There has been quite a remarkable thread on Judith Curry’s blog called quantifying the anthropogenic contribution to atmospheric CO2. Remarkable for a number of reasons. One being that Judith would post it without comment. What Judith says in the comments is also surprising. It’s also remarkable in terms of the the arguments some seem willing to make to justify a natural contribution to the rise in atmospheric CO2. It is, however, also remarkable in the sense that some people who I had never found particularly sensible, actually did seem to accept something that is virtually certain. A silver lining, shall we say.
What has prompted this post, however, is me remembering that Steve Bloom had pointed out that there is evidence that many people do not understand the concept of accumulation. It comes from a paper called why don’t well educated adults understand accumulation: A challenge to researchers, educators, and citizens, by Cronin, Gonzalez and Sternam (2009).
The basic issue relates to the difference between a stock and a flow or, what we in physics might call a quantity and a flux. A stock is the amount of something that exists in some volume/location, and a flow is the amount of that something that flows into and out of that volume/location, over some time interval. The crucial point is, though, that the rate of change of the something at that location depends on the net flow; i.e., the difference between how much is coming in and how much is leaving. A classic example would be debt versus deficit. A debt is how much is owed at any instant in time. A deficit is a situation where – over some time interval – the expenses exceed the income. If there is a deficit, therefore, the debt must be growing. The confusion, apparently, is that some think that if the deficit goes down, so does the debt. This isn’t correct. If the deficit goes down, then the rate at which the debt is growing decreases, but it is still growing. The deficit would have to go to zero for the debt to remain constant, and there would need to be a surplus (income exceeding expenses) for the debt to start dropping.
A similar problem seems to be encountered when considering the accumulation of CO2 in the atmosphere. It is accumulating because there is a net flow/flux of CO2 into the atmosphere. When we look at these flows, we can show that there is a net flow into the natural sinks (CO2 is accumulating in the oceans and biosphere), there is a net flow into the atmosphere (it is accumulating in the atmosphere), and there is a net flow out of fossil fuels (i.e., anthropogenic emissions are positive). In other words, the natural sinks are absorbing some of our emissions and, hence, the accumulation of CO2 in the atmosphere is anthropogenic. That seemed to be one thing that some on the Climate Etc. thread were unwilling to accept.
Another confusion, however, seems to relate to our emissions. There seem to be some who think that if our emissions go down, that atmospheric CO2 will go down. Again, this is wrong; if our emission go down, the rate at which it is accumulating in the atmosphere will go down, but it will still be accumulating. If we want CO2 to stop accumulating in the atmosphere, we need to stop emitting it, not simply decrease our emissions (okay, not quite true, but to stabilise atmospheric concentrations would require a dramatic reduction in our emissions). On that note, there is another interesting paper that explains this using a bathtub analogy (H/T Gavin Cawley).
Okay, I’ve written this all a bit quickly, but I do think it is an interesting issue. It does seem as though many don’t quite get the subtlety between stocks and flows, which is certainly important if you want to understand something like the accumulation of CO2 in our atmosphere. I thought I would finish with an example from the first paper I mentioned. The graph below shows the number of people entering (solid line) and leaving (dashed line) a store every minute. There are two questions to consider: during which minute were the most people in the store, and during which minute were the fewest people in the store? It’s explained in the paper, so if you want to try and answer in the comments, don’t cheat. Also, if you do provide an answer, don’t explain it; let others have a go too.
credit : Cronin et al. (2009)
...and Then There's Physics 
You are making it way too complicated with stocks and flows. This is a basic diffusion problem whereby the CO2 molecule diffuses to a permanent sequestration site. As it randomly walks its way down it will occasionally reappear in the atmosphere.
Sometimes you have to go with statistical physics instead of a deterministic viewpoint, which is what you are attempting to do. The heuristic BERN model describes the sequestration response and this is easily modeled as a solution to a dispersed diffusion equation.
13? [I have read the paper, but quite a while ago]
I think it is worth pointing out that there are some that can reason about stocks and flows quite well, …, until applied to the carbon cycle! ;o)
Nice summary
Dikran,
Indeed, we’ve just discovered that. Thanks.
WHT,
I think that’s a first – easier to forget stocks and flows and simply consider it as a basic diffusion problem? 🙂
I’m not sure I agree with you, though. The diffusion is more related to the steady state concentration, rather than to the accumulation. Our emissions far exceed the rate at which CO2 is being sequestered in rocks/deep ocean, so that I think it isn’t really relevant here – I think.
and 30?
A Mathematician, a Biologist and a Physicist are sitting in a street cafe watching people going in and coming out of the house on the other side of the street.
First they see two people going into the house. Time passes. After a while they notice three persons coming out of the house.
The Physicist: “The measurement wasn’t accurate.”.
The Biologists conclusion: “They have reproduced”.
The Mathematician: “If now exactly 1 person enters the house then it will be
empty again.”
Great post. I think we saw the confusion during the election. P.S. The Bathtub link went down the plug hole 🙂
So, my best attempt to understand the logic of [Mod: unnecessary] is that they see CO2 concentration as an equilibrium problem, more like water vapor than like methane. E.g., for water vapor, the total amount in the atmosphere is basically a function of global temperatures. So, even if for the sake of argument, we emit X gigatons of water vapor per year (by irrigation, combustion, whatever), and we observe an increase of X/2 in water vapor atmospheric loading, that doesn’t mean that the increase in atmospheric water vapor is due to the emissions.
Having said that, in the case of CO2 it is total baloney, and it is a field of science that has been understood for decades, and it should be really embarrassing to the Currys and Salbys and and Essenhighs and Wattses of the world (and occasionally the Spencers) that they keep falling for ridiculously simplistic correlation arguments that should be falsifiable with undergrad-level understanding of the carbon cycle. They just have no concept of what the research community is actually working on, often apparently thinking that the Bern Cycle model approximation is the forefront of knowledge: e.g., as one example of real research, http://onlinelibrary.wiley.com/doi/10.1111/j.1600-0889.2009.00446.x/pdf has a single, physically based model which can simultaneously explain C12, C13, C14, seasonal cycles, and geographic patterns. Unlike the Salby & Essenhigh approaches, which come up with different lifetimes for the different isotopes of carbon because each one uses a different global fitting parameter.
Does this mean that we understand everything about the carbon cycle? No! But the actual research frontier is lightyears beyond the simplistic understanding evinced by Curry’s reposting junk like this. It is like we are trying to design a Mars expedition, and Curry is reposting analyses that claim to prove that the moon landings were faked based on analysis of photos of the flag…
What Dikran said to part I and II. Here is one for you though, how do you use a balance to accurately determine when the minimum number of people in the store occurs?
It may be poor, but it’s not unusual for Curry to post nonsense from others without comment.
But “What Judith says in the comments is also surprising. in no way reflects the utterly surreal way she attempts to ignore the overwhelming evidence for the Co2 rise being anthropogenic.
She’s made some pretty bizarre statements in the past IMHO, but nothing comes anywhere close to this. It’s dumbfounding.
My experience is gained from the semiconductor world, and the BERN model of CO2 sequestration is really no different than the math that we use to model the incorporation of dopants into wafers. That is all transient analysis and is definitely not a steady-state solution, so you are wrong about that.
If process engineers hand-waved like you are doing with your stocks and flows they would be kicked to the side in deference to someone who knew how to work the math of diffusion.
Curry:
”
And finally, why does it matter whether we attribute 100% or 90% or 50% of the increase in atm CO2 to humans?
”
”
… the white part of the Italian flag is pretty big here.
”
Blog-science just don’t get any better than that.
WMC,
My daughter – who had a maths exam today – didn’t find that as funny as I thought she would :-).
Eli,
I don’t understand your question.
Richard,
Fixed, thanks.
VTG,
It is rather dumbfounding. What was irritating – amongst other things – was how people didn’t seem to get the difference between the long-term trend (anthropogenic) and the variability (natural).
MMM,
It really should be embarassing and it amazes me that it doesn’t seem to be.
TVRJH,
It is a classic.
Even if we want to ignore isotope, oxygen, and ocean/land sink mass budget issues, does anyone seriously think that the ~120 ppm increase since industrialization is just coincidence give the very small variations exhibited over the Holocene, and very slow changes on still longer timescales?
I don’t know why anyone finds Judith’s confusions surprising anymore, or her selective bits of what is “interesting” and lawyer-like tangents into multi-decadal variability. The rhetorical fallbacks onto magical and unfalsifiable modes of “variability” without any considerations imposed by observation and physics is not in the least bit compelling, and she only blames everyone else for not convincing any serious scientist about how to frame such issues. Yawn.
Salby should take Cronin et al’s test. His motivation for his anthropogenic CO2 denial is from the confusion about stocks and flows.
“…is just coincidence give the very small variations exhibited over the Holocene, and very slow changes on still longer timescales?”
Chris, Curry has a ‘solution’ to that, too: not believe those measurements. For example, she pointed out that there was a discrepancy between ice core and stomata proxies in CO2 levels. There you go, ice cores uncertain, and hence, therefore, thus…
Balance the graph at the intersection point and fill the space between the lines with CO2 …:)
richardt,
Good point. In fact, if one could withdraw a constant amount every month from Salby’s bank account, he wouldn’t notice.
I think the joke is better when the mathematician says, “Now there are -1 people in the house”.
Judith says: “I think it unlikely that 100% of the increase in atm CO2 is caused by humans. It is not unreasonable to start from a point of 50-50 (Fred’s conclusion) and see if you can falsify natural variability as large as 50%. It may not be 50%, but I don’t think it is 0%”
Isn’t it more reasonable to assume that our contribution is around 200% given that we emit twice as much as remains?
Rachel,
Is that how Ben tells it?
Any half-decent mathematician would say:
“Now there are e^(i*pi) people in the house.”
I just taken a look at what Curry posted and her comments.
Oh dear. It must take a great deal of effort to feign such ignorance, but the uncertainty monster must be fed. Despite the efforts of dissembling Salby and a dozen other cranks, the anthropogenic origin of the increase in atmospheric CO2 levels is probably the single best established fact in the climate change debate.
Minute 13th the most peops, minute 30th the least?
Couldn’t have guessed by just looking at the graph, had to do it minute by minute in excel.
It was either this or doing housework — you may get a call from my wife, ATTP :).
Yep!
Your wife and my wife may have quite a lot in common, then 🙂
Chris Colose says:
”
I don’t know why anyone finds Judith’s confusions surprising anymore…
”
Dr Curry is not confused. She is publicly declaring her almost complete ignorance of, and general lack of interest in, the relevant science. She’s made a blog-scientific career out of saying “We don’t know much, and everything will be fine”. Pretty straight-forward, if you ask me.
Confusion would require cognitive dissonance – and Curry very cleverly avoids that at all costs.
Cognitive dissonance may, however, afflict those who try to imagine that Dr Curry is a scientist.
lerpo-
50/50 is not a special number. There’s no reason this should be a null hypothesis. She played this game with warming attribution too, but Gavin among others have shown that she isn’t thinking about it correctly.
> nothing comes anywhere close to this.
I disagree:
Judy’s response:
Anything goes, as long as it doesn’t look like Skydragon stuff too much.
VRJH – an imaginary friend took the side entrance, pi eyed
ATTP –
Remember the joke about two comedians who live together and to make life easier they number all their jokes so if one says “#23” the other laughs “ha ha … love that one”.
One day one calls out “#143” and the other one goes “ha ha ha … I haven’t heard that one before”.
Replace ‘joke’ with ‘myth’ / ‘misunderstanding’. The catalogue of myths that the #crankattractors love are well documented, and recycled in true groundhog day style. “Ah yes, #myth43, risen from the dead for 21st time this year alone”.
I’d be surprised if you ever these days call out over your cornflakes “blimey … I haven’t heard that one before”. Or am I wrong?
Willard: “Anything goes, as long as it doesn’t look like Skydragon stuff too much.”
Yeah, but how long until she starts finding the Skydragon stuff “interesting” and fraught with too much uncertainty to really know for sure?
Curry seems to be following Salby’s sad path from academia to quackademia.
Occasionally at Skeptical Science we get angry comments from concerned members of the Society for the Protection of Cruelty to Dead Horses (trademark A Koestler).
“Surely nobody sane believes that any more. Stop the beatings immediately! Can’t you see that the poor beast has passed on?”
But every once in a while, as Richard Telford noted, the Uncertainty Monster requires feeding and somebody has to revive long-expired nags with names like “Volcano”, “Urban Heat Island”, “Cosmic Ray” or “Sea Foam”.
@Willard
The paper was generated by Mathgen, a piece of software that produces “Randomly generated mathematics research papers”.
http://thatsmathematics.com/blog/archives/102
I call it ‘The Micawber Effect’.
As an alternative to the bathtub, I also came across the metaphor of the young lad whose dad put a pound in loose change into his piggy bank each night and then the lad spent 99 pence a day on his bus fare to school, and whatever else little boys spend their money on. At the end of the year his piggy bank was full to the tune of £3.64.
Thanks, MikeH. Here:
Tumblr includes, cool!
Bartemis backtracked a bit:
It’s just a matter of time before he’ll go silent. My response is awaiting moderation:
My own preferred analogy is with a swimming pool – the greatest flow in to the pool being that from the filter return. Does a trickling hose raise the level significantly if it’s only a fraction of a percent of the total flowing into the pool from all sources, when all sources includes that filter return? The point, obviously, is that the water from the filter return came out of the pool, just as the CO2 released from vegetation came out of the atmosphere first. But, absent analogies, even a basic understanding of The Carbon Cycle short circuits the “too little to matter” arguments.
“Lord, enlighten thou our enemies. Sharpen their wits, give acuteness to their perceptions, and consecutiveness and clearness to their reasoning powers: we are in danger from their folly, not from their wisdom; their weakness is what fills us with apprehension, not their strength.”
— John Stuart Mill, “Essay on Coleridge”
Its kind of obvious in retrospect that someone goofed when they tried to justify the excess CO2 as anthropogenic by looking at the isotope ratios. That was a misguided move, but of course the agenda-driven skeptics held on to that like a pit-bull biting down on a bone. It’s sad that the skeptics use science to score points instead of for a deeper understanding, but they are political animals after all
> they are political animals after all
We all are, Web. We also are featherless bipeds.
A randomly walk through the carbon cycle is enough to dilute the vestiges of isotope origins. The smarter of the skeptics were able to see this but then aggressively use it to score points for their cause, not for once intending to further the understanding the science of diffusion.
> they are political animals after all
>> We all are, Web. We also are featherless bipeds.
MAN, n. An animal so lost in rapturous contemplation of what he thinks he is as to overlook what he indubitably ought to be. His chief occupation is extermination of other animals and his own species, which, however, multiplies with such insistent rapidity as to infest the whole habitable earth and Canada.
Ambrose Bierce – The Devil’s Dictionary
wilard
“Scientific arguments only need to be idiot proof. They don’t need to be closed under deduction, nor do they need to be daemon proof. Daemon-proof arguments are fun, but perhaps we should keep them for Gedankenexperimenten.”
I missed that.. Nice. I had to stop reading the thread.. too painful.
Lerpo: even leaving aside that aspect of it, the fraction is probably over 100%, but Curry seems stubbornly incapable of understanding that this is even a possibility.
http://initforthegold.blogspot.com/2015/01/more-than-all.html
I used to think this was obvious in the same way that ATTP seems to think it obvious. But then I realized it is not so simple. Calculating the fraction of anthropogenic emissions that remain in the atmosphere requires an implicit and unproven assumption: that absent anthropogenic emissions, CO2 would have remained rock steady at its pre-industrial value. But how do we know that is the case? Maybe if we had a control experiment the atmospheric concentration would have gone up anyway, and the anthropogenic fraction remaining in the atmosphere is smaller than it appears. Or vice-versa, as in the early anthropocene hypothesis. We don’t really know, and pooh-poohing such questions does nothing to convince skeptics (even genuine skeptics, not just pseudo-skeptics) otherwise.
When I look at the atmospheric CO2 load plotted vs. total emissions, I find it hard to believe that the one is not almost entirely due to the other. That is evidence, not proof. Accidental correlations do occur; when both quantities are steady trends, accidental correlations can appear to have very high statistical significance. There have been natural CO2 excursions comparable in rate, but not duration to what has been seen in the last century (the most recent in the 1940’s). There have been natural CO2 excursions comparable in extent, but not in rate, to what has been happening (and in those case the rate estimates are, I think, just lower bounds). So one can not just dismiss the possibility that some significant fraction of what has been happening is natural.
I remind you that if the ocean were to come into equilibrium with the atmosphere, the rise in atmospheric CO2 would be large compared to what has been observed in the last century.
Perhaps there is a reasonably convincing quantitative argument that can put upper bounds on the natural contribution. If so, I would like to be directed to it. If there is no such calculation, then the claim that “we don’t know” is logically valid. To simply dismiss it is to provide support for the skeptics charge that your opinion is based on belief, not science.
MT,
Indeed, that was a remarkable exchange with Judith Curry.
MikeM.,
I was going to try and illustrate why we think this is so, using a basic flux calculation, but I’ve just been for a run and don’t quite have the energy. I think what you would find is that if nature was slowly increase atmospheric CO2, our emissions would soon take over and dominate. If nature were increasing it rapidly, then we’d see atmospheric CO2 rising faster than our emissions. That it is rising slower than our emissions means that nature is a net sink and that we are the source of the long-term rise.
Not understanding the complaints about isotopes here.
The excess carbon is clearly of fossil origin. Isotopes are the chief weapon of deep paleoclimate and many other aspects of deep time studies.
http://www.realclimate.org/index.php/archives/2004/12/how-do-we-know-that-recent-cosub2sub-increases-are-due-to-human-activities-updated/
http://cdiac.ornl.gov/trends/co2/modern_isotopes.html
That the Wattsites get it wrong and then trumpet their errors wouldn’t much surprise me but that doesn’t mean that C-13 doesn’t tell us anything at all.
MT,
I don’t quite understand it either 🙂
There is much at Judith’s beyond any comprehension of mine.
Carbon isotopes is the least of it.
one random walk through a carbon cycle and the isotope fingerprint is gone.
Go with the diffusion argument — that at least works and can be applied to estimate how long the CO2 will stay around.
WHT,
No, I don’t think it is. It might be diluted, but it doesn’t go.
ATTP,
“I was going to try and illustrate why we think this is so, using a basic flux calculation”.
I think I know how that calculation is done. Imagine setting up the equations for that calculation; make it as simple or as fancy as you like. You get all the constants adjusted so that when you put in the actual amount of emissions to date, you get a mixing ratio of 400 ppm. Now redo the calculation with zero total emissions; what mixing ratio do you get? I’m guessing 280 ppm.
What information gives you that result? An implicit assumption. Unless, that is, every parameter in your model is obtained without making any use of actual atmospheric CO2 concentrations. Otherwise, how can you know what would have happened?
But without the correct diffusion kinetics, no one knows what the dilution is per cycle.
You have to start with the fundamental statistical physics. So it all hinges on a diffusion model
http://contextearth.com/2013/12/02/dealing-with-the-dynamics-of-diffusional-sequestration/
The heuristic Bern model is actually a diffusion model at its core — they make it into a series of damped exponentials to make it easier to work with. This is the real profile based on a maximum entropy range of diffusion coefficients:
MikeM,
It’s too late, and I’m struggling to set something up that makes sense.
WHT,
i don’t think that really matters if all you’re interested in is the origin.
Speaking of the Berne model:
The number of ze issues is amazing.
settin your sights kinda low, doncha know
ATTP, no reason to do much work yourself, Knorr (and others) have already done the work for you:
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.362.5329&rep=rep1&type=pdf
Marco wrote: “ATTP, no reason to do much work yourself”, apparently referring to ATTP’s statement “I was going to try and illustrate why we think this is so, using a basic flux calculation” which in turn was made in response to my claim “Calculating the fraction of anthropogenic emissions that remain in the atmosphere requires an implicit and unproven assumption: that absent anthropogenic emissions, CO2 would have remained rock steady at its pre-industrial value.”
Thanks to Marco for providing a reference in support of my claim.
I personally think the airborne fraction has historically been overused – it is a nice rule of thumb to say that historically, “about 40 percent of emissions have stayed in the atmosphere”, but because any trend (or lack of trend) in airborne fraction is a function of changes in both emissions and sinks, it is not really a physical indicator as such. For example, if we were to drop emissions by about 60% next year, the “airborne fraction” would probably jump instantly to near 100%. Similarly, if we were to double emissions next year, the airborne fraction would drop to 20%. This is because sinks are to a large extent a function of the difference between the concentration of carbon in the atmosphere and the concentration of carbon in the various sinks.
Mike M.: There is no doubt that 90% or more of the increase in CO2 concentrations is due to human emissions (both fossil and land use change) (with some remainder due to increased temperatures – which are likely a result of human emissions anyway – and some possibly due to natural variability). “blog proof” is always going to be limited: the best evidence is the long ice core record showing little variability in pre-industrial concentrations since the interglacial started, with arguments about mass balance and isotopes and such being supporting evidence. But for real proof, you actually need to read the papers by the real carbon cycle experts, who rarely engage in these arguments because they probably don’t even realize that this is still a thing: the paper I linked to in my earlier post is a good example. There, it isn’t just setting up an implicit assumption that in the absence of human emissions we’d go back to 280 ppm, it is modeling all the atmospheric and oceanic flows and using observations to show that the model is capturing all the details correctly (or, where there are deviations, trying to understand those deviations). Here’;s the paper link again: http://onlinelibrary.wiley.com/doi/10.1111/j.1600-0889.2009.00446.x/pdf
Marco,
Thanks.
MikeM.,
How does it support your claim?
MikeM “requires an implicit and unproven assumption: that absent anthropogenic emissions, CO2 would have remained rock steady at its pre-industrial value. But how do we know that is the case? ”
This seems to me to be a case of applying reasoning to the carbon cycle that wouldn’t be applied to other similar situations. If I had a joint bank account (I think I may have used this analogy before ;o) with my wife, which for the sake of argument attracts no interest or bank charges. The account started of with the £1,000,000 we already had (I wish! ;o). I pay in £1,000 a month and make no withdrawals, but notice that our balance rises only by £500. This means that I know that my wife is taking more money out of the account than she is putting in to the tune of £500 a month. Next we define my “bankbourne fraction” as being the proportion of my cumulative deposits that remain in the bank, and we can clearly see in this case this is 0.5. Now, in what way does this assume that our bank balance would have stayed rock steady at its initial value in the absence of my deposits?
Clearly it doesn’t depend on that assumption in any way, so why is the carbon cycle any different?
Note that the paleo record suggests that CO2 should have peaked at 280ppm in the holocene climatic optimum c.6000 years ago and have dropped substantially since then.
ATTP – “How does it support your claim?”. Look at their equations. In the absence of emissions, there are only small fluctuations around a constant mean. OK, that requires a slightly flexible definition of “rock-steady”, but in a way that that physicists should be comfortable with.
dikranmarsupial – “Now, in what way does this assume that our bank balance would have stayed rock steady at its initial value in the absence of my deposits?” Because it overlooks the £1,000 a month interest the bank is paying on your balance.
Angrew Dodds – “Note that the paleo record suggests that CO2 should have peaked at 280ppm in the holocene climatic optimum c.6000 years ago and have dropped substantially since then.” Glad that someone else sees the issue. We don’t know what would have happened if not for our emissions.
MMM – “it is modeling all the atmospheric and oceanic flows and using observations to show that the model is capturing all the details correctly”. But the models don’t reproduce the details over, say, a full glacial cycle. And I am reasonably certain that they don’t quantify the extent to which the near constant pre-industrial holocene CO2 has been due to anthropogenic activity. Otherwise Ruddiman’s early anthropocene hypothesis would be either accepted (if only partially) or dead. The models are not ab initio; they have to adjust all sorts of parameters to fit the data. And in doing that they have to make reasonable (not the same thing as valid) assumptions – such as natural pre-industrial sources and sinks in balance at 280 ppm.
“because it overlooks the £1,000 a month interest the bank is paying on your balance.”
So as my wife’s transactions represent all natural sources (deposits) and sinks (withdrawals) and my deposits represent all anthropogenic emissions, and the balance represents the carbon that ends up in the atmosphere, just what does the interest represent? If it isn’t nature and it isn’t anthropogenic, that leaves extraterrestrial and supernatural, which is it to be?
Mike M.,
Okay, I wasn’t quite clear. I don’t quite understand what your claim is. Could you repeat it?
“And in doing that they have to make reasonable (not the same thing as valid) assumptions – such as natural pre-industrial sources and sinks in balance at 280 ppm.”
Read chapter 6, Mike M:
Click to access WG1AR5_Chapter06_FINAL.pdf
Mike M. wrote: “because it overlooks the £1,000 a month interest the bank is paying on your balance.”
I wish! ;o)
Mike M.: You originally wanted an upper bound on the potential natural contribution. But you seem to confuse “we don’t know everything exactly” (e.g., “We don’t know what would have happened if not for our emissions”) with “we can’t place bounds”. We have 800,000 years of CO2 concentration data (http://www.epa.gov/climatechange/images/indicator_downloads/ghg-concentrations-download1-2014.png). In that entire time period, CO2 never exceeded 300 ppm. I think it is therefore pretty safe to say any exceedance of 300 ppm since the Industrial Revolution is manmade, even if we don’t know _exactly_ what would have happened in the absence of human emissions. So, that puts one conservative upper bound on the natural contribution of 17% (20/120).
Also, like many contrarians, you don’t seem to understand that there is a vibrant research community out there working on this, and assume if you don’t know about something, it doesn’t exist: for example, “But the models don’t reproduce the details over, say, a full glacial cycle.” Actually, there are plenty of papers applying carbon cycle models to glacial cycles that get most of the details right. Try google.
Also, while it is true that carbon cycle models are not ab initio – the last time I worked in the ab initio modeling field, the limit for ab initio modeling was, I believe, 2 atoms – that does not mean that they make any assumptions about sources and sinks being in balance at 280 ppm. This is like contrarians claiming that models assume a water vapor feedback. The balance at 280 ppm is a result of the model physics in combination with the observations, not an assumption, much like increasing water vapor with temperature is a result of model physics. And the fact that the models get SO MUCH RIGHT, for multiple isotopes, across geographic and temporal space, is evidence that there is some pretty good understanding there. And the result of that understanding is being able to pin down the anthropogenic contribution even tighter than the 17% I handwaved above. I don’t know exactly how far – I’m not personally a carbon cycle expert – but I wouldn’t be surprised if it was 8% or less, based on the 10 ppm upper bound for the carbon cycle response to a degree of ocean temperature rise.
And Ruddiman’s hypothesis is about the difference between 245 ppm and 280 ppm, and goes in the wrong direction to support a greater natural contribution to today’s CO2 levels.
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