How to bury carbon? Let plants do the dirty work

Plot twist: What we know as oil was buried by a former civilisation that fought climate change then went extinct. The trees we bury will be used as fossil fuels by the next civilisation.

I like to joke that oil is a renewable energy source, it's just that people tend to be too impatient.

There does appear to be a limit to carbon content in soil, though. It isn’t an endless sink.

Some interesting research which I can’t find great sources for indicates that microbial content of the soil can increase the carbon capacity, but I’d need to see better sources to verify that. An example of research around that would be BEAM/Johnson-Su bio reactors and the compost they use to improve agricultural yield. Part of their work indicates that the more they feed the soil with microbe and fungus-rich compost, the better the carbon-retention becomes. There are others like it, but there’s not a large enough body to work to show how well this scales and how well it works across different media.

If I had another life to live I think I’d love to do this kind of research.

One thing missing from this fascinating Azolla story: if there's a huge green hat on the world, wouldn't something try to eat it? You'd think some sort of Azolla-eating creature would have a great time munching it and keeping the carbon up on the surface.

Yes, but most plants we currently have are in a pretty much 100% cycle, that is, why the biosphere is so stable. Take the Amazon region, the top soil layer is like 50cm. There is no deposit of carbon into the ground going on and its one of the most lush growing regions. To have a net carbon capture, the decay of dead plant material needs to be cut down by at least an order of magnitude. The article names options for that.

Note: from what I read, our fossil fuels come not from a constant process but from certain times in the past where the natural cycling of e.g. wood was way less than 100% which created concentrated deposits of carbon. Which makes sense if you consider how concentrated they are, if it was a truly steady process, they would be much more spread out.

Fossil fuels largely come from the Carboniferous period. The reason for this is that during this period the trees which covered the planet's surface did not decompose, and became buried in thick layers, which became coal seams. Whatever the reason, we know that the carbon cycle was not so much of a cycle at this point--the fixed carbon was retained and not recycled back into the atmosphere, resulting in the accumulation of vast quantities of carbon.

In the current environment, breakdown is rapid and carbon is rapidly recycled. Accumulation is minimal and while some longer-term storage can occur in places such as peat bogs, these are not stable long-term stores--they can easily burn if they dry out.

Yes, this is addressed in the article. Please read the article before commenting.

But it does reach a steady state. All the carbon we are releasing comes from a time when Lignin essential did not decompose at all. And we are releasing orders of magnitude more of it than current gets captured and stays captured. Even large intact rainforests do very little to actively capture CO2, they cycle it and they keep a large chunk of existing Carbon bound but they do not capture new Carbon from the atmosphere since they already reached a steady state between growth and decomposition.

Decomposition might take a "long time" from a human perspective, but from a geological perspective it's nothing at all.

Fungi can break down wood chips in a very short time since the wood is easily accessible. A whole solid tree will take longer, but the timescale is only years, not decades or longer.

Any processing which requires specific processing and handling is ... likely to scale poorly. The quantities involved are simply enormous.

Human CO2 emissions are about 43 billion tonnes annually. Let's say we're hoping to just take a bite out of that rather than compensate the whole thing, so we'll aim for a bit over 10% with 5 billion tonnes of stored CO2, to make the maths easier.

Let's say we're making and storing hay bales as a way of sequestering carbon.

A hay bale 16" * 18" * 36" weighs about 50 lb. (40 * 45 * 90 cm and 22 kg respectively). That's what you'll see as a traditional human-handled hay bale.

I'm not positive of the precise chemical composition, but I'll assume a 20% moisture content (by mass), and the remainder consisting of cellulose (C6 H10 O5). That leaves about a 35% net carbon content.

Note that carbon itself is only 27% of the mass of CO2, with most of the molecular weight being oxygen, so to sequester 1 billion tonnes of CO2, we need only bury 260 million tonnes of pure carbon.

We want to sequester five billion tonnes CO2 here, so 1.3 billion tonnes of carbon, or in the form of hay, 3.7 billion tonnes of hay bales. Let's round again, for convenience, and call it 4 billion tonnes of hay.

One tonne is roughly 40 bales.

One billion tonnes is 40 billion bales.

Four billion tonnes is 160 billion bales.

If we stack these to a height of 10m (33 feet), roughly 20 bales high, each tower weighs a half tonne. We need 8 billion of those stacks. Simply taking squares, that's a stack roughly 90,000 bales on a side, and 20 bales tall. (89,442.719 bales on a side if you want to be precise, I'm ... not.)

90k * 40 cm is 36 km (22 mi).

90k * 90 cm is 81 km (50 mi).

You'd need to bale, stack, and rack that much hay in a year. And repeat it every year.

And you're only accounting for 10% of annual human carbon emissions.

A process which captures, converts, and buries the carbon on its own would ... likely be preferable.

Note as well: large stacks of hay have a strong tendency to self-ignite through metabolic action. Or by other mechanisms in places such as Gävle.

Hopefully one day I will be able to contribute to it!