Carbon isotopes do not show that humans’ climate impacts are too small to notice, despite the Daily Sceptic’s inaccurate claim

Carbon comes in three naturally occurring isotopes on Earth: carbon-12, carbon-13, and carbon-14. Different carbon sources bear different mixtures of the three, and studying these isotopic signatures can help trace how carbon moves across the Earth. For instance, carbon-13 and carbon-14 are less abundant in fossil fuels than in atmospheric CO₂, but the concentrations of the two isotopes in atmospheric CO₂ have both dropped since the mid-20th century. A large body of scientific evidence has long attributed these declines to extra CO₂ pumped into the atmosphere by humans burning fossil fuels^[1].

An article authored by Chris Morrison in The Daily Sceptic, an outlet with a track record of scientifically unfounded messages, makes a conflicting claim: that changing isotope signatures in atmospheric CO₂ result from the biosphere, rather than human causes like fossil fuel emissions. The article primarily cites “a new paper” published in February 2024 and supports its claim with a second paper published two years earlier. The article then upholds the two papers’ findings as scientific evidence that the human impact on climate change is “non-discernible”.

Below, however, we show that the Daily Sceptic builds its claim on a scientifically shaky foundation. The two papers are at odds with decades of scientific results showing that these isotope changes are the direct result of CO₂ emissions from human activity. Furthermore, both papers have received heavy criticism from climate scientists for drawing their conclusions from faulty analyses. Additionally, scientists contacted by Science Feedback emphasized that isotope concentrations are far from the only evidence pinning human activity as the primary cause of increased CO₂ in the atmosphere.

claim 1 (INCORRECT):

A lack of carbon-13 is the biosphere’s problem

The Daily Sceptic draws the core of its argument from a paper (“the Sci paper”) published in February 2024 in Sci, a journal published by MDPI, which has a reputation as a “predatory” publisher.  The Sci paper analyzed historic data showing a decline in the ratio of carbon-13 to carbon-12, the element’s two stable isotopes, and blamed the change not on fossil fuel emissions but on a “more productive and expanded” biosphere.

The carbon-13 to carbon-12 ratio is a common measurement in climate science. Carbon-12 accounts for about 99% of Earth’s carbon, and carbon-13 takes up almost all of the remaining 1%. Depending on the origin, the exact concentrations of each vary by a few fractions of a percent (Fig. 1). Climate scientists gauge these variations with δ13C, the deviation of a sample’s ratio of carbon-13 to carbon-12 from a standard benchmark originally derived from a particular type of limestone^[2]. In other words, the lower a sample’s δ13C, the higher its carbon-12 content.

δ13C is a telling indicator when used to measure CO₂ in the atmosphere (δ13CO₂). Plants prefer to photosynthesize carbon-12, making the biosphere’s δ13C lower than δ13CO₂. By extension, fossil fuels made from ancient biological matter also hold more carbon-12; when humans burn those fossil fuels, it also releases CO₂ with lower δ13C than δ13CO₂. The consequences are observed globally: δ13CO₂ has declined since the beginning of the Industrial Revolution (Fig. 2). Analyses have shown that, after accounting for exchanges between carbon in the atmosphere and carbon in the ocean and terrestrial biosphere, this decline matches the expected change from known fossil fuel emissions^[1].

Instead, the Sci paper tried to explain the decline with a scenario that entirely ignored fossil fuel emissions. The paper attempted to calculate the relationship between CO₂ and δ13CO₂ between 1520 and 1997 and noted that both measurements exhibited seasonal cycles as Earth’s biosphere became more active in the Northern Hemisphere spring, then declined in the Northern Hemisphere winter. The Sci paper then used this observation to claim that δ13CO₂ declined as a consequence of Earth naturally warming since the end of the Little Ice Age in about 1800, which the paper argues boosted the carbon cycle of the planet’s biosphere. The paper claimed the biosphere pumped low-δ13C greenhouse gas into the atmosphere, causing both a δ13CO₂ decline and a rise in atmospheric CO₂ levels.

When Science Feedback asked scientists who studied isotope ratios to comment on the Sci paper, they pinpointed flaws in the paper’s methodology. For instance, although it is true that CO₂ activity follows seasonal cycles, the paper’s model neglected to include several key processes impacting δ13CO₂. The paper treated the atmosphere as a closed CO₂ reservoir, ignoring two-way exchanges with the oceans and terrestrial biosphere that allow isotopic perturbations in the atmosphere to dissipate. More egregiously, the paper explicitly cited the known CO₂ emissions from fossil fuels and their isotopic signature^[4], then excluded them from its analysis and dismissed them as a driver of CO₂ concentration and δ13CO₂ changes.

“What is frustrating and confusing to me is that the author knows that human emissions have increased significantly during the industrial period, enough to explain the rise of CO₂,” Sourish Basu, a research scientist at NOAA Global Monitoring Laboratory and an expert in carbon cycle, told Science Feedback in an email. “Early on, the author erroneously concluded that the biosphere must be the main driver behind the atmospheric CO₂ budget and fossil fuel emissions must be negligible.”

Heather Graven, a climate physicist at Imperial College London, echoed this criticism of the Sci paper. “What he does is he just tries to estimate the isotope composition of the source using a flawed method,” she told Science Feedback via telephone. “He doesn’t really perform a simulation taking into account all the factors.”

Basu and Graven also questioned the validity of the Sci paper’s conclusion. “What we see in the atmosphere is because the biosphere and the oceans take up half of our emitted fossil CO₂. The biosphere is a net sink, not a net emitter. The author gets this basic fact wrong,” Basu told Science Feedback.

Therefore, the scenario that a more productive biosphere could simultaneously push down δ13CO₂ and increase the amount of CO₂ in the atmosphere makes little sense. “If [the carbon-12] were coming from the biosphere, we would have had to lose carbon,” Graven told Science Feedback.

The Daily Sceptic derives the core of its claim from a paper that begins with flawed assumptions and uses a flawed model. Essentially, the paper ignores fossil fuel emissions to argue that they are not the root cause of declining δ13CO₂. The paper then uses this flawed model to conclude that the biosphere is responsible for pumping CO₂ into the air at unprecedented rates, something that contradicts the majority of available evidence.

Meanwhile, numerous studies have explained that measurements of declining δ13CO₂ correlate with and are caused by an increase in anthropogenic CO₂ from fossil fuel emissions^[5-7].

claim 2 (incorrect):

Carbon-14 shows that fossil fuels emissions are a drop in the bucket

The Daily Sceptic supports its erroneous claim by citing a second paper (“the Health Physics paper”), published in February 2022 in the journal Health Physics, a publication that has no significant relevance to climate science. The Health Physics paper examines carbon-14 data to conclude that fossil fuels are responsible for only a small fraction of atmospheric CO₂.

Climate scientists do use carbon-14 as an indicator. The isotope is extremely rare: about one in every 1012 carbon atoms is a carbon-14 atom. Carbon-14 is radioactive, with a half-life of about 5,700 years, meaning that fossil fuel carbon, which is hundreds of millions of years old, contains almost no carbon-14 whatsoever (Fig. 3) Therefore, the absence of carbon-14 is a flag for the presence of fossil fuel emissions.

Specifically, climate scientists use a measure called Δ14C, which is calculated from how much a sample’s carbon-14 concentration varies from that of atmospheric air prior to the invention of nuclear weapons^[8]. A lower Δ14C indicates that less carbon-14 is present. In the 1950s and 1960s, carbon-14 released as fallout from nuclear weapons tests caused the Δ14C of atmospheric CO₂ (Δ14CO₂) to dramatically spike. But after the Partial Nuclear Test Ban Treaty largely ended above-ground tests in 1963, Δ14CO₂ began to decline just as drastically (Fig. 4).

However, Δ14CO₂ declined too quickly to be explained by carbon-14 decaying or being exchanged out of the atmosphere, indicating that carbon-14-free CO₂ entered the atmosphere from fossil fuel emissions^[1]. Multiple studies have clearly demonstrated that anthropogenic fossil fuel emissions are responsible for the ongoing decline in Δ14CO₂^[10,11].

The Health Physics paper’s three authors, none of whom have an obvious climate science background, created a model to match data on Δ14CO₂ dating from between 1750 and 2018. Their model assumed that fossil fuel CO₂ contained zero carbon-14 and that any given volume of CO₂ cycled out of the atmosphere in about 4 years. The authors’ analysis determined that fossil fuel emissions only accounted for 12% of global CO₂ as of 2018. The authors, then, concluded that fossil fuels could not have driven modern-day climate change.

Like the Sci paper, the Health Physics paper garnered criticism from scientists for making false assumptions. One published comment in the same journal pointed out a major flaw in the Health Physics paper’s methodology: It conflated atmospheric CO₂’s residence time (the amount of time a CO₂ molecule actually spends in the atmosphere before being exchanged with the land or the water, which is about 4 years) with its adjustment time (the amount of time an extra volume of CO₂ will stay in the atmosphere, which can be millennia).

As a result, the Health Physics paper authors drastically underestimated how long fossil fuel CO₂ would stay in the atmosphere. This is not a new error; as early as 1990, an IPCC report warned researchers against making it^[12]. Furthermore, the comment stated, “Throughout [the Health Physics paper] the authors have failed to cite numerous related and relevant earlier publications in this field and demonstrated a lack of fundamental understanding of biogeochemical carbon cycle processes,” the comment stated.

A second comment from a different group, published in the same journal, explained that — in addition to conflating residence time with adjustment time — the Health Physics paper used faulty Δ14CO₂ data and inadequately addressed the role of carbon-14 from nuclear weapons testing that remained in the atmosphere into the 21st century, both of which led them to further underestimate the fossil fuel contribution to Δ14CO₂ trends. This comment demanded that Health Science retract the paper, which the journal has not done.

Much like the Sci paper, the Health Physics paper derived its conclusions from a flawed methodology. Moreover, the paper ignores that the volume of CO₂ from fossil fuel emissions and its impact on the atmosphere at large are both very well-documented from methods such as air sampling^[4]. The concentration of CO₂ in the atmosphere has risen from about 280 parts per million (ppm) prior to the Industrial Revolution to about 420 ppm today; the Global Carbon Budget attributed about two-thirds of the excess to fossil fuel emissions and the remainder to human-caused land use changes like deforestation^[13].

The body of evidence points to fossil fuel emissions as the culprit for atmospheric CO₂ rise

Despite the two papers’ flaws and their public debunking by scientists, the Daily Sceptic article champions them to assert a lack of a “discernible” human fingerprint on the climate and claims that carbon isotope signatures are an “interesting branch of climate science to investigate”.

The reality is that it has been thoroughly investigated for decades. The study of carbon isotope ratios, in fact, predates any scientists reaching consensus about climate change. As early as the 1950s, chemist Hans Suess measured carbon-14 concentrations in wood and connected them to carbon-14-free fossil fuel emissions released starting from the Industrial Revolution^[14]. By the late 1970s, climate scientists had measured shifts in δ13CO₂ and Δ14CO₂ matched the predicted changes caused by then-known CO₂ emissions^[15].

It may be prudent to look at the isotope ratio changes in the bigger picture. We need a culprit that can explain all of the changes we have observed: a culprit that has lower δ13C than atmospheric CO₂, a culprit that is sufficiently old for its carbon-14 levels to have decayed to effectively zero, and a culprit that can explain the dramatic increase in atmospheric CO₂ concentrations since the industrial revolution. According to Basu, this allows us to rule out alternative carbon sources like volcanic outgassing, for example, which has a high δ13C; instead, the only suitable suspect is the emissions from humans burning fossil fuels.

Moreover, the study of isotope ratios is only one branch of many clearly pointing at a human origin for climate change. Supporting this idea, Ralph Keeling, a climate scientist at the Scripps Institute of Oceanography, told Science Feedback in an email:

“We don’t need to turn to measurements of isotopes to establish that the CO₂ rise is caused by humans. In fact, we know quite well how much CO₂ we’ve dumped into the atmosphere over the past 150 years through the burning of fossil fuels, and it’s more than enough to account for the observed rise. The rise started at the time of the dawn of the industrial revolution, and has accelerated since then. Overall, the CO₂ rise is a bit similar to the buildup of trash in a landfill. The trash is obviously of human origin, because we know we put it there. There’s not much sense in questioning its human origins.”

Indeed, scientists have clearly identified human causes as responsible for all of modern climate change activity^[16], as Science Feedback has previously covered.

REFERENCES

• 1 – Graven et al. (2020) Changes to Carbon Isotopes in Atmospheric CO₂ Over the Industrial Era and Into the Future. Global Biogeochemical Cycles.
• 2 – Hoffman and Rasmussen (2022) Absolute Carbon Stable Isotope Ratio in the Vienna Peedee Belemnite Isotope Reference Determined by 1H NMR Spectroscopy. Analytical Chemistry.
• 3 – Graven et al. (2017) Compiled records of carbon isotopes in atmospheric CO₂ for historical simulations in CMIP. Geoscientific Model Development.
• 4 – Ritchie et al. Key Insights on CO₂ and Greenhouse Gas Emissions. Our World in Data.
• 5 – Clark et al. (2021) SuessR: Regional corrections for the effects of anthropogenic CO₂ on δ13C data from marine organisms. Methods in Ecology and Evolution.
• 6 – Francey et al. (1999) A 1000-year high precision record of δ13C in atmospheric CO₂. Tellus B.
• 7 – Böhm et al. (2002) Evidence for preindustrial variations in the marine surface water carbonate system from coralline sponges. Geochemistry, Geophysics, Geosystems.
• 8 – Shervette et al. (2021) Radiocarbon in otoliths of tropical marine fishes: Reference Δ14C chronology for north Caribbean waters. PLOS One.
• 9 – Xiong et al. (2021). Time series of atmospheric Δ14CO₂ recorded in tree rings from Northwest China (1957–2015). Chemosphere.
• 10 – Graven (2015) Impact of fossil fuel emissions on atmospheric radiocarbon and various applications of radiocarbon over this century. Proceedings of the National Academy of Science.
• 11 – Yu et al. (2022)  Estimation of Atmospheric Fossil Fuel CO₂ Traced by Δ14C: Current Status and Outlook. Atmosphere.
• 12 – Watson et al. (1990) Greenhouse Gases and Aerosols. Climate Change: The IPCC Scientific Assessment.
• 13 – Canadell et al. (2003) Global Carbon Budget 2023. Earth System Science Data.
• 14 – Suess (1955) Radiocarbon Concentration in Modern Wood. Science.
• 15 – Keeling (1979) The Suess effect: 13Carbon-14Carbon interrelations. Environment International.
• 16 – Anderegg et al. (2010) Expert credibility in climate change. Proceedings of the National Academy of Sciences.