Arguments
The Independence of Global Warming on Residence Time of CO2
Posted on 1 March 2012 by Dikran Marsupial
A hearty congratulations from the SkS team to our own Dikran Marsupial for getting a response to Essenhigh (2009) published.
A climate myth that crops up far more often than it should is that the rise in atmospheric CO2 since the industrial revolution is not caused by anthropogenic carbon emissions, but is instead a natural phenomenon. This has been addressed repeatedly on SkS, for example see CO2 increase is natural, not human-caused. An example of this argument is found in the paper "Potential Dependence of Global Warming on the Residence Time (RT) in the Atmosphere of Anthropogenically Sourced Carbon Dioxide" by Prof. Robert Essenhigh that appeared in the journal Energy and Fuels in 2009. The argument is easily refuted by the observation that the rate at which atmospheric CO2 levels are rising is less than the rate at which we are releasing CO2 into the atmosphere from fossil fuel use, which implies that the natural environment must be a net carbon sink, taking in more carbon each year than it emits.
More formally, let Ea represent annual carbon emissions from anthropogenic sources (fossil fuel use and land use change), En represent the carbon emissions from all natural sources (the oceans, soil respiration, volcanos etc.) and Un represent the uptake of carbon by all natural carbon sinks (oceans, photosynthesis, etc.), Ua would be the uptake of carbon due to anthropogenic activities, but this is essentially zero, so we can safely exclude it from the analysis. Then assuming that the carbon cycle obeys the principle of conservation of mass (any carbon emitted into the atmosphere that is not taken up by natural sinks remains in the atmosphere), the annual change in atmospheric CO2 is given by:
C' = Ea + En - Un
This can be rearranged to give an estimate of the difference between annual emissions from all natural sources and annual natural uptake by all natural sinks.
En - Un = C' - Ea
We have accurate, reliable data for the growth of atmospheric CO2 and for anthropogenic emissions (for details, see Cawley, 2011). Both of these are displayed below, along with an estimate of the net natural carbon flux En - Un. The fact that the net natural flux is negative clearly shows that natural uptake has exceeded natural emissions every year for the last fifty years at least, and hence has been opposing, rather than causing the observed rise in atmospheric CO2.
Given that this myth is so easily refuted, it is not perhaps surprising that it occurs much more rarely in scientific journals than in the media and the blogsphere, as the reviewers used by an appropriate journal are likely to be sufficiently familiar with the workings of the carbon cycle to readily identify the flaw in the reasoning (n.b. Prof. Essenhigh's paper was published in Energy and Fuels, a journal that is perhaps only tangentially concerned with carbon cycle issues).
Here at SkS, we like to be able to support our arguments with references to papers that have been published in peer-reviewed journals. Unfortunately, as far as I was aware, there were no such papers in this case; that anthropogenic emissions are responsible for the post-industrial rise in atmospheric CO2 is so self-evident that ordinarily a paper demonstrating that fact would be too trivial to be publishable. So I decided to write one! I am pleased to announce that my rebuttal of Prof. Essenhigh's paper has recently appeared in the journal:
Gavin C. Cawley, "On the Atmospheric Residence Time of Anthropogenically Sourced Carbon Dioxide", Energy & Fuels, volume 25, issue 11, pages 5503–5513, 2011.
I hope that the existence of a peer-reviewed refutation will be of some use in preventing this myth from propogating further. I hope to be able post a longer blog article at some point, setting out the arguments in more detail.
I am very grateful to the anonymous reviewers, Robert Essenhigh, Philip Goodwin, Riccardo Reitano, Nigel Franks, Andy Skuce, and John Cook for their helpful discussion and comments (sorry if I have missed anybody out) and to all at SkS generally for the work they have put in to refuting incorrect arguments that crop up in the climate debate. Thanks also to Ferdinand Engelbeen for his tireless efforts in addressing this myth in various climate blogs.
@CBDunkerson No faults in the logic were identified, IIRC most of the reviewers comments were related to adding caveats and clarifications (the models used were very crude, and only really represents the initial fast response of the carbon cycle, so the true adjustment time is likely to be longer than the estimate of about 74 years given in the paper). Essentially the caveats explained why the models were unduly optimistic about the rate at which CO2 levels will fall following a cessation of anthropogenic emissions (most of this is in the penultimate section "limitations of the one-box model", Philip Goodwin's help was invaluable in writing that section!).
@Dana1981 I should add that my email correspondance with Prof. Essenhigh was entirely good-natured; while I didn't convince him that his conclusion was incorrect, he was unfailingly a gentleman in setting out his position and in the subsequent discussion.
From a comment at Eli's
Thanks Dikran for doing this.
If I take the plug out of the handbasin in my bathroom I can turn on the cold water tap to a point where the basin remains half full - ie the water runs in from the tap at the same speed that it escapes through the plug hole. If I then turn on the hot tap a little the water will rise in the basin and eventually overflow. The residence time in the basin is very short, the percentage of hot water in the basin at any time will be very low but I defy anyone to argue that the overflow was not caused by my turning on the hot tap.
Hypothetically imagine a natural carbon sink that emits Co2 if current concentrations are above a specific equilibrium level, and absorbs Co2 if it is above this level. And assume that this equilibrium level increases as temperatures increase, and that the carbon sink acts quickly enough to maintain Co2 concentrations close to the equilibrium level.
Then it would be possible for man to emmit Co2, this hypothetical sink to be a net absorber of Co2, and the overall increase in Co2 be determined not by man's emissions but by the hypothetical sink.
The ocean does act a little like this, and has a preferred equilibrium for Co2 concentration that depends on temperature, and will tend to absorb or emit Co2 as required to maintain this equilibrium.
Some more understanding of what Co2 is doing in the ocean is required to rule out natural sources of the Co2 increase.
Fortunately, we are already very well aware that the oceans are taking up CO2 at a very high rate, leading to an ocean acidification which is effectively unprecedented in geological history. There is a long series here on Skeptical Science going through the chemistry which demonstrates this is the case.
This is despite the rise in temperatures, which would normally result in the oceans outgassing CO2.
The oceans have (as per the ice core record) absorbed or release ~90ppm of CO2 over a 5-6C temperature swing, with a time delay of 500-800 years. Not due to atmospheric CO2 concentration changes, mind you, but due to solubility changes with temperature. The oceans therefore do not fit your hypothetical, as atmospheric concentrations have changed in sync with our emissions for the last 150 years or so, with natural sinks (primarily the oceans) absorbing only half of our emissions.
In regards to the oceans, I would suggest reading the OA Is Not OK series, Why ocean heat can’t drive climate change, and What is causing the increase in atmospheric CO2 to review the mass balance issues.
We certainly understand enough of what CO2 is doing in the oceans to determine that the oceans are not the source of CO2 increases over the last 150 years.
His claim that CO2 levels spiked 1000% and declined in 16 years only measured the rare C14 values, not the CO2 well-mixed global levels. Those levels showed no spike at all to nuclear testing.
He connects CO2 levels as a consequence of warming while skipping two basic tests. First, there's no record of a similar CO2 rise of 25%/century anywhere in the geological or historical record. Second, if .7dC warming can push CO2 levels up 40% (his claim), there would be a GHG record that looked like a seismograph output showing extremely unstable values for atmospheric CO2. Basically, he's failed to review the data, and he's missed the cacophony claiming CO2 is rising WITHOUT temperature rising.
Then he constructs a one-way model that has no relevance to the carbon cycle - he missed the basic difference between residence time in one regime (atmosphere, land, ocean) that averages about 8 years; and the residence time of the system load in the biosphere as a unit - which is in the 50 to 200 year range the IPCC outlines.
Feel sorry for a man when the need to invent a paradigm, that embarrasses academic degrees and a professional status, that fails to listen and learn before it talks and teaches.
The '1000%' spike source
Anthropogenic CO2 levels are currently controlled by anthropogenic emissions, the net environmental sink is demonstrably unable to cope with our current level of emissions, so if we carry on at the current rate or higher, atmospheric CO2 levels will continue to rise. If we cut emissions sharply enough that the net environmental sink outstrips anthropogenic emissions, then atmospheric levels will fall. The choice is in our hands.
The equilibrium concentration of the ocean is also determined by the difference in partial pressure between the surface waters and the atmosphere, not just temperature. Our emissions have increased the CO2 content of the atmosphere and that has changed the fluxes into and out of the oceans so that the ocean is now a net carbon sink.
If someone wants to argue that the rise in temperature is causing the rise in CO2 (e.g. due to ocean degassing) they need to be able to explain why the oceans are so much more temperature sensitive now than they were at the end of the last major glaciation. At that time there was a similar rise in CO2 levels (although much slower) but the change in temperature was about ten times that we have observed since the end of the industrial revolution.
"Some more understanding of what Co2 is doing in the ocean is required to rule out natural sources of the Co2 increase."
While more understanding is always a good thing, this statement is simply false. If the observed rise were due to natural sources, the annual rise would be greater than anthropogenic emissions, instead of less. This is a simple matter of accounting, and is valid assuming conservation of mass, which seems a pretty reasonable assumption.
The C14 in the atmosphere is created in the upper atmosphere by the action of cosmic rays on nitrogen. This means that the exchange fluxes replace C14 in the atmosphere with lighter isotopes of carbon, rather than with more C14. This means that C14 analysis only tells you about the residence time, but not the adjustment time.
I think that the confusion arises from not appreciating the difference between residence time and adjustment time, and the effects of the vast exchange of carbon between the atmosphere and oceans/terrestrial biosphere. It took me some time to fully appreciate when I first came across this argument.
Please ignore whoever argues about the importance of residence time and CO2.
Dikran, good work!!! It is however, akin showing that the earth is spherical to flat-earthers....
The important half-life is the one relating to the ocean sequestration until saturation which is in the order of hundreds of years, assuming emissions of CO2 ended today.
In your exchanges with him, what arguments did Essenhigh use to support his position. I can't imagine a defensible argument. I mean, this distinction between residence time as applied to tracers, and net uptake as applied to total CO2 concentrations and the role of the ocean has been well stablished since Suess and Revelle. As you show it is a simple matter of accounting. This was established science before I was even born!
Not if the equilibrium level is determined by temperature, and temperature is rising rapidly.
As an example, consider a see-saw with a small bucket on one side (atmosphere) and large swimming pool on the other side (ocean). Connect them with a pipe, and poor water slowly into the bucket, while tilting the see-saw so that the bucket gets lower and the swimming pool gets higher. With the right balance, it would be possible to pour water into the bucket at a rate faster than the water level in the bucket is rising. Water would be flowing through the pipe from the bucket to the swimming pool. But the level in the bucket would not be determined by the amount of water poured in, but by the tilt of the see-saw.
I certainly do not believe that the oceans are responsible for the increase in Co2, but pointing out it is theoretically possible, unless you consider additional facts beyond what is presented in your argument.
http://mtkass.blogspot.co.nz/2011/08/end-of-ice-ages.html
However, isotopic evidence from CH4 in ice bubbles points to this being of swamp or oceanic source rather than clathrate.
Fischer et al 2008
or
Hinrich Schaefer, Michael J. Whiticar, Edward J. Brook, Vasilii V. Petrenko, Dominic F. Ferretti, Jeffrey P. Severinghaus 2006 Ice Record of 13C for Atmospheric CH4 Across the Younger Dryas-Preboreal Transition, Science 25 August 2006: Vol. 313. no. 5790, pp. 1109 - 1112
We report atmospheric methane carbon isotope ratios (13CH4) from the Western Greenland ice margin spanning the Younger Dryas–to–Preboreal (YD-PB) transition. Over the recorded 800 years, 13CH4 was around –46 per mil (); that is, 1 higher than in the modern atmosphere and 5.5 higher than would be expected from budgets without 13C-rich anthropogenic emissions. This requires higher natural 13C-rich emissions or stronger sink fractionation than conventionally assumed. Constant 13CH4 during the rise in methane concentration at the YD-PB transition is consistent with additional emissions from tropical wetlands, or aerobic plant CH4 production, or with a multisource scenario. A marine clathrate source is unlikely.
[DB] The Schaefer study can be found here (free download after free registration):
http://www.sciencemag.org/content/313/5790/1109.abstract
"If we cut emissions sharply enough that the net environmental sink outstrips anthropogenic emissions, then atmospheric levels will fall. The choice is in our hands."
There's no evidence that such a process or balancing act exists. If the sinks had that magical power and capacity, atmospheric levels would not have stayed at interglacial peaks of 280ppm cycle after cycle.
The dance-partners are temperature and moisture. The true significance of the pollution problem has been diagrammed out repeatedly over the last decade - temperature/GHG levels will remain charged for centuries, and future interglacial cycles will respect the additional GHGs in the biosphere. There's a research paper around somewhere that calculates real sequestered-removal is in the order of 2 ppm per cycle.
The natural carbon cycle has a stable equilibrium as the result of opposing feedback mechanisms balancing. If the equilibrium is peturbed, the balance is lost, and act is to restore the equilibrium. Such dynamic equilibria are common in the natural world.
If we peturb the equilibrium with anthropogenic emissions, then the feedbacks will no longer be balanced (e.g. because carbon flux between the atmosphere and oceans depends on the difference in partial pressure between the surface waters and the atmosphere), and the natural environment becomes a net carbon sink, trying to retore the equilibrium.
As for glaciation, the peturbation started by Milankovic cycles cools the oceans, which take in more CO2, which reduces the GHE slightly, ice caps grow, increasing abedo, which also results in cooling. The carbon cycle does re-establish the equilibrium, but it takes thousands of years to do so, just as it will take thousands of years to reestablish the equilibrium after we cease anthropogenic emissions (stabilisation will ocurr more quickly).
The fact that atmospheric CO2 levels are rising more slowly than anthropogenic emissions established beyon reasonable doubt that the natural environment is a net sink hand has been opposing the rise. Any argument that suggests that the oceans were degassing would require the terrertial biosphere to be taking up half of anthropogenic emissions AND the CO2 released by ocean degassing in order for mass to be conserved. Even in that case, the natural environment is still a net sink and is opposing the rise in CO2.
Now as it happens, scientists have monitored the oceans, and we know that it is a net carbon sink. See the work of e.g. Corrine Le Quere.
"owl905 The fact that CO2 levels had been pretty stable between 180ppm and 300ppm for the last 400,000,000 years, even during glaciations, is a pretty good indication that a balancing process exists."
Simply untrue. That oscillation is a reflection of the CO2 load, and the biosphere context during some of the recent cycles. The claim of an intervening set of forces to return it to a previous state (after the GHG proportions are changed, the oceans warmed, and the biosphere disrupted) is nothing more than wishful fantasy. It ignores the geological record of different patterns in different ages.
While not trying to be abrasive, you're trying to work both sides of the street arguing in favour a return to natural levels in a useful period of time while confusing this by discussing multiple millenniums. Frankly, stating that "the carbon cycle does re-establish the equilibrium" is refuted by the track record - episodes of massive GHG injections into the biosphere have upset things for very long periods - the Permian, the KT, and the PET juncture. The 400k record isn't even the major story - the record shows much longer interglacials before that time - so your 'natural balance' claim consists of 3 out of the last 8 or more inter-glacials. The geology record shows no period where GHG levels fell 40% in less than a century.
Additionally, your remark to MHauber that the missing CO2 accounting is virtual proof of a mystery sink is far beyond what the accounting, the chaotic nature of the pollution, or the science, supports.
And that ocean 'carbon sink' is so poor, ocean acidification has become one of the great pollution problems. That's exactly why suggestion of oceanic sequestering of a decade ago went to the trash heap.
Even the PETM resulted, eventually, in a stabilized climate, as the excess CH4 and CO2 were (over tens/hundreds of thousands of year) absorbed by weathering and the carbon pump to the ocean depths. The geologic balances are certainly not speedy, but they are there, and cannot be ignored.
"...the record shows much longer interglacials before that time..." - Yes, driven by the Milankovitch cycles. Those provide the time frame for the glacial intervals.
I don't believe Dikran (or anyone else, for that matter) is stating that CO2 levels, ocean acidification, or other climate change effects will "...return to natural levels in a useful period of time..." - but rather that they are stating that over the long term view the climate will find another equilibrium. Although perhaps one we don't find particularly pleasant...
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Whether we enjoy the process or not, the carbon cycle will stabilize over the long term - certainly after we've burned all the fossil fuels. But in the meantime, we're looking at a very long and unpleasant interval with global warming, mass extinctions, agricultural losses, and other issues that can be (hopefully) mitigated to some extent by acting while we're not over-committed to warming.
(i) The ice core data shows that even through an event as extreme as a major glaciation, the response of the natural carbon cycle is a change of only about 90-100 ppm, and in each case, an equilibrium is restored on the scale of tens of thousands of years. This is true, even though CO2 is thought to have provided positive feedback amplifyng the change in forcings. So why doesn't this positive feedback result in runaway warming or a snowball earth? Simple, there are other feedback mechanisms that exist, that oppose it. David Arher's primer on the carbon cycle is well worth reading, one of the things that it explains is why CO2 can provide positive feedback on short (by geological standards) timescales and negative feedback on longer timescales (via the "weathering thermostat")
(ii) You wrote "arguing in favour a return to natural levels in a useful period of time" I am arguing no such thing. The adjustment time estimate of 74 years is likely an underestimate of the most rapid response of the oceans to an increase in CO2. This won't bring us back to pre-industrial levels. For that you need the slower response processes by which carbon is taken down into the deep ocean (thousands of years) and a full return to pre-industrial conditions is only possible by permanent sequestration of the carbon back into the lithosphere by weathering, which will take tens to hundreds of thousands of years. This is mentioned in my paper and links are given to more authoratative works. We can however stabilise CO2 levels much more quickly by cutting emissions to a point where they match environmental net uptake, which will limit the change to our climate.
(iii) Additionally, your remark to MHauber that the missing CO2 accounting is virtual proof of a mystery sink is far beyond what the accounting, the chaotic nature of the pollution, or the science, supports."
The sink isn't a great mystery, we know that the carbon is taken up by the other reservoirs, but we don't know exactly how much is going into each reservoir. However the point is that assuming conservation of mass, we don't need to know where in the natural environment the carbon is going to know with high confidence that it is being taken up by the natural environment. If you think this is beyond science, then the mass balance argument has appeared in several papers, at least one of which was referenced in my paper. I suspect the reason it doesn't appear more often is that it is so obvious as to be taken for granted. If you think it is false, then please do point out which step is incorrect.
(iv) "And that ocean 'carbon sink' is so poor, ocean acidification has become one of the great pollution problems. That's exactly why suggestion of oceanic sequestering of a decade ago went to the trash heap. "
Actually that is not correct. The surface waters are still taking up substantial amounts of anthropogenic emissions (note the evidence of an increase in the airborne fraction is highly equivocal, which suggests that the surface ocean has not yet saturated). The thing that limits the rate at which the oceans can take up CO2 on longer timescales is the transfer of CO2 from the surface waters to the deep ocean. On long timescales, the oceans are still very powerful carbon sinks.
i) the discussion abour runaway or not isn't part of the discussion
ii) You still miss the point and try to argue both sides of the street - Dikran15's claim was that levels would start to fall at the point of anthro-cutoff. You said it will and it won't. It comes across as noise.
iii) Same thing -
iv) Yes it is true. It's a poison sink. Again, you don't appear to understand the issue.
"The sink isn't a great mystery, we know that the carbon is taken up by the other reservoirs, but we don't know exactly how much is going into each reservoir."
More pedantic double-speak. And you carry on the assumption that the accounting is correct and that background measurement is globally representative.
@KR - Dikran15's statement was exactly that levels start falling when the AG CO2 stops: "If we cut emissions sharply enough that the net environmental sink outstrips anthropogenic emissions, then atmospheric levels will fall."
Your response that turns a comment about 'inter-glacials' into a Milankovich dump about glacial cycles is off-topic.
Go back and read Dikran's comment restated here. And then stop rephrasing it into something he didn't say as a rebuttal.
You (in your post here, and IMO) appear to have overstated the case - "The claim of an intervening set of forces to return it to a previous state (after the GHG proportions are changed, the oceans warmed, and the biosphere disrupted) is nothing more than wishful fantasy." - this is not correct, the various feedbacks on the carbon cycle will act upon any disturbance. Just, unfortunately, over a rather long time period that will be very unpleasant for us. My post here was an attempt to point out those issues. Perhaps it was not stated very clearly (in which case mea cupla), but that was indeed the intent.
"While not trying to be abrasive..." - While feel I agree with you on the basic issues of anthropogenic global warming, I would submit that you are not succeeding in this aim. You have a tendency to be harsh on anyone with a differing view, when it may well be a difference of emphasis or presentation - and that comes across as disdainful.
You write: "And you carry on the assumption that the accounting is correct and that background measurement is globally representative."
The accounting of anthropogenic emissions would have to be wrong by a factor of about 0.5 to change alter the conclusions. It is extremely unlikely for the error to be anything like as much as that. If nothing else, governments monitor fossil fuel use for the purposes of taxation, so there are very good records.
Regarding background measurements being globally representative, well while I have used the Mauna Loa record, there is a global network of monitoring stations, which show that the background level that is pretty much the same, regardless of where you are. There is a difference of IIRC about 2ppm between the North and Southern hemisphere (because most anthropogenic emissions are from the Northern hemisphere and it takes time to equalise globally). Satellite data (AIRS) suggests that CO2 in the atmosphere is not completely well mixed, but the differences do not alter the conclusions of the mass balance argument. This is because the mass balance argument depends on the rate of change of atmospheric CO2, not on its absolute level at any particular location. As it happens the Mauna Loa observatory is in the tropical region where levels are comparatively stable.
If I look at the RSS AMSU temperature series next to the Mauna Loa CO2 emmission series (http://www.esrl.noaa.gov/gmd/ccgg/trends/), I have the impression there is some correlation. I searches for a lag-time for a CO2 effect and didn't find it.
Ofcourse the commulative effect is the most important but am I seeing spooks in this more immediatebut perhaps illusionay correlation?
However, taking the difference between this years CO2 and last years to get the annual rise totally eliminates the linear trend, so the correlation only really tells you about the year to year variability, but tells you nothing about the long term trend.
So in short, you are seeing a genuine correlation between temperature and CO2, but the correlation is most noticeable in the variability, and a long term rising trend in temperatures is exactly what you would expect if CO2 levels are rising.
HTH
It certainly does. Thank you, for this answer but more particularly for your posts in general.
See: The Carbon Cycle Response to ENSO: A Coupled Climate–Carbon Cycle Model Study -Jones (2001).
"Climatic changes over land during El Nino events lead to decreased gross primary productivity and increased plant and soil respiration, and hence the terrestrial biosphere becomes a source of CO2 to the atmosphere. Conversely, during El Nino events, the ocean becomes a sink of CO2 because of reduction of equatorial Pacific outgassing as
a result of decreased upwelling of carbon-rich deep water. During La Nina events the opposite occurs; the land becomes a sink and the ocean a source of CO2."
@Rob, many thanks for the reference, it'll be useful if I ever find the time to write up an advanced version of the residence time rebuttal!
To continue the bath tub: What if the sink can widen or narrow, according how far you open the hot water tap only, but the cold water tap opens, too? This is a reactive system and the mass balance seems not to apply so front-up as explained here.
So we could have the situation that cold water increases its flow, you also add some droplets of hot water, and the sink expands to flush an amount equivalent to the additional hot water, but not to the arbitrary additional cold water. Water level would rise.
If you stopped the hot water, the sink would narrow back to normal, but the water level would still rise because of the arbitrary additional amounts of cold water. Right? Wrong?
Again, this thought experiment is just to point out that mass balance is not an argument for itself that rising CO2-levels in the atmosphere automatically attribute to man made CO2. Which means we need more linked evidence to that.
Explanation by analogy is very useful. But reasoning from an analogy back to a system under investigation is only plausible if that portion of the analogy holds - it's much better to work matters out directly in the system of interest. In this case expansion and contraction of the bathtub is something you have not mapped to the carbon cycle.
I would note, in addition, that mass balance/CO2 discussions are more relevant in the How do human CO2 emissions compare to natural CO2 emissions or possibly the Murry Salby CO2 source threads. This discussion is on CO2 rise rates and interactions with atmospheric residence time...
No. The mass balance argument is sufficient, in and of itself, to show that rising atmospheric CO2 levels have been caused by humans.
For each of the past ~50 years we know the amount of CO2 released by human fossil fuel burning and the amount of CO2 increase in the atmosphere. In all of those years the amount we released has been greater than the amount which accumulated. Thus, the various 'analogy possibilities' you raised are not not possibilities. The only way that human emissions exceed atmospheric accumulation each and every year is if the net of other (i.e. natural) factors is taking some of the carbon out each year. It doesn't matter if the various components increase, decrease, or do the lambada... so long as we emit more than accumulates we are unquestionably responsible for 100% of the accumulation.
KR, CBDunkerson, the flexible sink would react only to fossil, not to natural CO2 input. That is where I would map the analogy. E.g. because of a yet unknown mechanism, sinks are taking up fossil sources faster than from natural sources, and in place of fossil CO2, the increase in atmosphere really comes from some more arbitrary methane belching. This is of course speculation, as there is no knowledge about something like this, and I only do this in order to understand sceptics like Julian. It is sort of an "maybe we don´t know everything about it" point, which you can never ever totally deny.
Your response here is, well, we do know a lot and nothing makes this kind of speculation very probable. E.g. the mechanism of oceans' uptake could be not so selective that it would priority-process the fossil CO2; the increase is proportional to increased fossil output so that breaking that obvious connection would require more than just abstract speculation; etc.
That´s why I agree, it is indeed about LGM or Leprechauns. But who knows, maybe they are real, anyways... ;)
A sink that only reacts to fossil, not natural CO2 is not speculation - it's fantasy. When the the CO2 from this "natural" source has the exact same characteristics (isotope ratios, O2 reduction, etc., which are required as a result of the other evidence) as the fossil source, how is the natural sink supposed to know to suck up the fossil CO2 and not the natural source? "Not very probable" is in the rhealm of "when monkeys fly out of my butt".
However, even if it weren't inherently impossible... it would be contradicted by half a dozen other lines of evidence. That is, if a magical human CO2 sink existed then the strong correlation between human CO2 emissions and atmospheric accumulations (both in timing and rate) would be anomalous. Likewise, the C12/C13/C14 ratio changes indicate that fossil fuels are the source of the atmospheric increase... which would be odd if a magical sink were removing all the fossil fuel carbon. Et cetera.
So even if we allow for one 'leprechaun' to invalidate the mass balance argument we'd still need to throw in a pixie, a couple of unicorns, and a bandersnatch in order for it to hold up in the face of the other evidence.
'It is not technically impossible'... well, no... but you'd have to be crazy to think otherwise.
1) The mass balance argument is an inductive, not a deductive argument. That is, its premises can be correct and its conclusion false.
2) The same is true of the other nine arguments I presented leading to the same conclusion. This is true both separately and conjointly.
3) The same is also true for the various arguments that the Sun rather than the Earth is at the center of the Solar System, that Relativistic, not Aristotelian, mechanics govern the behaviour of bodies, and indeed, that the Earth is not flat.
4) The question of interest is not whether the truism that scientific evidence is not deductive is true, but whether an alternative explanation to anthropogenic emissions as the dominant cause of the increase in atmospheric CO2 can be found. Any such explanation must be more elegant than the anthropogenic explanation otherwise it is a worse explanation and should be rejected. The alternative explanation should also make novel falsifiable predictions, and not resort to magical thinking (such as relying on physical sorting mechanisms that can distinguish between CO2 molecules which are physically identical, but have a different origin).
No such alternative explanation has ever been proposed. Deniers have not even come close to proposing one. The nearest they have come is to propose an alternative that explains just one aspect of the evidence and rigorously ignores all other evidence and which makes no novel empirical predictions. Every proposed denier explanation I have seen has been contradicted by known facts, and makes fewer empirical predictions than does the theory of the anthropogenic cause of the increase in CO2 concentration. Frankly, I have seen more coherent and better worked out theories of geocentrism than any theory of natural increase in CO2 proposed by deniers.