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Comment by Perz1val | Hacker News Reader

The carbon emission is not a problem, the imbalance is. When we burn fossil fuels, we use carbon that was stored by dead plants being buried. We take carbon from the ground and dump it in the atmosphere. Plants take carbon from the atmosphere. If were to bury the equivalent amount of wood, we would be equal. Buried wood is not useful, but building from it also locks the carbon somewhere else that isn't an atmosphere. So... there actually is a finite number of wooden airports that completely offsets airplane carbon emissions.

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1 kilo of wood sequesters (temporarily for the most part) about 2kg CO2 equivalent, so for the GP's example of a single seat one-way from Boston - NYC, you'd need to sequester 350kg of wood.

To sequester the ~800 Mt of CO2 emitted by aviation annually, you'd need to sequester about 400 billion kilograms of wood. So a finite but absurd amount.

Not only that, you have to build new wooden airports every single year to keep up.

Or you transition the airline industry to biofuels that are inherently carbon neutral.

There's a simple way to think about this that doesn't require doing a ton of research and number crunching. Think about the volume of fuel in a single airplane, about 7,000 gallons for a 737, or about 100 55 gallon drums. It's nowhere near exact, but you can roughly equate that volume of fuel to the volume of volume of wood to compare carbon - I'm pretty sure this is being very generous to the energy density of the wood, but it gives you an upper limit on what the wood could be sequestering (I think in reality jet fuel is something like 2-3x as energy/carbon dense per m^3). Now think about how many planes take off every day from an airport, and the volume of n*100 drums.

It's just nowhere near in reality, and we're not going to make a large dent via sequestration, the "finite" airports is functionally infinite for our purposes. We have to start making carbon neutral fuel.

That said, manufacturing concrete is incredibly carbon intensive, so avoiding making the amount we would've needed for this building is a pretty good win.

> If were to bury the equivalent amount of wood, we would be equal.

Burying wood that would have taken decades to decay is not equal to instantly releasing the equivalent amount of carbon. I'm not sure it's physically possible to remove CO2 from the atmosphere at the same rate a plane emits it, which would be the only way to genuinely "offset" the emissions.

Does burying wood actually work? I'm assuming it's a little more complicated than literally burying it, cause wouldn't it decay and release carbon at some point anyway?

Without oxygen, any decay is going to be incredibly slow. They're still pulling usable timber from trees that sank in rivers after being cut in the southeastern US 200 years ago. It's not cheap, but if you want some real old-growth oak, it's a viable source. And that's not a completely anaerobic environment.

200y is not really an incredibly long time at all

No, it's not, but that's without any noticeable decay. A tree that's been felled in that environment and left above ground will rot very quickly.

As most of the heavy construction and earth moving equipment you'd use to bury the wood will be burning diesel and releasing carbon the entire time, you enter some new Tyranny of the Rocket/Wagon equation.

Interesting point!

It looks like Skanska is GC for the Project, and cites it as a 9 acre (!) lumber roofing system[SK], and that it uses 3.5M board foot of Douglas Fir Project Lumber[PL].

Douglas Fir is 3.2 pounds per board foot, or 1.45kg [DFM]. So 1.45 * 3.5M = 5Mkg of lumber for PDX airport.

DF has an Embodied CO2 of 1.6kgCO2/kgLumber [DFC]. A little hard to believe? But maybe that's bc a lot of the mass of a tree is left in the ground. Worth following up.

1.6kgCO2/kgLumber * 5MkgLumber = 8MkgCO2 = 8KtCO2 embodiment/sequestration from the PDX roof project. (tho there's a lot more to the project that probably goes in the other direction)

Global CO2 emissions from commercial flights is ~60MtCO2/month [CF], so we need roughly 12,000 airports per month, 144,000/yr, to offset flight CO2 emissions.

There's 9000 commercial airline airports [NA] (tho obv many smaller than PDX, but they would also represent less CO2 from their flights), so 144,000/9000 is a 16x annual airport rebuild rate we'd need to offset CO2 emissions from the flights they service.

So yeah, this is absurd on the face of it.

But, how much of the mass of an airport is the roof? If it's like 1/100th the total mass, and you start building airports with all wood (foundation, runways, etc) you'd get to 16% annual rebuild rate to offset flight emissions. Still too high to be plausible. But another 10x somehow and you get to ~1% range of annual airport rebuild rate to offset emissions.

Then you'd have something.

[SK] https://www.usa.skanska.com/what-we-deliver/projects/278172/...

[PL] https://www.forbes.com/sites/johnoseid/2024/08/19/portlands-...

[DFM] https://www.globalwood.org/tech/tech_wood_weights.htm

[DFC] https://www.douglasfir.co.nz/net/environment/carbon-footprin...

[CF] https://www.statista.com/statistics/1500409/global-aviation-...

[NA] https://sentinel-aviation.com/blog/over-40000-airports-in-th...

> tho there's a lot more to the project that probably goes in the other direction

I mean, this is obviously why this is just a fun math exercise and not much else. Building an airport, even if you build part of it out of wood, is not net carbon-negative.

Just finished the full edit.. check it out. If the full airport is made out of wood, seems like it's getting towards plausible, or at least not obviously wrong

Most uncertainty is how much mass the ceiling is compared to rest of airport. Maybe it's more like 1/10th. Hmm

Apparently they used 4.4 million cubic yards of concrete during the construction of Denver airport around 1993.

That's 3.364×10^6 m3, 1 m3 of concrete weighs 2.4 tonnes (I'm sure it varies), so roughly 8 million tonnes of concrete.

https://www.concreteconstruction.net/_view-object?id=0000015...

I'm not convinced the wood used in the Portland airport project is net negative in itself, once you factor in the emissions of harvesting, processing and transporting it. I.o.w. leaving the ugly old roof in place would probably have been better, in terms of CO2eq emitted.

I found a more authoritative reference, from the Institute of Structural Engineers, which appears to be a major international organization [ISE].

The Embodied Carbon figure they use is 1.64kgCO2/kg timber as a rule of thumb [ISE-EC], and agrees with what I posted above

For processing that yields built lumber, they account in stages, with % CO2 emissions added:

A1) Raw Material Extraction, 20-25% A2) Transport to Facility, 8-10% A3) Manufacture, 5-10% A4) Transport to Site, 50-55% A5) Construction, 10-15%

A1 to A3 reduce sequestration by 0.28, for a net of 1.36. They then say A4&5 account for 1.5x more emissions than A1-3, so .42kg total factor, for a net sequestration of 1.22kgCO2/kgBuiltLumber. They separate these as the transport is the largest variable between projects.

These figures are from Austria to UK. From the reporting, the PDX project is using mostly local wood.

So I think they're getting a net sequestration for the roof project.

It's really interesting that building with wood has this major sequestration factor. It'd be really something if we could build our way out of the environmental crisis just by switching to wood! :)

[ISE] https://en.wikipedia.org/wiki/Institution_of_Structural_Engi...

[ISE-EC] https://www.istructe.org/IStructE/media/Public/Resources/ARU..., p17: "The amount of carbon sequestered can be assumed as -1.64kgCOe per kg of timber when product-specific data is not available". I take the e to be emission, so a negative is a sequestration.

The e is short for equivalent.

https://chancerylaneproject.org/glossary/carbon-dioxide-equi...

thanks. Also forgot to note size difference between those airports.. DEN is #4 in the US with 38M passenger boardings/year, PDX #33 with 8M. So maybe 4x larger

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

Based on the discussion in the descendent thread with morsch, it seems like the runways are the real story, at 1000x the mass of the airport roof.

But concrete is not so CO2 intensive. Lumber has a +1.6 sequestration factor of CO2 emitted vs built mass, compared with concrete at -0.8.

So we'd need runways made mostly of wood, or combined with a Woodcrete that was net sequestering, and then maybe there's a way to make even our most CO2 intensive industries net neutral so long as we rebuild continuously.

Also, since construction is about 40% of global CO2 emissions, if it could become a net sequestration as a whole, maybe it could flip the sign to -40% and offset most of the rest of our industrial emissions.

This also got me interested in what's a good number for rebuild rate.

Found a study that concludes the "Apparent ecosystem carbon turnover time [T]" is 43±7 years.

So maybe we should be rebuilding our built environment at 2.3%, or probably higher since species have evolved to be more energy intensive, humans especially.

[T] https://essd.copernicus.org/articles/12/2517/2020/essd-12-25...

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