Direct inventory method

To calculate the carbon stocks of buildings, we used the same direct inventory method as the one presented in our previous study [17]. The target regions and methods used in the estimations are described in detail in the following subsections.

Target regions, period, buildings, and HWPs

This study covered all 47 prefectures in Japan from 1990, the base year for the first commitment period of the Kyoto Protocol, to 2019, the latest year with available statistical data [16]. The target buildings were classified as wooden, steel-reinforced concrete, reinforced concrete, steel, and concrete blocks/others. We used statistical data of the floor area of existing buildings from the Summary Report of Fixed Assets [16] for both taxable and non-taxable buildings (subject and exempt from property taxation under the local tax law, respectively). The targeted HWPs included log, sawnwood (including glued laminated lumber), plywood (thickness lower or higher than 6 mm), and wood board (particle board, hard board, medium-density fiberboard, and insulation board). We included both domestic and imported HWPs of buildings to emphasize the quantification of total HWPs used in buildings. Under the Paris Agreement, countries not using the production approach (which considers the HWPs derived from logs harvested from the country’s own forests) are requested to report the annual changes in HWP carbon stocks using the production approach as supplementary information in their NIR. Moreover, countries are also free to choose any approach that covers both domestic and imported HWPs, including the stock change approach, which covers HWPs consumed in the country [2]. Furthermore, countries can also choose any approach for nationally determined contributions (NDCs) [19].

Estimation of carbon stocks in HWPs of buildings

For the direct inventory method, we used the same formula and parameters as those presented in our previous study [17]. Since that study was written in Japanese, the method is described in English here. The carbon stocks of buildings were estimated using the direct inventory method, as shown in the following equation:

$$ {text{CBD}}left( {i,s,j,p} right) = Sleft( {i,s,p} right) times Vleft( {i,s,j} right) times Dleft( j right) times {text{CF}}left( j right), $$

(1)

where (i) represents the year, (s) is the structural category of buildings (wooden, steel-reinforced-concrete, reinforced concrete, steel, concrete blocks/others), (j) is the HWP category (log, sawnwood, plywood, wood board), (p) is the prefectural category, ({text{CBD}}left( i right)) is the carbon stock of buildings (t-C) at the beginning of year (i), (Sleft( i right)) is the total floor area of existing buildings (m2) at the beginning of year (i) [16], (V) is the HWP input per unit floor area (m3/m2) [14, 20], (D) is the wood density (oven-dry mass over air-dry volume; t-d.m./m3) [4, 5], and ({text{CF}}) indicates the carbon fraction of oven-dried wood (t-C/t-d.m.) [4, 5].

The total floor area of the taxable buildings from 1990 to 2019 [16] was obtained from the Summary Report of Fixed Assets by submitting a request for the disclosure of relevant administrative documentation to the Ministry of Internal Affairs and Communications, by prefecture, year of construction, and structure. The total floor area of non-taxable buildings was obtained from the website of the Summary Report of Fixed Assets [16].

Because the HWP input per unit floor area indicates the volume of HWPs used in constructed buildings in a year, we separated the total floor area of existing buildings by year of construction and applied the HWP input per unit floor area corresponding to each construction year. For taxable buildings, as the total floor area can be determined based on construction year (i.e., buildings constructed before January 1, 1963, and every 3 years thereafter, such as buildings constructed between January 2, 1963 and January 1, 1966), the HWP input per unit floor area corresponding to each construction year was used for the estimation. However, the data on the total floor area of existing buildings by year of construction referred to only two categories: wooden and non-wooden buildings, and a further breakdown of non-wooden structures was unattainable. Therefore, based on the percentage of the floor area of non-wooden buildings by structure applied to the floor area of newly constructed buildings [21, 22], we estimated the floor area of existing non-wooden buildings by year of construction by classifying them into steel-reinforced concrete, reinforced concrete, steel, and concrete blocks/others. For non-taxable buildings, because it was not possible to determine the total floor area of existing buildings by year of construction and structure, the minimum and maximum values of carbon stocks were estimated by assuming that all buildings were either steel-reinforced concrete or wooden structures, which have the lowest and highest HWP inputs per unit floor area, respectively. The HWP input per unit floor area for these two structures was the annual average of the recorded values. In 2019, taxable and non-taxable buildings accounted for 95.5% and 4.5% of the total floor area of existing buildings.

For the HWP input per unit floor area, the log, sawnwood (including glued laminated lumber), and plywood values were obtained from the Survey on Labor Demand and Construction Materials [20]. This survey was conducted approximately once every 3 years, and only the aggregate data for each fiscal year (from April to March) were published, so we considered these data as the annual value (from January to December). In addition, materials subject to loss (temporary materials that are discarded after construction) were not included in this study, because they are not expected to remain in the buildings. For the HWP input per unit floor area of wood board, we used values from 1976 to 2012, when data on the volume of wood input to buildings were available [14], and to be consistent with the Survey on Labor Demand and Construction Materials and the total floor area of existing buildings by year of construction, the values for every 3 years were used. The HWP input per unit floor area of wood board was calculated by multiplying the annual shipment of wood board according to type (including domestic and imported) [14] by the percentage of shipment for each use in the corresponding year [14] to obtain the input of wood board by type and use, and then dividing the input for building use by the floor area of newly constructed buildings in the corresponding year [21, 22]. For some years, the HWP input per unit floor area for log, sawnwood, and plywood were missing, and they were supplemented by the average of the previous and following years. For wood boards, the HWP inputs per unit floor area for insulation boards were missing for 1976–1985, so the values for 1988, the year closest to the survey year, were used to supplement the missing years. As a reference, Table 1 shows the HWP input per unit floor area for log, sawnwood, and plywood, and Table 2 shows the HWP input per unit floor area for wood boards.

Table 1 HWPs (log, sawnwood, and plywood) input per unit floor area by structure (m3/m2)
Table 2 HWPs (particle and fiber board) input per unit floor area (m3/m2)

We used density and carbon fraction values reported in the 2019 IPCC guidelines [4] and used in the NIR of Japan [5]. Because conifers account for most of the domestic supply of sawnwood materials [23], the log and sawnwood used in the buildings were judged to be mostly from conifers, so their densities were estimated based on the values for conifers.

Flux-data method

Target regions, period, buildings, and HWPs

The target regions, period, and HWPs were the same as those used in the direct inventory method. In addition, because the flux-data method estimates the carbon stocks at the beginning of year i + 1 using the HWP inflow to carbon stocks during year i (see Eq. (2) below), the HWP inflow must include data that go the farthest possible into the past, preferably prior to 1990, to provide more accurate estimates. Therefore, in accordance with the 2006 IPCC guidelines [3], 1900 was adopted as the starting year of the estimation. The target buildings were almost the same as those used in the direct inventory method. However, the statistical data [21, 22] on newly constructed buildings used in the flux-data method excluded buildings with a floor area of 10 m2 or less, because the Building Standards Law stipulates that submission of a Notification of Building Construction is not required for such small-sized buildings. Consequently, there was an error in this respect compared to the buildings targeted by the direct inventory method. Moreover, because there was no classification of taxable and non-taxable buildings in the statistical data used in the flux-data method [21, 22] and data by structure could be obtained for all buildings, we used the same estimation method for all buildings.

Estimation of carbon stocks in HWPs of buildings

The flux-data method estimates the carbon stocks base on FOD, which is the default decay function suggested by the IPCC guidelines [3, 4] and is widely used internationally. Carbon stocks in buildings under the flux-data method applying FOD were estimated using Eqs. (2)–(4):

$$ {text{CBF}}left( {i + 1,s,j,p} right) =,e^{ – kleft( j right)} times {text{CBF}}left( {i,s,j,p} right) + frac{{1 – e^{ – kleft( j right)} }}{kleft( j right)} times {text{Inflow}}left( {i,s,j,p} right), $$

(2)

$$ {text{Inflow}}left( {i,s,j,p} right) = Wleft( {i,s,p} right) times Vleft( {i,s,j} right) times Dleft( j right) times {text{CF}}left( j right), $$

(3)

$$ kleft( j right) = frac{{{text{ln}}left( 2 right)}}{{{text{HL}}left( j right)}}, $$

(4)

where ({text{CBF}}left( i right)) is the carbon stocks of buildings (t-C) at the beginning of year (i), ({text{Inflow}}left( i right)) is the carbon inflow of HWPs to buildings in year (i) (t-C/year), (Wleft( i right)) is the floor area of newly constructed buildings in year (i) (m2/year) [21, 22], and ({text{HL}}) is the half-lives of HWPs. We adopted 1900 as the initial year, for which the carbon stocks (({text{CBF}}left( {1900} right))) were set to 0.

For the estimation of carbon inflow (({text{Inflow}})) after 1961, the floor area data of newly constructed buildings from 1961 to 1988 were obtained from the Building Statistical Annual Report [22], and data for after 1989 were obtained from the Newly Constructed Building Statistics [21]. The same values as those in the direct inventory method [see Eq. (1)] were used for density, carbon fraction, and HWP input per unit floor area.

The ({text{Inflow}}) for 1900–1960 was estimated using Eq. (5), according to the 2006 IPCC guidelines (Eq. 12.6) [3]:

$$ {text{Inflow}}left( i right) = {text{Inflow}}left( {1961} right) times e^{{left( {U times left( {i – 1961} right)} right)}} , $$

(5)

where ({text{Inflow}}left( i right)) is the carbon inflow of HWPs to buildings in year (i) (1900–1960) (t-C/year), and (U) is the estimated continuous rate of change in industrial roundwood consumption between 1900 and 1961 (hereinafter called EI), for which the default value for Asia (0.0217) given in the 2006 IPCC guidelines [3] was used.

The half-life values of HWPs were set to 35 years for log and sawnwood, and 25 years for plywood and wood board, referring to the default values given in the 2019 IPCC guidelines [4].

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