In light of the recent Nature study I discussed yesterday in Settled Science Springs a Leak, it’s time to revisit a position I’ve long held—and, it turns out, one that now requires correction. Specifically, the long-standing assumption that carbon isotope ratios (δ¹³C and Δ¹⁴C) provide unambiguous proof that the rise in atmospheric CO2 is almost entirely anthropogenic.
For years, I’ve maintained that anyone challenging the anthropogenic origin of the CO2 increase had to address the isotope fingerprint argument before their work could be taken seriously. Submissions that didn’t engage with the Δ¹⁴C or δ¹³C evidence were declined, often with little further discussion. That confidence, I now recognize, was misplaced.
The recent study, Old carbon routed from land to the atmosphere by global river systems, has fundamentally altered the context in which the isotopic attribution arguments operate. According to the authors, 59% of global riverine CO2 emissions are sourced from old carbon, meaning millennial-aged carbon from deep soils, sediments, or rock weathering—not recently photosynthesized biomass.
These emissions are radiocarbon-dead, lacking the Δ¹⁴C signal, and are often depleted in δ¹³C—precisely the isotopic traits long attributed to fossil fuel combustion. Until now, these natural contributions were omitted from carbon cycle models and largely absent from attribution logic.
“This previously unrecognized release…equates to 1.2 ± 0.3 Pg C year⁻¹… and 41 ± 16% of river CO2 emissions…could contain recent anthropogenic-derived carbon.”
The implication is unambiguous: the very isotopic signatures used to claim anthropogenic dominance in atmospheric CO2 are now also emerging from newly recognized natural processes at global scale. These rivers are essentially exhaling ghosts—carbon that predates the industrial era by centuries or even millennia—and doing so in quantities on par with major biospheric fluxes.
The Isotope Argument Just Got Muddy
The confidence behind isotopic attribution rested on the idea that only fossil fuels could be responsible for the observed depletion in δ¹³C and Δ¹⁴C. This paper shatters that by showing that the Earth has its own large, continuous natural source of isotopically similar carbon.
Previously, isotope-based attribution was treated like a fingerprint match: the crime scene carbon looked like fossil fuel carbon, and fossil fuel emissions matched the rising levels in the air. Case closed.
But now we know there’s another suspect. And he’s been walking around in the open the whole time—we just didn’t test the rivers.
A Second Look, Long Overdue
To be clear, this doesn’t prove the rise in atmospheric CO2 is not anthropogenic. But it does mean that the evidentiary strength of isotopic data as a sole or dominant indicator of that origin is now substantially weaker.
Climate science has always leaned heavily on isotope ratios because they offered a veneer of mathematical certainty. With the carbon mass balance deeply uncertain, and the models riddled with arbitrary assumptions, the isotopes offered something that looked solid.
Now that, too, is compromised.
To those contributors whose submissions I rejected on the basis that they did not sufficiently address the isotope argument: I owe you an apology.
You raised objections in good faith. Many of you suspected that natural processes were more complex and underappreciated than the models allowed. This paper has proven that instinct correct. You didn’t need to disprove the Δ¹⁴C narrative outright; you simply needed the science to catch up to the question.
It just did.
Confidence Requires Humility
This study is a stark reminder that confidence in science is no substitute for humility in the face of uncertainty. That applies as much to editorial policy as it does to modelers, activists, or policymakers. In this case, the error was not in demanding rigor—but in assuming it had already been achieved.
Carbon isotope ratios remain valuable tools, but they are no longer unimpeachable witnesses. They are part of a broader, far more uncertain picture of how carbon moves through the Earth system—a picture we now know was missing a major river-fed chapter.
A Final Word
The takeaway is simple: no one gets to claim the science is settled when it just got rewritten.
And so, to every researcher, independent thinker, or persistent submitter who brought forth alternative views and was met with the standard rebuttal—“Come back when you’ve dealt with the isotope ratios”—you were due more openness than you received.
For that, I offer a strong, clear, and unreserved apology.
Discover more from Watts Up With That?
Subscribe to get the latest posts sent to your email.
Dear Charles, you don’t need to apologize for things you (and the climate community) didn’t know…
That being said, that finding doesn’t change the case for the human contribution of the current rise of CO2 in the atmosphere with one per mil at all…
The mistake of many in this case is that one confuses a more or less constant natural input (even if that is huge) with a change in input from an internal or external source…
The drop in 13C/12C ratio (δ13C) since about 1850 completely parallels the human contribution at about 1/3 of the value, if all that fossil CO2 remained in the atmosphere.
If the contribution of the δ13C drop was partly natural, then the natural source would have mimicked the human contribution at the same pace, which would be very remarkable.
Here for the δ13C drop in ice cores – firn – atmosphere compared to the drop in ocean waters as found in coralline sponges:
There is no way that a change in natural contribution is the cause of that drop, as the net effect of the human contribution in the atmosphere would be already three times what is observed.
Nature only replaces 2/3 of the fossil input in the atmosphere with CO2 from other reservoirs.
So where is the rest? Part of fossil CO2 is redistributed by the fast (seasonal) cycles into the ocean surface and vegetation, but that isn’t enough to explain the “dilution” of the human fingerprint.
The main CO2 flux that reduces the human δ13C comes from the deep oceans: what goes into the deep is the isotopic composition of today (minus the air-water discrimination), but what comes out is the isotopic composition (minus the water-air discrimination) of ~1000 years ago, long before the human use of fossil fuels or atomic bomb tests…
One can estimate the deep ocean CO2 fluxes, based on the diluting of the fossil “fingerprint” and about 40 PgC/year exchange with the deep oceans would do the job:
The discrepancy in the years before 1990 is thought to be from vegetation that was a slight source of CO2 at these decades, but from 1990 on a small, but increasing CO2 sink.
In conclusion: the new finding doesn’t show that the natural contribution of ancient carbon has any influence on the change in isotopic composition of the atmosphere, which is almost entirely from the human use of fossil fuels.
I hope you are right, and I suspect so. We humans, instead of wailing and gnashing our teeth should be CELEBRATING our contribution of the entirely beneficial, life-giving gas, CO2. It can use even more, much more than we can actually give.
Completely agree: until now, the contribution of more CO2 only had benefits: the earth is greening and the yield of all grains and other crops only gets higher with the years.
Besides somewhat more heat waves (but less cold waves!) there are no significant changes in weather extremes or sealevel increase…
Until now?! It still is.
Unless you don’t like food, shelter, central heating, motorized transport, air conditioning, telecommunications, internet, electricity, pharmaceutical products, need I go on?
Let’s just hope that this publication is left to stand as it is without some wealthy pressure group
(e.g. Mann & co.) menacing to sue the journal to force it to retract the paper.
I’m also so happy that people like Cook and others can take their “consensus” to a place
where the sun doesn’t shine … This is a perfect case, where one argument can turn everything
upside down. Trump & Seldin are currently clearing the regulatory space and I hope they’ll get the job done eliminating all these policies that had no scientific basis at all. No apology is needed. that’s just science as it’s supposed to be.
Eric,
Except that this work doesn’t throw the facts upside down. There is a find of an until now unknown source of low-13C and zero-14C, which adds to our knowledge of the natural carbon cycles.
Except if that source increased in exact ratio to human emissions since about 1850, that may have any influence on the enormous drop in 13C since then. If it is a rather constant source, that has zero influence on the isotopic balance, which was quite constant over the past 800,000 years…
The fact is, we don’t know for sure. That’s the ultimate problem with the entire agw narrative; so much of it is based on assumptions, with no or very shaky evidence and a lot of modelling. There might be some mechanism for this new source to be the cause of the isotope ratio change, but we don’t know.
It’s also based in things that have been happening for eons, that we just started studying, and have become alarmed about.
Archer, we are very sure that if all fossil emissions remained in the atmosphere, the drop in δ13C would be three times larger than observed. And we are very sure, that if all fossil CO2 remained in the atmosphere, the increase in mass would be two times faster.
So that leaves very little room for doubt of the cause of the increase in mass or decrease in δ13C.
Even if the just discovered source of low-δ13C increased over time, that only adds to the natural removal rate as mass and exchange rate for the low-δ13C molecules…
On only has to wonder why the Keeling curve isn’t divided into natural and anthropogenic CO2 to understand that no one can practically tell the difference.
Quite simple: the seasonal variability is mainly from deciduous forest in the NH, the trend is for 90% human and 10% from warming oceans.
Here without the seasonal swings, the human contribution, the increase in the atmosphere and the influence of ocean surface temperatures since 1850:
With yearly human emissions that are twice the increase in the atmosphere, there is no room for a huge natural source: both oceans and vegetation are proven net sinks for CO2…
Charles,
Good for you. Finding and absorbing new information is what science is all about. I know I am Planck advocate, but his treatises always have numerous references to other facts and theories from other scientist’s work. That is the true mark of a scientist, a constant search for new information!
No apology needed for being a good scientist.
You isotope believers had your suspect and like every bad detective never looked for another …. bad detective work … Horrible science …
The isotope “believers” look at the facts and see that human emissions alone would give a drop in 13C/12C ratio of three times what is observed. That proves that nature replaces about 2/3 of the original fossil CO2 molecules in the atmosphere by CO2 molecules from other reservoirs.
The new finding doesn’t change that fact with one per mil δ13C…
In my view a field of study where the scientific method cannot or is not being applied is not science. It is just a field of study. It may or may not have merit, but it cannot claim to be science.
Science requires the hypothesis to be tested in a controlled experiment.
Just my opinion, but I don’t think just made it up myself.
No empirical experiment was even done and a confounding variable was not even know. That is called science? It is just hypothesis forming.
Charles Rotter, a question if I may.
Your analysis is on rivers. Well done.
Can it be extended to large lakes (Great Lakes or glacier carver Finger Lakes for example)?
Also oceans.
Erosion, currents moving silt, etc. as well as storms and tides.
I recall in my youth spending summers in the Finger Lakes and heavy rains would transport large amounts of silt down the gullies into the lake.
And, the other question is how much of the residual river born CH4 and CO2 end up in the oceans and lakes?
Without isotopic measurements from lakes themselves, any extension is speculative.
As for oceans, they represent a different beast. While coastal zones receive enormous carbon inflow from rivers, the open ocean is likely dominated by different cycling mechanisms—driven by the biological pumps, upwelling, and air-sea gas exchange. Perhaps, in nearshore and estuarine environments, similar processes of aged carbon release occur.
Thank you.
I suspect there is still a lot to learn.
The science definitely is not settled.
We needn’t rely on isotopic analysis to know that human emissions are the cause of the ongoing (beneficial!) increase in atmospheric CO2. Simple “mass balance” arithmetic proves it beyond legitimate dispute:
tl;dr:
Mankind is adding CO2 to the atmosphere.
Nature (the net sum of all natural “carbon fluxes,” positive & negative) is removing CO2 from the atmosphere.
Mankind is currently adding CO2 faster than Nature is removing it, so the amount of CO2 in the air is increasing.
tl;dr + numbers:
Mankind is currently adding CO2 faster than Nature is removing it, so the amount of CO2 in the air increasing. Measurements show that it is increasing by about 2.5 ±0.1 ppmv/year. (1 ppmv CO2 = 7.8024 Gt CO2 = 2.12940 PgC.)
Mankind is adding 4.7 ± 0.5 ppmv/year of fossil CO2 to the atmosphere, plus 0.5 ±0.3 ppmv/year CO2 from “land use changes” (clearing forests and draining swamps). That increases the amount of CO2 in the air by 4.4 to 6.0 ppmv/year. (The fossil CO2 figures are calculated from economic data: the amount of coal, oil & natural gas produced and burned.)
The difference between those two numbers is the rate at which nature is removing CO2 from the atmosphere: (5.2 ±0.8 ppmv/year) – (2.5 ±0.1 ppmv/year) = 2.7±0.9 ppmv/year.
The isotope data is consistent with that. To learn the details see:
Engelbeen F, Hannon R, Burton D (2024). The Human Contribution to Atmospheric Carbon Dioxide. CO2 Coalition. https://doi.org/10.31219/osf.io/het6n
The higher atmospheric CO2 levels go, the faster natural “carbon sinks” remove it from the atmosphere. That is a powerful “negative feedback,” which helps to stabilize the Earth’s climate.
The two most important of those sinks are marine uptake and terrestrial “greening” / soils. They both accelerate approximately linearly as the CO2 level rises.
W/r/t absorption of CO2 by water, that linearity is probably obvious: the more CO2 molecules there are in the air, the more frequent are their collisions with liquid water.
Terrestrial biosphere uptake is less obviously linear, but it we know from agronomists’ studies that “CO2 fertilization” enhances C3 plant growth nearly linearly to above 1000 ppmv, and the plants that sequester most carbon (trees, sphagnum moss) are C3 plants.
So it shouldn’t surprise you to learn that the net rate of natural CO2 removals from the atmosphere is an approximately linear function of the CO2 level in the atmosphere. In fact, the rate of natural CO2 removal from the atmosphere accelerates by about 1 ppmv/year for every 50 ppmv rise in the atmospheric CO2 level (about 1/50 = 2%). That means the effective atmospheric lifetime (“adjustment time”) of CO2 added to the atmosphere is about 50 years.
(Aside: the IPCC’s Second Assessment Report acknowledged that fact, in a roundabout way; it reported that, “Within 30 years about 40-60% of the CO2 currently released to the atmosphere is removed.” [SAR WGI TS B.1 p.15] That implies a half-life of 23 to 41 years, which implies an adjustment time (effective atmospheric lifetime) of halflife/ln(2) = 33 to 59 years. That means the rate of annual natural CO2 removals accelerates by 1/59 to 1/33 of the increase in CO2 level, which is 1.7% to 3.0% per year.)
That means the atmospheric CO2 level cannot rise indefinitely. The level is currently rising by about 2.5 ppmv/year. So if anthropogenic emissions were to continue at the current rate for the long term, the atmospheric CO2 level would plateau at just 2.5 × 50 = about 125 ppmv above the current level.
That’s a mere 37% of a doubling. For comparison, the Earth has already seen 60% of a doubling since the “preindustrial” 1700s, with only benign effects, so there’s no reason to suppose that another 37% would be harmful.
Attempting Mass Balance, without known masses, is silly.
Too many unknowns and not enough equations gives a donkey theorem type problem with multiple soln’s… so they just pick the one they like.
DonM. depends of what is known…
Most CO2 fluxes are known within very wide limits, based on O2, δ13C and other measurements. That is for individual natural fluxes in and out the atmosphere.
Two parts of the carbon mass balance are quite exactly known: human emissions of fossil fuels and CO2 in the atmosphere.
The first is based on fossil fuel sales (taxes!) and burning efficiency, the latter is based on very accurate measurements in several “baseline” monitoring stations.
That is all you need to make the mass balance. Humans emit a lot of CO2 from fossil fuel use, one-way directly into the atmosphere: some 10 +/- 1 PgC/year.
The increase in the atmosphere is about 5 +/- 0.4 PgC/year. Even with these margins of error, it is clear that humans are fully responsible for the CO2 increase in the atmosphere and that nature, whatever the height of the natural fluxes, is a net sink for CO2.
That was the case for the past 67 years, with a few borderline (El Niño) exceptions.
Dear Charles,
I am quite surprised that you were convinced of a problem with the isotopic carbon balance, which is in fact no problem at all.
There are lots of natural carbon/CO2 inputs and outputs of the atmosphere, each with their own isotopic “fingerprint”, but the changes in isotopic composition of CO2 in the atmosphere in the past 800,000 years were very modest at -6.5 +/- 0.4 ‰ δ13C, despite huge changes of CO2 levels between glacial and interglacial periods.
So were the Δ14C levels quite constant, only varying with the solar cycles.
What does the finding tell us? That there is a source of extra low-δ13C and 14C-free CO2 that may have been underestimated. So what?
If there are indications that the emissions of that source increased over time (e.g. since 1850), then the impact may be important. If the emissions were relative constant, then they are not relevant at all.
Take volcanic emissions: going up and down, about as long as the earth exists: Higher in δ13C than the atmosphere and completely depleted from 14C. Besides the fact that these are only 1% of human emissions, they are a “constant” in the carbon cycle and their impact on CO2, δ13C and Δ14C levels is simply included in the total carbon and isotopic balances.
The same for huge coal seams fires, where lots of coal burn already for millennia: the same low-δ13C and zero 14C as for human coal burning today. Also incorporated in the pre-1850 δ13C and Δ14C levels.
The same for carbonate rock weathering: its δ13C level is the same as for the oceans, near zero. When dissolved in rainwater with CO2 at low pH that reacts to form bicarbonates with the same higher than the atmosphere δ13C. When that gets released, some fractionation takes place and we should see a release of CO2 with a δ13C level of around -6.5 ‰, higher than in the current atmosphere at -8.2 ‰. Again wit zero 14C.
The impact of all these natural fluxes on the δ13C and Δ14C levels was established millennia ago and shows very little variability over the past 800,000 years. Thus are of no interest for the cause of the enormous drop in δ13C since 1850. That drop still has one certain cause: fossil emissions…
I would humbly suggest an apology is only necessary if you engaged in ad hominem, or some other logical fallacy to dismiss someone’s claims(which is something I would not expect given your posting style).
Simply pointing out that a particular conclusion is not supported by data (yet) is a valid scientific observation.
What I think is expected is exactly what you have done with this article, ‘Acknowledgement and recognition’. Also, as you point out, there are rarely ‘daggers through the heart’ of these issues. If people who are worried about the damage humans cause to the environment were as honest and self evaluating, I think we’d be significantly more successful in conserving (different than preserving) our world.