The climate crisis places time at the heart of our thinking. 2050 (our net zero target year) is 26 years away. 1990 (the benchmark year that is used for progress in reducing emissions) is 34 years ago. Fossil carbon was deposited c. 300 million years ago. It takes 50 years (or 12 or 120 depending on your geographical region and the species) to grow a tree. It might take only minutes to burn it to generate electricity, if it is chipped for bioenergy, displacing fossil fuels. Not only time, but also rates. How much will our planet change by 2100? How much can we slow the warming process to avoid a tipping point?
It is increasingly recognised that forests and wood can provide a three ‘S’ solution for carbon emissions: sequestration, storage and substitution. Carbon sequestration happens in the forest, where trees convert carbon dioxide and water into their biomass. Substitution relates to the displacement of high ‘embodied carbon’ materials[1] (such as concrete and steel) by lower embodied carbon materials such as timber, especially in the context of the built environment. Measuring the effect relies on life cycle assessment (LCA) methods to quantify greenhouse gas emissions of each product within the system.
The third S is storage, and works hand in hand with the carbon sequestration done by the tree in the forest. The harvested timber leaves the forest carbon pools, but enters the harvested wood products pool (HWP[2]). We can find good estimates to the quantity of timber entering the HWP pool in national statistics and FAOSTAT. The more challenging aspect is estimating how long the carbon stays in this pool. Modelling the benefit of carbon storage in wood products requires knowledge about duration. The quantity of carbon stored in my desk today is exactly the same as it was ten years ago when I bought it. The benefit delivered by the desk is to store the carbon out of the atmosphere; it does not claim to offer any flux of carbon. There are two elements: the longer the better, and the larger the pool the better. The duration question is a rich source of discussion in attempts to quantify carbon benefits of using timber in long-life products.
This paper will consider three of the currently used methodologies relating to carbon storage in harvested wood products:
· The LCA approach – which handles biogenic storage but not the time component,
· the dynamic LCA approach – which models the benefit of duration of storage using different functions
· and the flux data method – which uses modelling to consider the flow of inputs and outputs from the HWP pool, in order to quantify change in pool size accounting for time.
Comparing these three concepts is of particular interest to me as I would like to quantify the benefit of the duration carbon is stored in wood materials and products. Especially as we seek to shift towards increased reuse and recycling of wood. The storage benefit may be spread across multiple product lives or recycling events. A calculation method must be able to handle the timing of processing events to properly quantify the benefits. It we are to model effects and make strategic choices to stabilise our climate, a method must be able to handle duration as well as quantity.
Keywords: greenhouse gas, carbon storage, duration of storage, modelling, harvested wood products (HWPs)
Authors
Morwenna Spear
The BioComposites Centre, Bangor University, United Kingdom
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