> They are only a net store of carbon while they're alive.

Is wood not a carbon store?

When or a tree dies, or burns, during the decomposition process it releases the carbon back to atmosphere. Maybe you could bury them somewhere where decomposition it's slower or more difficult but hard to imagine doing it at scale.

Surely lumber, treated cellulose, contains a high % of the original carbon content (minus water)?

Lumber isn't a permanent removal from the carbon cycle. Wooden structures have a shelf life, and eventually the wood decomposes and re-enters the carbon cycle. The only way to sequester carbon via lumber would be to bury it in a way that prevents decomposition. I don't think this is an especially efficient method of carbon capture.

By the same measure, CO2 is not a permanent state either, but it's an issue because of the imbalance in dynamic processes - The carbon stored by lumber is the amount of lumber in the world minus decaying lumber.

36 billion tons of CO2 came from industry and fossil fuel combustion in 2019. [1] CO2 is about 27% carbon by mass. Cellulose is about 44% carbon by mass [2]. That means about 22 billion tons of cellulose needed to offset one year's worth of CO2 emissions.

In 2014, use of wood products in all notable categories combined was about 1.25 billion tons [3]. Including just sawnwood and wood paneling, it was 0.827 billion tons. Since there's no plausible way to use 2500% as much lumber for structural purposes as we do now, and just harvesting wood and leaving it outdoors will lead to decomposition on short (decades) time scales, people consider charring it and/or burying it underground. Just building more structures out of wood is barely going to put a dent in the problem.

Likewise, dynamically shifting the equilibrium toward wood from CO2 doesn't help much if the wood persists for only decades. The time scale for CO2 to naturally return to its pre-industrial equilibrium is more like 100,000 years [4]. We need sequestration measures that prevent CO2 from returning to the atmosphere over longer time periods than wood takes to decay naturally.

[1] https://www.statista.com/statistics/276629/global-co2-emissi...

[2] https://en.wikipedia.org/wiki/Cellulose

[3] https://www.forest2market.com/blog/fao-report-highlights-res...

[4] https://www.amazon.com/Long-Thaw-Changing-Climate-Princeton/...

True, but this proves that wood alone isn't a total solution. Along with reduction in FF dependency and other forms of carbon capture, it could be. "only decades" is enough time to come up with better solutions, including bio masses with a better per $ rate of carbon capture.

Only until it decomposes, which is usually relatively quickly after it dies.