Estimation of carbon stocks in wood products for private building companies

Abstract

Wood products function as carbon storage even after being harvested from forests. This has garnered attention in relevance to climate change countermeasures. In the progress of efforts toward climate change mitigation by private companies, the effective use of wood products has been an important measure. However, the methodology for accounting carbon stocks in wood products for private companies has not been established. Therefore, this study investigated methods for estimating carbon stocks in wood products used in wooden houses built by private enterprises, targeting a major company in the Japanese building industry. The results indicated that both the direct inventory method and flux data method (FDM) were applicable for estimating the carbon stocks. These two methods use data that can be obtained from many other building companies, thus, indicating high versatility. The log-normal, Weibull, normal, and logistic distributions, in descending order, proved to be suitable lifetime functions of wooden houses under the FDM, with a half-life of 66-101 years. It is important to continuously acquire time-series data on the floor areas of both newly built and existing houses and the amount of wood products used to improve the accuracy of estimates and explore future predictions.

Figure 1

Estimated lifetime functions of wooden houses. Note: “Actual” indicates the actual remaining fraction calculated based on the number of existing and built wooden houses (see the “Flux data method” section). “Exponential,” “Logistic,” “Normal,” “Log-normal,” and “Weibull” represent the estimated remaining fraction based on parameters that minimize the residual sum of squares (RSS) between the actual values and the estimated values for each lifetime function.

Figure 2

Estimated carbon stocks in wood products used in existing wooden houses. Note: “DIM” indicates the estimated results using the direct inventory method (DIM). Each lifetime function denotes the estimated results based on the parameters in Fig. 1 and Table 1 using the flux data method (FDM).

Figure 3

Future prediction of carbon stocks in wood products used in existing wooden houses until FY2050. (a) Business as usual scenario: The yearly average number of wooden houses built for the past five years during FY2016–FY2020 (8348 houses/yr) was assumed to continue to FY2050. (b) Goal achievement scenario: The target company’s goal of the number of domestic wooden houses built in FY2030 (10,000 houses/yr) was assumed to be attained and it continued with the same increasing trend till FY2050.

Figure 4

Number of wooden houses and average floor area per house built by the target company.