Counting carbon in timber construction
In the quest to decarbonise almost everything, few construction strategies offer more obvious appeal than growing trees, and finding ways of extending the life of “harvested wood products”.
But “nothing is as simple as it looks,” and carbon stored in timber is no exception.
The basic logic is simple enough: trees use photosynthesis to convert carbon dioxide from the air into carbon compounds in the wood (“sequestration”). Roughly half the dry weight of timber is carbon, which for NZ radiata means almost 250 kg of carbon per m3 of timber (having come from almost a tonne of CO2 in the air - 1000 kg OF CO2 contains 272 kg of C).
As long as the tree is growing, the process continues (though eventually slows down), but once the tree is harvested two broad changes occur: a) the sequestration stops (it is very variable but in NZ is of the order of 50 – 100 kgCO2/adult tree/year) and b) carbon in harvested timber begins a journey to eventually return to the atmosphere
(a) It is critical new trees replace harvested ones (otherwise the process increases deforestation and worsens the climate crisis). This is why certification like “FSC” (Forestry Stewardship Council) is important, and one reason why crediting “biogenic carbon” is not automatic in, for example, MBIE’s carbon assessment methodology.
(b) The “journey” back to CO2 in the air can vary from hours (if slash is burned) to millions of years (via coal formation), and may be direct (e.g. via burning) or via, say, decay and methane production (with an accompanying big but temporary global warming hit).
For normal “harvested wood products” we must strive to avoid short-lived uses (packaging, pallets etc.) and maximise longer-lived products (e.g. buildings, furniture). Importantly, we should also maximise the ability for further effective use at the “end of life”, to delay as long as possible the eventual return of the carbon to the atmosphere.
Arguably the only way to ensure almost perpetual carbon storage in timber is by burying it deep underground (although addition to soil via biochar can be very beneficial).
The uncertainty over end-of-life conditions is why the NZ Green Building Council does not recognise biogenic carbon at all in their Homestar certification.
More information: