Is It Really Our CO₂?

New Evidence Questions Humanity’s Role in the Carbon Cycle

Questioning the Fundamental Basis of Climate Science and Policy

As an isotope geochemist, I’ve spent years studying the subtle signatures that reveal Earth’s hidden stories. Isotopes, variants of elements like carbon, act as chemical fingerprints, tracing the movement, age, and origin of materials through time. They don’t lie, and they don’t bend to narratives. Early in my career, I was struck by the Suess Effect, a shift in atmospheric carbon isotopes that seemed to pin the rise in CO₂ squarely on fossil fuel combustion. The evidence was compelling: burning ancient coal and oil, devoid of radiocarbon (¹⁴C) and depleted in ¹³C, was diluting the atmosphere’s isotopic ratios. It didn’t make me fear catastrophic climate change, but it convinced me humans were reshaping the atmosphere.

Now, I’m not so sure. Recent discoveries and lingering contradictions suggest the story isn’t as clear-cut as we’ve been told. At the heart of climate science lies a critical assumption: we fully understand the carbon cycle, the complex dance of carbon through air, oceans, plants, soils, and rocks. This assumption underpins every climate model, policy, and trillion-dollar investment. But what if the foundation is shakier than we thought? What if nature is playing a larger role in rising CO₂ than we’ve accounted for?

Let’s explore the cracks in this narrative and ask a critical question: Are we really responsible for the carbon in the atmosphere?

Cracks in the Foundation

The carbon cycle is Earth’s grand accounting system, tracking how carbon moves between vast reservoirs: the atmosphere (850 petagrams of carbon, PgC), oceans (38,000 PgC), soils, vegetation, and fossil fuels. According to the IPCC’s Sixth Assessment Report (AR6, Working Group 1, Chapter 5), natural processes like photosynthesis, respiration, and ocean-atmosphere exchange shuffle hundreds of gigatons of carbon annually, orders of magnitude more than human activity, which adds roughly 9.5 PgC per year through fossil fuel combustion and land-use changes. Figure 5.12 in the IPCC report illustrates this beautifully, with yellow arrows showing natural fluxes and pink arrows marking human contributions. The deep ocean alone holds 40 times more carbon than the atmosphere, and natural fluxes dwarf our emissions.

Figure 5.12 | Global carbon (CO2) budget (2010–2019). Yellow arrows represent annual carbon fluxes (in PgC yr–1) associated with the natural carbon cycle, estimated for the time prior to the industrial era, around 1750. Pink arrows represent anthropogenic fluxes averaged over the period 2010–2019. The rate of carbon accumulation in the atmosphere is equal to net land-use change emissions, including land management (called LULUCF in the main text) plus fossil fuel emissions, minus land and ocean net sinks. Source: https://www.ipcc.ch/report/ar6/wg1/figures/chapter-5/figure-5-12

This isn’t to say human emissions are trivial. Even a small imbalance can tip the scales, gradually increasing atmospheric CO₂. But the sheer scale of natural processes raises a question: How precisely do we understand these fluxes? If our models miss even a fraction of nature’s contributions, the entire carbon budget, our estimate of sources and sinks, could be off. And with it, our confidence in attributing CO₂ rise solely to human activity.

The Isotope Fingerprint... and Its Smudge

The Suess Effect has long been climate science’s ace in the hole. As fossil fuels burn, they release CO₂ with no ¹⁴C (due to radioactive decay over millions of years) and less ¹³C than atmospheric CO₂. This dilutes the atmosphere’s isotopic ratios, a pattern observed since the Industrial Revolution. The IPCC cites this as definitive evidence that fossil fuels drive the CO₂ increase, and I once agreed. The isotopic math seemed airtight.

But here’s the wrinkle: other natural sources can mimic this signature. Permafrost thawing in the Arctic releases ancient organic carbon, low in ¹⁴C and ¹³C. Petrogenic carbon from weathered sedimentary rocks does the same. Deep ocean CO₂, upwelled from centuries-old waters, carries a similar isotopic profile. These sources aren’t new, but their contributions are poorly quantified. If they’re larger than assumed, they could blur the line between “human” and “natural” CO₂. The Suess Effect remains strong evidence, but it’s not the unassailable fingerprint we once thought. Could we be overconfident in our attribution? (I discussed this in more detail in my article on the Suess Effect.)

Nature’s Lung Capacity... Bigger Than We Thought

Recent research deepens these doubts. A 2024 study in Nature Geoscience used carbonyl sulfide (COS), a chemical absorbed by plants, to estimate global photosynthesis. (You can read more about that in my critique here.)

The result? Terrestrial ecosystems are soaking up 31% more CO₂ than previously modeled. This suggests land plants are a more powerful carbon sink than we realized, which could shrink the net human contribution to atmospheric CO₂. If our models underestimated this sink, what else have we missed?

Source: https://scitechdaily.com/scientists-were-wrong-plants-absorb-31-more-co2-than-previously-thought/

Oceans, Warming, and Henry’s Law... A Chemical Contradiction

Then there’s the ocean, which climate models treat as a massive carbon sponge. But a basic principle of chemistry, Henry’s Law, complicates this picture. As liquids warm, they release dissolved gases. If global oceans are warming, as climate models predict, they should be outgassing CO₂, not absorbing it. Yet the IPCC insists oceans remain a net sink, driven by biological processes and circulation patterns. This isn’t impossible; complex systems like oceans don’t follow simple rules, but it highlights a tension. Warming should reduce the ocean’s carbon-holding capacity over time. Are our models fully accounting for this, or are we leaning too heavily on oceans to balance the carbon budget? (I unpacked this paradox in this article.)

All of this leads to a bigger question: If we are not the sole or even primary contributor to rising atmospheric CO₂, then why are we spending trillions to mitigate it?

Now comes what may be the biggest challenge to the narrative yet. A new paper in Nature...“Old carbon routed from land to the atmosphere by global river systems”...has identified an enormous, previously underestimated natural source of atmospheric CO₂: rivers. Not only are rivers emitting CO₂, they’re releasing old carbon...carbon that predates the industrial era by centuries or millennia. In other words, this carbon was never part of the post-1850 human carbon budget, and yet it’s influencing the modern atmosphere.

Could this be the overlooked force that has muddied the isotopic fingerprint? Could this be the missing piece that topples the entire "settled science" claim?

In the subscriber section below, I’ll dive into:

• The numbers: how much ancient carbon rivers emit

• The isotope data: why this old carbon looks like fossil fuels

• What this means for the IPCC’s attribution of CO₂ rise

• And how this might force a total rewrite of the modern carbon budget

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