Developing a carbon stocks and flows model for Australian wood products

Abstract

This paper describes the development of a model for estimating Australia's stocks and flows of carbon in harvested wood products, including estimates of atmospheric emissions. The model estimates emissions in various forms, including those from wood products contained in Australia, encompassing both domestically produced (net of exports) and imported wood products. This estimate is the basis of Australia's National Greenhouse Gas Inventory report on wood products. The model can also estimate emissions from all (and only) wood products produced in Australia, and a third variant that presumes emissions from wood products at the time of harvest. The model represents a collaborative effort, involving relevant Commonwealth and state government agencies, industry groups and research bodies. The model uses available statistics on log flows from forest harvest and estimates of the carbon content of the various wood products processed (for example, sawn timber, plywood, pulp and paper and woodchips) to determine carbon inputs to wood products. The model uses estimates of the decay period of various classes of wood product to calculate the pool of carbon in wood products. Crosschecking with independent input data was done wherever possible to test the robustness of various input data used in the model development. The model is built in Microsoft Excel with all rate and age parameters easily accessed and varied for sensitivity testing using the @Risk software. Wood products in use are assigned to young-, medium- and old-age pools. Simulated losses of wood products from their service life occur from each of the young-, medium-and old-age pools. Material leaving service is either transferred to bioenergy, added to landfill, recycled or emitted to the atmosphere. Losses of carbon can also occur from the landfill pool. The recorded imports and exports of wood products are used to calculate emissions under two approaches. The first is from wood products produced in Australia (but not necessarily remaining within Australia), and the second from wood products stored in Australia (wherever they were produced). Further simulations, with and without consideration of storage and emissions from landfill, are then run for each approach. The results show that an accounting approach that presumes emissions from wood products at harvest over-estimates emissions to the atmosphere when compared with approaches that consider the service life of wood products. The storage of wood products in landfill is also significant.