Whose C02 is it Anyway?

Written by Dr Jennifer Marohasy on April 11, 2025. Posted in Current News

Picture this: it’s a hot day, and you grab a soda can that’s been in the sun. You crack it open—psssht—and CO₂ fizzes out, tickling your nose, maybe spraying your shirt if you’re slow

It’s a tiny chaos, a burst you can’t control. Now imagine that fizz across the ocean’s sun-warmed surface, covering 71 percent of Earth, bubbling CO₂ into the air we breathe.

Wild, right? A bit mad. I reckon it’s a missing piece of the climate puzzle.

The IPCC pins it all on smokestacks—11 billion tonnes of carbon a year from ‘fossil fuels’. Even skeptics like the CO₂ Coalition echo this, leaning on guys like Ferdinand Engelbeen who do their maths by the consensus numbers on this issue of CO₂ origins.

Greg Wrightstone writes to me about how they have multiple lines of evidence. But they might have their evidence back to front.  They might also be leaving out ocean chemistry and biology. I’m convinced they are.

The Keeling Curve—CO₂’s climb from 280 to 420 ppm—carries their blame. But what if the ocean’s fizzing more than they think? Their rock-solid evidence could be mostly myth.

I’ve been digging into this with Ivan Kennedy, my second guest for the webinar series Towards a New Theory of Climate Resilience. That was back in February and I’m still to process the audio from this discussion.

So much thanks to everyone who has been a part of the Zoom sessions so far.  Instead of learning how to expertly edit this audio, my focus has been on writing technical papers.

Ivan and I are working through a hypothesis that could perhaps flip the climate script. 

Engelbeen claims fossil fuels’ isotopic fingerprint—light ¹²C (isotope C12) dragging the air’s ¹³C-to-¹²C ratio from -6.5 percent (per mille)** to -8.5 percent since 1850—is proof of coal and oil’s guilt. Ocean CO₂, averaging zero percent from deep waters, should nudge it up—not down. Case closed.

Except. That ¹²C/¹³C tale’s shakier than they admit. What if the ocean’s surface, warmed by the sun, fizzes CO₂ richer in ¹²C than the deep oceans zero percent?

Calcification—limestone forming in seawater—might churn out CO₂ at -10 percent or lower, diluting that delta 13 signal just like ‘fossil fuels’. It’s not the deep ocean I’m on about—it’s the top 65 meters, the mixed layer, where sunlight and warmth cause biological action.

So much action that it has built the biosphere’s great carbonate deposits, even the White Cliffs of Dover.

Ivan and I talked some of this over—Great Barrier Reef, North Pacific—during our webinar (soon my first podcast—thanks for waiting!). Calcification’s no sleepy trick; it’s a biological buzzsaw—corals, algae, phytoplankton like coccolithophores churning limestone.

In summer blooms, they might pump out tonnes of CO₂, light on ¹³C. Our Thermal Acid Calcification (TAC) hypothesis says nature’s pitching in more than you might think.

Ponder this next time you sip a soda: could the ocean be bubbling up a CO₂ twist? 

TAC’s perhaps the second plank in my New Theory of Climate Resilience.

** When we say deep ocean carbon is zero percent (per mille), we’re talking about its carbon isotope ratio, specifically the δ¹³C value. This is a measure of how much carbon-13 (¹³C) is present relative to carbon-12 (¹²C), compared to a standard reference.

In this case, zero percent doesn’t mean there’s no carbon-13 in the deep ocean—it means the ratio of ¹³C to ¹²C in deep ocean dissolved inorganic carbon (DIC) is about the same as the standard reference, which is usually the Vienna Pee Dee Belemnite (VPDB). A δ¹³C of zero percent indicates no enrichment or depletion of ¹³C relative to that standard.

See more here jennifermarohasy.com

Header image: iStock

About the author: Dr Jennifer Marohasy is an Australian biologist, columnist and blogger. She was a senior fellow at the free-market think tank the Institute of Public Affairs between 2004 and 2009 and director of the Australian Environment Foundation until 2008. She holds a PhD in biology from the University of Queensland.

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