From the DSS, the Department of Settled Science, we hear that those carbon cycle cartoons made readily available by the NOAA and Wikipedia and that crowd are… um… wrong. At least, waaaaay more carbon is going down the river and into the sea than we thought. As usual the details are complicated, involving the ratio of carbon 12 to carbon 13. But the bottom line is that instead of about 600-900 million tons of carbon going down the creek a year, it’s … more. They say 900 to 1900 million tons a year. Give or take.
The finding might seem dull. After all, the carbon cycle involves a gazillion tons of carbon and most of it is the bad person-made kind, correct? Well, no.
The general estimate is that humans emit just under 12 gigatonnes of carbon a year one way or another. That’s 12 billion tonnes, next to which 600 million is not a rounding error. And while it’s not a huge slice either, note that the uncertainty range was 1,000 million tons, aka a gigatonne. If the top-end nearly two gigatonnes of carbon a year is going into the sea, it’s 1/6 of the human total. And if as, for instance, Wikipedia asserts, “The uncertainty in the atmospheric CO2 growth rate is very small, ±0.02 gigatonnes of carbon per year” but “the ocean gross fluxes” have risen to “90 gigatonnes of carbon per year to account for the increase in atmospheric CO2” then not being sure where a gigatonne went is a big deal in terms of overall understanding.
As so often when it comes to climate, it’s really important not to believe the people who tell you the science is settled and precise. When it comes to the carbon cycle specifically it’s even funny, if you look closely at the various graphics that insist that nature is in carbon balance but our stuff smells bad and is often rejected, that they all deliver the same message but with different numbers.
Wikipedia for instance uses an image from NASA showing a total of 800 gigatonnes in the atmosphere while EarthHow has 750; likewise NASA’s deep ocean has 37,000 gigatonnes and EarthHow 38,100, while NASA’s soil contains 2300 and EarthHow’s just 1580. And if they seem to have compared notes on air-sea gas exchange, 90 out and 92 in, on photosynthesis NASA says 123 (3 of them our red kind) and EarthHow 121.3.
Well, that’s just two graphics. Maybe EarthHow is out to lunch or has outdated numbers. But if you go over to PBS, their LearningMedia site shows 77.5 Gt escaping the seas each year and 80 going in instead of the 90 and 92 both the previous sources agree on. They also have humans adding 9 from fossil fuels and 1 from land use changes, and 120 Gt a year coming naturally from plants while photosynthesis absorbs 122.5. (EarthHow seems to think humans emit 5.5 from chimneys and 1.6 from land use, which is much lower than most estimates.)
Meanwhile over at a Penn State site with a NASA logo, we get numbers for the amount in the atmosphere that seem to agree with EarthHow with some additional items like “Mantle huge amount” and “Sedimentary Rocks 1,000,000 GT” which seems like a lot too. And when you get down to their carbon cycle, they’ve got 6 GT from fossil fuels, the ocean air-sea exchange at 90 each way, farming in red with no number and photosynthesis at 110.
We are not complaining that they are not all chanting in unison. On the contrary, we find when that happens it is unsettling and antiscientific. Rather, and commendably, this study indicates once again that the science is not settled.
The subsidiary point, worth noting given the often spurious precision of various entries in the discussion, is that people including “climate scientists” don’t have little carbon-counters clicking every time a molecule flits through the gate going out of a chimney or into a tree. Numbers acquire a certain spurious credibility in public debate just by being repeated, especially that 97% thing. But in fact it’s all highly speculative reconstruction, far too often including assumptions like Prince Charles’s infamous one that nature can absorb exactly half of our CO2 and not the other half (in the course of which he said humans were emitting about 36 GT not 12).
Indeed, the more evidence there is of massive greening, for instance NOAA’s recent finding about “Global plant growth surging alongside carbon dioxide”, the more evidence there is that the complex feedbacks in the global carbon cycle do mean nature gobbles up a lot of ours too. And that previous estimates, however prettily charted, that assumed otherwise were guesses that were wrong.
Will that riverine carbon change the big picture on climate? Maybe. Probably not. What’s a gigatonne among ecosystems? But it’s one more reason to keep looking and not believe the science is settled.