The Limits of Biochar

I recently did some basic equations about Biochar. If Biochar were the sole way that carbon is removed from the atmosphere, how much land would be needed to store the stuff? The calculations ended up being very pessimistic indeed. Armed with these calculations I sent off an email to the boffins at Real Climate. NASA scientist Gavin Schmidt responded and pointed out that I had wildly overestimated the amount of anthropogenic carbon in the atmosphere but, when taken with carbon that had been sequestered into the ocean, the final figure ends up being 400 billion metric tonnes - an amount that doesn't take into account future emissions.

How much Biochar can be used as a soil amendment? According to this, the amount is around 23.2 metric tonnes per hectare. It was just a matter of then dividing the amount of anthropogenic carbon by 23.2 and see how much land is required to sequester Biochar. The result is very depressing:

In short, 23.2 metric tonnes per hectare is not enough. Even if every single hectare of above ocean land mass is sequestered with 23.2 metric tonnes of biochar, the result is not enough to remove anthropogenic carbon. In reality, sequestering of biochar could not be achieved over the entire earth's surface, so I've given figures there for 10% of the earth's surface as well as 5%, which would require a Biochar sequestering of up to 23 times what is recommended.

So, the questions are:
  1. What amount of sequestered Biochar is too much? At what point will it turn from being a soil amendment and become toxic to plant growth?
  2. What would be the effects of deep Biochar sequestering, whereby Biochar is sequestered up to 10 metres underground rather than just existing within the 1-2 metres?
  3. Is it viable to use carbon as a resource to replace current commodities such as iron, aluminium, glass and so on?
The good news, I suppose, is that a cylindrical storage container 50 metres high and 18.25 kilometres wide could effectively store all 400 billion metric tonnes of carbon (at 2.267 grams per cm³ = 90,680,00 km³, volume of cylinder = πr²h) if necessary. NOTE: My spreadsheet let me down in its maths here. The real figure would be 500 metres high and 7600km in diameter: Not good news.


Nichol Brummer said...

Nice post! I think it helps to separate two things: first, it seems good for soil to have a certain amount of carbon in it. This should clearly be done. And it would sequester a good amount of carbon.

Storing that huge amount of carbon as a 'cylinder' in open air would be rather risky, as it could start to burn. But it should be simple to cover up a carbon dump with a good layer of soil and/or have most of it under (ground)water.

Could one make artificial reefs in sea using wood (or biochar), compacted or weighted down so it stays at the bottom of the sea?

And then, I wonder how long it would take for the CO2 dissolved in oceans to come back out again. The 'short term' need is to reduce the CO2 in our atmosphere only. That reduces the amount by a lot (for first).

Eclipse Now said...

Page 19 of this PDF from Eprida explains that:

Utilizing 1/3 of Crop Productivity for Bioenergy and Carbon based fertilizers and no-till, est ~ 6.5 Mg C ha-1, Land required to offset 1.9 Gt C/yr = 2.2E+8 ha (3xFrance)

In other words 6.5 tons of biochar per hectare would sequester 1.9 GT per year and still only use an area 3 times the size of France. Space is not an issue! Meeting demand is!

If you look at the graphic it explains that the burgundy piece of the pie-chart is the biochar, and the purple is the microbial fungi which sequesters about 5 times more carbon than the actual biochar! (I finally found that reference, and it is on this graphic.)

Not only that, but the IBI states:

Experiments have found that rates between 5 – 50 t/ha (0.5 – 5 kg/m2) have often been used successfully.

So in some cases you can at least double the amount of biochar you can add per hectare, and multiply the biochar by 5 times to get the actual carbon sequestered after all that microfungi have grown. Or look at it this way. All the coal we dig up uses up a fair bit of land, and does go pretty deep. But it's nothing compared to the enormous quantities of arable land we use. It's the equivalent of converting all that nasty coal into biochar and spreading it thinly over our farmlands. Then let nature's soil doctors, the glomalin from fungi, multiply that biochar by 5, and the job is on the way to getting done.

The challenge is not finding somewhere to put the biochar. The challenge is gearing up our industries to produce enough to meet demand, and at the right price. The last thing the biochar industry needs is questions being raised about how we're going to avoid being swamped in charcoal!

Eclipse Now said...

Wow, I must have got the HTML links wrong. Try these.

PDF with biochar micro-fungi ratio graphic on page 19

IBI FAQ that says sometimes 50tons / hectare is used.

Eclipse Now said...

I guess the last comment is to try to think of it in terms of lowering annual emissions, not trapping the whole amount at once! Sure it would be *nice* to return the atmosphere to pre-industrial levels of carbon dioxide, but that's not necessary.

1. Preindustrial levels were around 280 ppm, but James Hansen and peers want us around 350.

2. The job of adding biochar to the soil will be a continual one because we lose some biochar over time, some soil over time, and basically need to keep topping up the supply of carbon. Erosion has always been a terrible issue in farming.

3. There can be other uses for the char such as smelting steel, etc. (I think).

4. It's a useful wedge that could scale up to 60 gigatons over 40 years. Look ahead 4 centuries and there's 600 gigatons! But some of the original 60 gigatons will have decomposed and evaporated back into the atmosphere.... it's not a clear cut case of 'solved' and 'stop', but a useful annual process that might slowly help reverse climate change if we were to rapidly shift to GenIV nuclear reactors that can provide abundant cheap baseload electricity from nuclear waste.


Eclipse Now said...

Can I just say that I'm glad you wrote this article because I never asked the basic question: how much Biochar would we get if we fixed global warming? The answer? A LOT!

But even if the farming market for it was eventually saturated in a few generations, I can see other markets for it opening up like the steel industry. One day we're going to run out of coking coal. Biochar to the rescue of the steel industry!