ornl.gov/trends/co2/well-gr.html

Up until about 20 years ago, it was considered an established fact that CO2 had a 5-10 year lifetime in the atmosphere, established by a number of means. (See, for example, this short discussion:: http://www.financialpost.com/story.html?id=433b593b-6637-4a42-970b-bdef8947fa4e)

Now, however, that would be a truly “inconvient truth”, as it would directly imply that Human-created CO2 (being created over 100 years) could only account for a few percent of the observed rise in concentration. Hence it has become necessary to promote, with a straight face, complicated theories, unsupported by any actual measurements, that “prove” a long lifetime for CO2. In general, those who promote these theories don’t attempt to explain what is wrong with the actual measurements of decades ago — they simply ignore them.

It really helps to have a grasp of the big figures, to show just how pifflingly small is the human contribution, and how much evidence there is for living processes involved in maintaining homeostasis. But finding the figures and the science… they are scattered here like gold dust… and sometimes they seem questionable… for instance…

Gary Gulrud, you said “Daily variance of CO2 measured by AIRS in mid-troposphere and at 10,000? at Mauna Loa is on the order of 10^1 ppm, this corresponds to a daily fluence between ocean and atmosphere of on the order of 100 Gtons!”

But my calculations go thus: 380ppm CO2 in the air… http://www.grida.no/publications/vg/climate/page/3066.aspx shows a total atmopsheric CO2 weight of c. 750Gt. Maths: 380ppm=750Gt, thus 1ppm=~2Gt. Therefore you figure of 10ppm corresponds to 20Gt.

I’ve put the figures as best I could, graphically, on my primer (link thru name). These figures were taken from places like Glassman’s and Anthoni’s websites, both of whom I have a high regard for, science-wise. But they are not infallible either. ALL CORRECTIONS WELCOME!