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Comment by 6nf | Hacker News Reader

Every time one of these articles come up I wonder about the price per tonne of CO2 removed by simply growing wood and not burning it. I'm not really sure what that price is, but it seems to be around $20-$40 per tonne? Does anyone have a more accurate number?

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I think all these comments about wood eventually releasing the carbon back into the atmosphere are missing the point. We don't need to capture it forever, we just need to get the ratio between carbon output and input under control in the next couple of decades. That should give us enough time to come up with longer term and sustainable ways of our planet... until we encounter the next problem, and so on.

"Solutions" don't have to be perfect forever. They just need to work long enough till better solutions are discovered.

Additionally, if you permanently dedicate 1 acre of land to growing trees, then an acre of trees' worth of wood is permanently captured - though the individual trees may die and rot, more will take their place.

The only problem is that we don't have nearly enough space for that.

An acre of trees stores ~393 tonnes of carbon dioxide permanently (in Vermont anyway, other areas probably vary a lot) [1]

Humans generate ~40 Gtonnes of CO2 a year. That means we need to be adding ~100 million acres of trees every year to capture that. There's currently 4 billion hectares of forest cover - that's around 10 billion acres.

If we replanted every area which has seen deforestation since 1990 we'd get another 400 million acres, or 4 years of emissions. After that, we're fighting an uphill battle. Forests currently cover 31% of the earth. 29% of that surface is desert, 3% is urban areas and 10% is covered in ice. To keep on top of emissions we'd need to add 0.3% of the total each year globally, or 1.2% of the land which could actually grow trees.

That might be doable in 5 years, but 10 years? 15 years? 30 years?

Re-forestation may be part of the solution, but it can't be the only part.

[1] (PDF}: http://fpr.vermont.gov/sites/fpr/files/Forest_and_Forestry/T...

[2]: http://www.earth-policy.org/?/indicators/C56/

There is a lot more area in the ocean and seaweed grows very quickly. Perhaps we could even accelerate the sequestration by building large seaweed farms over the deep ocean, then sinking the seaweed once it grows large enough, & repeat.

I think this is going to have to happen for bio capture to work at all. We need land for food, and we'll need increasingly more of it in the next decade thanks to overfishing, and population growth

Or use it to build homes, which still keeps it out of the atmosphere

And if we could forest up some places where there's no forest at the moment, a large fraction of that carbon would be tied up there basically as long as that forest stays up.

For the most part we have been doing that for decades. The US has a lot more forests now that 100 years ago (the height of logging).

It is hard to know for sure, but there is reason to believe we have more trees than even 1000 years ago. We are not allowing forest fires to clean out the dead trees which allows them to build up. This is a negative, regular forest fires burn colder and leave a lot of charcoal in the soil, while the less regular forest fires burn hotter and so release that carbon back into the atmosphere.

So I didn't believe this statement (eg: more trees than 100 years ago), bc I'm the skeptical sort. You're right though. I looked at "US Forest Resource Facts..." page 8 and the numbers are higher for millions of acres across different types. https://www.fia.fs.fed.us/library/brochures/docs/2012/Forest...

You could also just bury it. That’s effectively putting coal back into the ground.

I have a better idea. Leave the coal in the ground. You get the same result with literally no work!

Way too reasonable, it'll never happen.

Building with it is a lot more effective as there's economic incentive in preventing it from decaying back to base elements

Good point.

Wood is not dense enough for this to be cost effective.

If a woodpile isn't dense enough, then surely building homes and furniture out of wood isn't dense enough either.

Can you think of a better way to store carbon!

Not very dense compared to the cost of digging a big hole in the ground with Caterpillars and the number of truck loads of pellets you would need to bring in. This is all assuming that just covering the wood over with dirt is enough to keep it sequestered, a big pile of biomass could start emitting methane.

> Can you think of a better way to store carbon!

BECCS like they referred to in the article.

You could burn it – the ash from burning a tree takes up only a fraction of the original volume.

Don’t know if this is sarcasm, but burning the wood releases most of the carbon dioxide back into the air. The ash is just the unburnt carbon residue and other leftovers.

Yes, it was a joke. Sorry for that.

Jokes aside, you could partially burn it, into charcoal, then grind down the charcoal, soak it in urea, and mix it with soil. The charcoal bits remain stable in soil for quite a long time, and helps retain plant nutrients.

For comparison. A ton of CO2 is roughly the equivalent of a London-NYC roundtrip in the economy class. https://calculator.carbonfootprint.com/calculator.aspx?tab=3

A 1 hectare (10k m^2) poplar plantation yields 30 tons of wet biomass per year; average carbon content of wet wood is 15-20%; that would translate into 4.5 tons of co2 sequestered per year (assuming that all the carbon comes from the atmosphere).

To offset 5GT of carbon dioxide emissions per year, you'd need 110 million hectares, or 1.1 million square km, that is 13 times the surface of Kansas. Then of course every year you'd have to cut down something like 1/7th of the trees, throw them in a big hole, and replant them. Just about doable, I'd say. :)

~$6 per (short) ton ($0.10 per tree planted, which removes ~34 lbs of carbon per year). More here: http://shindyapin.tumblr.com/post/141034501197/climate-chang...

Taking your estimated numbers it's interesting to add that cost to the price of fuel and see where it gets you. A gallon of gasoline on average produces about 20 pounds of C02, so 100 gallons gives a ton of CO2.

You can then calculate the cost of adding capture to gas prices.

* At $200 per ton gas prices increase $2 per gallon.

* At $100 the increase is $1 per gallon.

* At $40 the increase is $0.40.

* At $20 the increase is $0.20.

It looks as if we need to get numbers into the low tens of dollars for this to be feasible on a widespread basis.

The numbers are optimistic since they don't cover the CO2 from refining and transport. That seems to be around $2 but it's somewhat hard to get accurate values.

Yea that's what I mean. It seems that cheap fast growing timber can be purchased for $20 per tonne. Dry timber is about 50% carbon and CO2 is about 30% carbon, so that means we can grow CO2 out of the atmosphere at a price of around $12 a tonne. We'll need to stack the wood somewhere but I don't think that will be terribly expensive.

The problem we're seeing in the US West is that wood has an increasing tendency to burn up. For this to work at least where I live we need to reforest using species that grow fast and are also relatively fire-resistant.

It seems people have been thinking of the fire-resistant part for a while: http://blog.davey.com/2017/08/fireproof-landscapes-with-fire....

Carbon in trees is still part of the carbon cycle, it will eventually be released again though it may be buffered for a time. That's merely neutral as opposed to negative that you get with sequestration.

True, and the current CO2 problem is exactly because of that eventual release - it's just that the CO2 was locked in oil and coal from millions of years ago. Growing trees would work, but only as long as their decomposition can be stopped (like it did when coal formed) or at least limited so that not all their CO2 is released and a big part of it returns into the ground.

But, major tree growing operations are already being done, lots of companies are into the CO2 trading business where growing trees = an X amount of CO2 emission rights they can sell to CO2 producing companies / factories.

If only they had a soft cap and let people trade credits to try to keep emissions under that cap to incentivize sequestration and efficiency innovations.

You know what's the problem with that idea? There are no "sequestration and efficiency innovations".

Sequestration doesn't work and takes too much energy, and efficiency cannot be improved much thanks to thermodynamics.

Besides, if the IPCC is correct about how bad CO2 is, the only sensible cap for emissions is zero. You don't get there with tiny improvements in efficiency.

I'm sarcastically saying that we had proposals for a cap and trade system that would have mitigated it but it was destroyed for political reasons.

Obviously. I'm non-sarcastically saying that cap-and-trade won't work, because any meaningful cap is unrealistic to achieve. The political reason is: We want cheap energy. The politically acceptable choices boil down to burning coal or gas, no matter how bureaucracy is layered on top.

There's got to be something that grows faster, and is easier to sequestrate, than trees. Bamboo, maybe? Seaweed? Algae? Dry it off in salt pans and dump it in the deep, cold ocean?

That's just dumping more carbon into one of our natural sinks, I think, rather than trying to create a new artificial sink. Even just using it as fertilizer has got to be an improvement over using inorganic fertilizers.

Iron fertilization

https://en.wikipedia.org/wiki/Iron_fertilization

Cool idea, I'd never heard of this. TL-DR is that there are huge portions of the ocean that are nutrient-rich but have little life, and one hypothesis is that iron is the limiting factor here. Since various phytoplankton need iron to survive, why not dump iron in these places, watch the phytoplankton now thrive, and enjoy their carbon sequestration?

The answer seems to be "well, we don't really know what the effect would be". Some algae blooms are not good (red tide), deliberately altering an ecosystem is generally dangerous, it's not clear whether the positive local effect on carbon has some side effect elsewhere (maybe we're just moving phytoplankton around the ocean). Even doing experiments on this is difficult (the world's oceans are weirdly regulated), and current financial incentives don't make it all that enticing either.

But a cool idea.

Thank you for posting this - the link is very interesting.

If the purpose is carbon sequestration, you'll have to include the cost of storing that wood. Indefinitely. Won't be cheap.

You could sell it, maybe trick people into doing stuff with it, like making buildings or durable goods.

This was interesting: https://www.ted.com/talks/michael_green_why_we_should_build_...

Best comment I have read all day.

Doesn’t a forest tie up the carbon in live trees? A mature forest will be more or less carbon neural but a certain amount of carbon won’t be available to the atmosphere.

The trees will die eventually. The problem is that we've released a lot of CO2 from trees that died millions of years ago.[0]

If you want to get that out of the atmosphere, it's not good enough to grow trees today because in 50-100 years (or whenever a tree dies) you're right back where we are today.

[0] https://www.nationalgeographic.com/science/phenomena/2016/01...

The tree which dies is replaced by a new tree.

You're right that we've added more carbon than can be stored in living forests. But reversing deforestation can & should be part of the solution.

Burn them for their energy and keep more coal in the ground seems like a good first step.

but wood burns so much more dirtier than coal, and leaves much more uncombusted remains that then have to clean.

The problem is energy (or lack thereof) - and so why not solve it directly by adding ever larger sources of renewables, which eventually will becomes enough that there's no need to burn any fossil fuels!

Is that so? Even when ground fine and with enough oxygen? For local heating, I heard good things about the efficiency and cleanliness of wood pellet heaters.

Did you hear those things from either people trying to sell you a wood pellet burner, or people who've just spent a lot of money money on a wood pellet burner?

Personal anecdote- i bought a house with a wood pellet stove as a primary heat source. There was smoke damage to the paint everywhere.

You'll also need to store it in a way that does not release the wood's CO2 back to the atmosphere by biodegradation.

If you turn it into charcoal you keep most of the carbon locked up in a non-biodegradable form.

And you can add it to soil where it works wonderfully at increasing biomass, reducing water runoff and increasing the number and variety of microorganisms in the soil, sequestering more carbon.

http://en.wikipedia.org/wiki/Terra_preta

If you use it to build homes you provide shelter for more people.

Building houses is great, but if you're growing the trees in order to sequestrate carbon, you'd better be reasonably sure a decent proportion of the wood doesn't release it's carbon within the next fifty years. I don't live in a country where wood is a major building material, but I'm imagining that most wood that goes into building gets discarded within a few decades and ends up decomposing or being burnt.

If treated and maintained right it can last hundreds of years. So this is not really an issue and if you replace it with new wood it will still store carbon.

> if you replace it with new wood it will still store carbon.

Good point, although you need to be sure you aren't counting the wood you replace it with as more carbon storage. Every wooden house essentially provides a fixed amount of carbon storage, no matter how much of it is replaced, and only as long as it stands.

and wood doesn't make for a building material that can go higher than a few stories.

Here's a 14 story wood-based apartment building in Norway:

https://www.youtube.com/watch?v=e5XsqauBCX4

http://treetsameie.no/

There are many other wood-based buildings higher than that being planned. The key for most of them is the use of various hybrid wood-laminates.

Of course you're not getting skyscrapers from them but nowadays you can build at least five floors which is not that bad. Most buildings are below that I think.

We need to unburn roughly as much coal as we've burned. We're talking about hundreds of billions of tons. You could probably plaster the whole surface of the planet with homes and not have used enough wood.

Wood used in construction seems to degrade very slowly. I've lived in houses whose the floor alone was a couple hundred years old.

That's in dry conditions though, it'd probably be gone by now if there wasn't a roof.

Scorched timber can last a hundred years. I have seen a cabin from 1740 made of timber from trees that were scorched in a forest fire but remained alive.

We can bury it until it becomes oil again.

Well, according to Energy and Civilization forest stores an average of 1 watt per square meter in burnable wood. Wood burning is pretty carbon intensive as energy goes so I imagine that each square meter of forest, if the fallen trees are buried deep underground, could offset several watts of internal combustion engine use.

Nope. The energy density of gasoline and wood per carbon atom is about the same. So a watt is a watt, and forest are inefficient (about 0.4%) at converting sunlight into chemical energy.

I've often wondered (half jokingly) if a solution to CO2 in the atmosphere is to grow trees, chop them down and bury them in disused coal mines.

Rotting wood releases carbon dioxide too.

You don’t let it rot. You mill it, and scorch it. That will give you a couple hundred years of protection. Then you start stacking the timbers in a pyramid. As you add more and more layers to the pyramid, the bottom layers compress, eventually becoming totally uninhabitable by bugs and bacteria. Over time you end up with a giant pyramid made of coal, and possibly diamond. (haven’t done the math on how tall you have to stack coal to get diamond)

this is starting to sound like minecraft

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