Regional and temporal scope
This study covers all 235 countries and regions available from the forestry module of the FAOSTAT database [43] from 1900 to 2019.
Differences and similarities between the approaches in the IPCC guidelines
The SCA, AFA, and PA were described in the 2006 Guidelines [12], the PA13 was introduced in the 2013 Guidance [27], and the SCA19, AFA19, and PA19 were indicated in the 2019 Refinement [38]. In our analysis, we apply these approaches, which we consider the most likely choices of countries and the IPCC guidelines for their reporting under the Paris Agreement.
To calculate the HWP stocks and emissions/removals, we follow the respective IPCC guidelines ([12] in the case of SCA, AFA, and PA; [27] in the case of PA13; and [38] in the case of SCA19, AFA19, and PA19). The IPCC guidelines [12, 27, 38] suggest that one of three methods (Tiers 1–3) should be followed. Both the Tier 1 method in the 2006 Guidelines and the 2019 Refinement and the Tier 2 method in the 2013 Guidance apply first-order functions as one of the simplest ways to describe decay processes by using exponential functions, default carbon conversion factors and half-lives of HWPs, and activity data on the HWPs obtained from the FAOSTAT database [43] for each country. To maintain consistency across countries, we used the Tier 1 method in the 2006 Guidelines for the SCA, AFA, and PA and the 2019 Refinement for the SCA19, AFA19, and PA19, as well as the Tier 2 method in the 2013 Guidance for the PA13 as default methods for the respective guidelines. The details of these default methods are explained in this section, as well as in Table 1, emphasizing their differences and similarities. Although the simple-decay approach is also described in the 2006 Guidelines (and the 2019 Refinement), it is excluded from our analysis, as the default (Tier 1) method relevant for the approach is the same as for the PA (and the PA19) [12, 38]. The carbon stock changes in forests, which are also included in the respective IPCC method descriptions, are disregarded in this study. This is because the study looks into the differences in carbon removals/emissions due to differences in the accounting approaches in the IPCC guidelines for HWPs and found that forest carbon stocks would be the same regardless of the approach. We also excluded changes of carbon stocks in waste wood in landfills from our analysis because treatment of waste wood in solid waste disposal sites (SWDSs) is inconsistent in the three IPCC guidelines: waste wood in SWDSs is treated in the waste sector in the 2006 Guidelines and the 2019 Refinement, while it is accounted as instantaneous oxidation in the 2013 Guidance. The following descriptions adopted from Pingoud et al. [6] and the IPCC [12, 27, 38] explain the core differences and similarities between approaches.
The SCA and SCA19 aim to estimate the actual changes in the carbon stocks of wood products within the boundaries of the reporting country, as wood products are accounted for in the country they are used. Assuming that the carbon in wood products is immediately oxidized when these products are discarded, carbon emissions/removals correspond to annual net stock changes, that is, the difference between stocks in the considered year and the subsequent year. The SCA and SCA19 use different half-lives, initial year for carbon stocks, targeted HWPs, and carbon conversion factors (see Table 1).
The AFA and AFA19 are based on the same rationale regarding the carbon stocks of wood products, but the stock changes due to imports/exports are not considered for removals/emissions. Hence, the “carbon burden” of the wood material that is ultimately released into the atmosphere is shifted from exporting to importing countries. For exports and imports, all wood-based materials are considered in the AFA, while only feedstock for wood products are included in the AFA19. Additionally, the AFA and AFA19 use different half-lives, initial year for carbon stocks, targeted HWPs, and carbon conversion factors (Table 1).
The PA, PA13, and PA19 follow the accounting for products made from wood harvested in the reporting country. As such, these guidelines do not estimate the actual carbon stock in the reporting country, but the stock of products made from domestically harvested wood. The PA, PA13, and PA19 use different methods for calculating the share of domestic feedstock, half-lives, initial year for carbon stocks, targeted HWPs, and carbon conversion factors (Table 1).
Implementation of the IPCC guidelines
The carbon removals by HWPs in each country and for each approach were estimated using Eqs. (1)–(3):
$$CR_{SCA/SCA19} (i) = \Delta C_{DC} (i),$$
(1)
$$CR_{AFA/AFA19} (i) = \Delta C_{DC} (i) + E(i) \, {-}I(i),$$
(2)
$$CR_{PA/PA13/PA19} (i) = \Delta C_{DH} (i),$$
(3)
where CRSCA/SCA19(i), CRAFA/AFA19(i), and CRPA/PA13/PA19(i) (tC year−1) are the carbon removals during year i under the SCA (and SCA19), AFA (and AFA19), and PA (and PA13/PA19), respectively. ∆CDC(i) (tC year−1) is the change in the carbon stocks in HWPs consumed in a country during year i. E(i) (tC year−1) is the carbon transfer in the form of wood-based materials exported from the country during year i, while I(i) (tC year−1) is the carbon transfer in the form of wood-based materials imported into the country during year i. Note that the AFA covers all wood-based materials, including HWPs and feedstock for the HWPs [12], while the AFA19 targets only feedstock for the HWPs [38]. ∆CDH(i) (tC year−1) is the change in the carbon stocks in the HWPs that originate from a country’s domestic harvest during year i.
For the SCA, SCA19, AFA, and AFA19, we estimated the changes in the carbon stocks of domestically consumed products (Eqs. (1) and (2)) using the first-order decay functions described in Eqs. (4)–(6):
$$\Delta C_{DC/DH} (i) = C_{DC/DH} (i + 1) \, - C_{DC/DH} (i),$$
(4)
$$C_{DC/DH} (i + {1}) = e^{ - k} \cdot C_{DC/DH} (i) + \, \left[ {({1} - e^{ - k} )/k} \right] \, \cdot Inflow_{DC/DH} (i),$$
(5)
$$k = {\text{ ln}}\left( {2} \right)/HL,$$
(6)
where CDC(i) (tC) represents the carbon stocks in the HWPs consumed in the country at the beginning of year i. CDH(i) (tC) is the carbon stocks in the HWPs derived from domestic harvest at the beginning of year i. InflowDC(i) (tC year−1) is the carbon inflow to the carbon stocks in HWPs in the form of domestically consumed wood products during year i. InflowDH(i) (tC year−1) is the carbon inflow to the carbon stocks in HWPs in the form of wood products derived from domestic harvest during year i. k (year−1) is the decay constant of the first-order decay function and HL (year) is the half-life for HWPs.
For the SCA and AFA, the initial year in carbon stocks was suggested to be 1900 and the carbon stocks in that year (CDC(1900)) were assumed to be 0, according to the 2006 Guidelines. From 1961 to 2019, InflowDC(i) was calculated using production, import, and export data for each HWP category for each country from the FAOSTAT database [43] to determine domestic consumption (= production + imports − exports). For countries that became independent after 1961 and do not have data before independence in the FAOSTAT database, more recent production, import, and export data were extended back to 1961 using the rate of annual change in each dataset for the old country that the newly independent country was a part of [12] (see Additional file 1). From 1900 to 1961, InflowDC(i) was estimated using Eq. (7):
$$Inflow_{DC/DH} (i) = Inflow_{DC/DH} (1961) \cdot e^{{U\left( {i - {1961}} \right)}} ,$$
(7)
where U represents the annual rates of increase in the HWP consumption by world region from 1900 to 1961, as per the guidelines [12].
For the SCA19 and AFA19, the default method in the 2019 Refinement suggested 1990 as the initial year for carbon stocks, and that the equation with the average value of InflowDC(i) be used over the first five years from 1990 (1990 to 1994). Although the 2019 Refinement also suggested 1961 as the initial year, it simultaneously requested to verify that any historical trends in the relevant statistics for wood commodities reflect actual changes, rather than changes in the coverage of these statistics. In addition, it pointed out the uncertainties associated with the estimate by using earlier initial years (e.g., 1961) [38] (see section "Global perspective"). Therefore, this study applied 1990 as the initial year for carbon stocks and estimated the carbon stocks in that year (CDC(1990)) by using Eq. (8):
$$C_{DC/DH} (1990) = \left[ {\mathop \sum \limits_{i = 1990}^{1994} Inflow_{DC/DH} (i)} \right]/5k,$$
(8)
In the case of the PA, PA13, and PA19, we calculated the changes in the carbon stocks in the HWPs derived from the domestic harvest (∆CDH(i) in Eqs. (3) and (4)) by using InflowDH(i) in Eqs. (5)–(9):
$$Inflow_{DH} (i) = P(i) \cdot D_{D/W/P/R} (i),$$
(9)
where P(i) (tC year−1) is the carbon from the production of HWPs during year i. DD/W/P/R(i) is the share of feedstock for the production of HWPs originating from domestic forests during year i. We assumed DD/W/P/R(i) to be 0 when it took a negative value, according to the guidelines [27, 38].
For the PA, the share of domestic feedstock (DD(i) in Eq. (9)) was estimated using Eq. (10):
$$ \begin{aligned}D_{D} \left( i \right) & = IRW_{DP} \left( i \right)/\left[ {IRW_{DP} \left( i \right) + IRW_{IM} \left( i \right)}\right.\\&\quad\left.{ \, {-}IRW_{EX} \left( i \right) + WCP_{IM} \left( i \right){-}WCP_{EX} \left( i \right)}\right.\\&\quad\left.{ + WR_{IM} \left( i \right) \, {-}WR_{EX} \left( i \right)} \right],\end{aligned} $$
(10)
where DD(i) is the share of feedstock for the production of HWPs originating from domestic forests during year i. IRWDP(i), IRWIM(i), and IRWEX(i) (tC year−1) refer to the carbon in production, imports, and exports of industrial roundwood (FAOSTAT categories), respectively, during year i. WCPIM(i) and WCPEX(i) (tC year−1) denote the carbon in imported wood chips and particles and exported wood chips and particles, respectively. WRIM(i) and WREX(i) (tC year−1) are the carbon in imported and exported wood residues, respectively.
Under the PA13, the calculation of the carbon inflow was divided into wood and paper product components (InflowDH = InflowDH,wood + InflowDH,paper). The share of domestic feedstock (D(i)W/P in Eq. (9)) was estimated using Eq. (11) for sawnwood and wood-based panels and Eq. (12) for paper and paperboard (note the subtraction of exports in the numerator):
$$ \begin{aligned}D_{W} \left( i \right) & = \left[ {IRW_{DP} \left( i \right){-}IRW_{EX} \left( i \right)} \right]/\left[ {IRW_{DP} \left( i \right) }\right.\\&\quad\left.{+ IRW_{IM} \left( i \right){-}IRW_{EX} \left( i \right)} \right],\end{aligned} $$
(11)
$$ \begin{aligned}D_{P} \left( i \right) \, & = \, \left[ {IRW_{DP} \left( i \right) \, {-}IRW_{EX} \left( i \right)} \right]/\left[ {IRW_{DP} \left( i \right) \, }\right.\\&\quad \left.{+ IRW_{IM} \left( i \right) \, {-}IRW_{EX} \left( i \right)} \right] \, \\&\quad \cdot \, \left[ {WP_{DP} \left( i \right) \, {-}WP_{EX} \left( i \right)} \right]/\left[ {WP_{DP} \left( i \right) \, }\right.\\&\quad \left.{ + WP_{IM} \left( i \right) \, {-}WP_{EX} \left( i \right)} \right],\end{aligned} $$
(12)
where DW(i) is the share of feedstock for the production of sawnwood and wood-based panels originating from domestic forests during year i. DP(i) is the share of feedstock for the production of paper and paperboard derived from domestic forests during year i. WPDP(i), WPIM(i), and WPEX(i) (tC year−1) are the carbon in the production, imports, and exports of wood pulp, respectively, during year i.
Under the PA19, the share of domestic feedstock (DW/R(i) in Eq. (9)) was estimated using Eq. (11) for sawnwood and wood-based panels and Eq. (13) for paper and paperboard, which was revised to include recovered paper:
$$\begin{aligned} D_{R} \left( i \right) \, =& \, \left[ {IRW_{DP} \left( i \right) \, {-}IRW_{EX} \left( i \right)} \right]/\left[ {IRW_{DP} \left( i \right) \, + IRW_{IM} \left( i \right) \, {-}IRW_{EX} \left( i \right)} \right] \, \cdot \, \{ {1 }{-} \, [RP_{DP} \left( i \right) \\ &\; + RP_{IM} \left( i \right) \, {-}RP_{EX} \left( i \right)\left] / \right[RP_{DP} \left( i \right) \, + RP_{IM} \left( i \right) \, {-}RP_{EX} \left( i \right) + \, WP_{DP} \left( i \right) \, + WP_{IM} \left( i \right) \, {-}WP_{EX} \left( i \right)]\} \, \\ &\;\cdot\left[ {WP_{DP} \left( i \right) \, {-}WP_{EX} \left( i \right)} \right]/\left[ {WP_{DP} \left( i \right) \, + WP_{IM} \left( i \right) \, {-}WP_{EX} \left( i \right)} \right] \, + \, \left[ {RP_{DP} \left( i \right) \, + RP_{IM} \left( i \right){-}RP_{EX} \left( i \right)} \right] \, \\ &\;/\left[ {RP_{DP} \left( i \right) \, + RP_{IM} \left( i \right){-}RP_{EX} \left( i \right) + \, WP_{DP} \left( i \right) \, + WP_{IM} \left( i \right) \, {-}WP_{EX} \left( i \right)} \right] \, \cdot \, \left[ {RP_{DP} \left( i \right){-}RP_{EX} \left( i \right)} \right] \, \\& \;/RP_{DP} \left( i \right) \, + RP_{IM} \left( i \right){-}RP_{EX} \left( i \right)], \\ \end{aligned}$$
(13)
where DR(i) is the share of feedstock for the production of paper and paperboard derived from domestic forests during year i. RPDP(i), RPIM(i), and RPEX(i) (tC year−1) are the carbon in the domestic supply, imports, and exports of recovered paper, respectively, during year i.
For the PA and PA13, 1900 was the initial year for the carbon stocks in Eq. (7) by using InflowDH(i). Additionally, the 2013 Guidance [27] suggested that the PA13 use another method of estimating the carbon stocks in the initial year by using Eq. (8), based on the average of InflowDH(i) over the first 5 years for which statistical data are available. Therefore, in the modified version of the PA13 (i.e., PA13i), we also used 1961 as the initial year, which was the first year in the FAOSTAT database [43] (see Table 1) and applied Eq. (8) based on InflowDH(i) between 1961 and 1965. Meanwhile, the PA19 used 1990 as the initial year and applied Eq. (8) using InflowDH(i).