Wednesday, June 7, 2017

Carbon Reduction vs. Carbon Sequestration

Look, no bare ground under these cows' hooves
I recently had a discussion online about the feasibility of sequestering carbon in the soil vs. just reducing how much we produce. I have seen many articles lately where scientists say that it just isn’t feasible to sequester carbon in the soil as the soil just doesn’t hold enough carbon. I contended that this is incorrect and that it indicates a lack of understanding of grassland ecosystems. See, the grasslands rely very, very heavily on the carbon in the soil.

Unlike trees, which can suck up huge amounts of water when it rains to help them survive dry times, grasses don’t really have the luxury. They need to have the water in the soil where they can use it. They do this by having deep roots that they store energy in as the growing season ends and use as a food source during the beginning of the next growing season. The tissues that are left behind rot and add to the carbon content of the soil. Some prairie grasses can have roots that run as deep as twelve feet, but will routinely run three to four feet deep in most cases. The carbon, when added to the soil, helps the soil act more like a sponge, soaking up rains and causing very little runoff. This water is stored in the soil where the grasses can access it to extend their growing season.

Naturally the conversation led to Holistic Management and Allan Savory’s methods and I shared his TED video. One person in the discussion shared in turn a take-down of said TED talk and I found it rather interesting. Again, the rebuttal says that it is simply not feasible to store the carbon in the soils. So, naturally, being the nerd that I am, I thought I’d do a quick back-of-the-envelope calculation on this. How much land would we have to holistically manage to bring carbon back to pre-industrial levels?

So first of all, let me pull a couple of numbers from the two sources themselves.

Allan Savory says in the video that about 2/3 of the world’s grasslands are desertifying. My research tells me that the earth’s landmasses are 57.5 million square miles. Two thirds of that is 38.3 million square miles. He also says that we can restore half of the world’s grasslands and bring the carbon in the atmosphere back to pre-industrial levels. So half of that is 19.2 million square miles. So we have 19.2 million square miles to restore.

Switching to the article rebutting the video, let me pull the next number from there. The article says that there are currently 240 Petagrams (a petagram is 1 trillion Kilograms) more of carbon in the atmosphere now than in pre-industrial times. So this means that we need to find a way to sequester 240 trillion kilograms of carbon into 19.2 million square miles. Sounds daunting, but let’s make some assumptions and run the numbers.

First of all, we need to set some numbers relating to carbon in soil. So let me start by saying that as a percentage of total soil, carbon percentage will be calculated by volume, not by weight. This is typically how that’s done. So how much does it weigh? I did a little research (okay, I asked a friend who has a PhD in soils engineering) and what is commonly called “muck soil” or 100% organic matter, has a specific gravity of 0.25 grams per cubic centimeter. Yup, it is pretty light. But it isn’t 100% carbon. It is composed of  a few other elements, mostly hydrogen and oxygen. Even though carbon dioxide also contains oxygen, let’s make this a little more conservative and assume that only 50% of that muck soil is actual carbon.

Now to depth. That’s the tricky part, isn’t it? We tend to think that an inch of topsoil is good. But that is more a forest environment. Grassland environments tend to pump the soil much deeper. The bunching grasses that make up those environments almost universally send their roots three feet deep or more, sometimes over ten feet. But let’s make the calculation conservative again, shall we? Let’s only count the top foot of soil in this calculation.

First, let’s narrow this down. How much carbon do we need to sequester per acre of soil?

240,000,000,000,000 kg C / (19,200,000 square miles x 640 acres/sq mi) = 19,500 kg/acre

Reducing our per acre number down to a single square foot:

19500 kg/ac x 1ac/43560 sf = 0.45 kg/sf = 0.45 kg/cu ft (since it is 1’ deep)

Suddenly this doesn’t sound so bad, does it? The big numbers sound scary until they get divided by equally big numbers.

Now let’s calculate the percentage of carbon the soil would need to be increased by. Let’s start with a unit conversion. Remember, we are halving 0.25 g/cu cm.

0.125 grams/cubic centimeter x (2.54 cm/in)^3 x (12in/ft)^3 x (1 kg/1000g) = 3.54 kg/cu ft.

So what percentage soil carbon is that?

0.45 / 3.54 = 12.7% soil carbon.

So 13%, and a conservative estimate at that. It can take a while to reach that level. I think 1-2% is reasonable in the first year. Maybe in a decade or two we can reach 13%, maybe even 20%. But let’s talk context a little here. First of all, the biggest impediment of this kind of solution is not the enormity of the task. It is that, for some reason, people who want to help, people who want to do the right thing, are throwing their might against this idea. They are dismissing it out of hand and saying that it just won’t work. No, this isn’t the only solution, but it is a big one. It can do a huge amount of good. So why are people spending so much time opposing it?

Second, right now, those same soils are degrading at a rapid rate and turning into deserts. To do that, they have to lose the carbon they once stored. So those same soils are currently contributing to the carbon in the atmosphere at a rate that I don’t think is fully understood. Just reversing that and turning them from a source to a sink would make a huge difference.


This is a subject I have read a lot about, studied extensively. And the second article, the one that is attempting a take-down of Holistic Management, falls prey to a logical error, and it is a big one. The first is that grassland ecosystems and forest ecosystems work the same. They don’t. The grasslands function very differently. The article tries to simplify the functioning of an entire environment down into a few simple formulas. It isn’t that simple, though, and can’t be simplified like that. The cycle of the animal impact is intimately connected with the water cycle and the mineral cycle. Without that understanding, any logical argument about why this solution won’t work is fundamentally flawed. 

Ultimately, though, we can reduce the carbon output all we want, but as long as nothing is sequestering the carbon away, we are fighting a losing battle. We need to be finding ways to lock the carbon back away where it belongs: in functional, living ecosystems.

2 comments:

  1. ok one question. You calculated the total available square miles as 2/3rds of the total land mass on earth. Wouldn't that imply that we'd be planting grassland on most of the available land?

    Perhaps an easier calculation:
    This article notes that there is about 26 tonnes per Ha of carbon stored in soil depths to 1 m.

    http://onlinelibrary.wiley.com/doi/10.1111/gcb.13246/full

    So we'd need 240 * 10^9 tonnes CO2, which is only 65*10^9 tonnes C, or 2.5 *10^9 Ha. There is about 3.5 *10^9 ha of grassland now, so we'd need to either increase the amount of carbon stored in current grasslands by 60%-ish percent, or increase the amount of grassland ecosystems by 60%-ish percent. Or some combination. Not impossible, but not easy.

    Of course, this is a bit of a crazy discussion in the first place. No single technology or seqestration system can get us to the carbon reduction goals we need, but that doesn't mean that all of these technologies are failures.

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    1. I'm not saying we'd have to plant grassland, but rather that about 2/3 of the land area of the planet is either grassland or degraded grassland already. Most deserts are degraded grasslands.

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