Improvement of Biogenic Carbon Accounting in the Life Cycle of Wood used in Construction in Canada

Abstract

In Canada, wood is commonly used as a building material throughout the construction sector. However, the climate impacts of wood construction materials currently have limitations in how they are accounted for in life cycle assessment (LCA). Typically in LCA, a biogenic carbon neutral assumption is used for biomass and wood products, due to the fact that the carbon sequestered by biomass is equal to the carbon eventually released by that biomass. Given the dynamic nature of carbon dioxide emissions and the resulting effect on the greenhouse gas effect and subsequently on climate change, the simplistic paradigm that carbon neutral equals climate neutral is being questioned. There is an increasing body of scientific evidence that the actual climate impacts are dependent on many factors, such as storage time and emissions timing, the type of forestry management practiced, and the end-of-life treatment of the wood product. The overall object of this PhD work is to develop a method that consistently accounts for the uptake, emission and storage of biogenic carbon in the life cycle assessments of wood used in buildings by 1) developing temporally differentiated carbon flux profiles of the forestry carbon dynamics, 2) developing temporally differentiated carbon flux profiles from the point of harvest through to end-of-life, and 3) applying dynamic life cycle assessment to cradle-to-grave temporally differentiated carbon flux profiles of wood products. Most wood LCAs do not consider the forest management of wood products. This research work aims to improve the biogenic carbon accounting of the forestry phase of the life cycle of softwood products in Canada. This involves specifically modelling carbon fluxes as a function of tree species, growing conditions and forest management practices, from Canadian managed forests. Overall, the results show that for most forest landscapes, harvesting wood in the Canadian boreal forest results in net negative emissions. The research work also yields carbon fluxes, termed as ecosystem carbon costs (ECC) for most softwood species used in Canadian construction that can be used to model the forestry ecosystem carbon associated with the wood product. The implications of these results are that the sustainable harvesting of wood from most Canadian forest landscapes show a net sequestration, beyond what is already sequestered in the harvested wood itself. Considering the beneficial effects of sustainably harvesting forests on the overall biogenic carbon balance for wood products, forestry carbon dynamics should always be included in the life cycle assessments of wood products. Biogenic carbon accounting is currently not considered throughout the lifespan of wood products in LCA studies. This work aims to improve the accounting of carbon storage and fluxes in long-life wood products in LCA. Biogenic carbon from harvested roundwood logs were tracked through wood product manufacturing, building life and end-of-life phases, and carbon stocks and fluxes to the atmosphere were estimated. The results show that the degree of postponement of end-of-life emissions is highly dependent upon the wood product type, region and building lifespan parameters. This work develops biogenic carbon profiles that allows for modelling dynamic cradle-to-grave LCAs of Canadian wood building products. The implications of the results are that the biogenic carbon from wood processing to end-of-life can have variable positive carbon emissions, which are dependent on the specific building parameters. The final element in considering the biogenic carbon emissions, storage and uptake in wood product LCAs, is to integrate the timing of greenhouse gas emissions. The objective of this work is to calculate a database of temporally differentiated life cycle inventories (LCI) and dynamic climate change impacts of wood products, for different use contexts across Canada. The results encompass all the elements of this research work, allowing for the cradle-to-grave climate change impacts of wood products to be evaluated. In doing so, they allow for a verdict to be made on the relevance of biogenic carbon neutrality of wood products. In all but potentially the most outlying cases where ECC scores are positive or have very low levels of sequestration, the overall net life cycle climate change impacts of wood products are negative. This implies that using a carbon neutrality assumption for biogenic carbon would be a conservative assumption by overestimating overall life cycle climate change impacts. The frameworks established within this doctoral research allow for a full cradle-to-grave assessment of climate change impacts of wood products in the context of the Canadian construction sector. The climate change impact results themselves show that most wood products have net life cycle carbon sequestration and thus life cycle biogenic carbon emissions do not cancel themselves out. These findings add to the mountain of evidence in the literature that help in dispelling the myth that wood products should be considered biogenic carbon neutral in LCA.

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