Comment by pmayrgundter | Hacker News Reader

Comment by pmayrgundter | Hacker News Reader

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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...

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...

> 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...

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