## 2 Materials and Methods
### Inventory Analysis
#### 2.3.2 Usage Process  
This process consisted of the usage of data center, communication networks, and listening and/or watching by the users. (1) Usage of data center  
Table 2 shows the data utilized during data center usage. CO2 emissions derived from the usage of the data center were calculated by multiplying the average total floor area of the data center with CO2 emission intensity. Afterwards, this estimation was divided by the average number of racks and storage capacity, as shown in Table 1, to obtain CO2 emissions per 1 MB of communication volume associated with the usage of the data center. (2) Usage of communication networks  
For CO2 emissions derived from the usage of communication networks, the estimate (2.48 \(\times\) 10\({}^{-3}\) kg-CO2/MB) made by the Japanese Environmental Management Association for Industry [41] was used. However, this was calculated based on the data of 2003. However, CO2 emissions derived from the usage of the communication networks might \begin{table}
\begin{tabular}{l l c c c} \hline \hline  & **Item** & & **Unit** & **Source** \\ \hline \multirow{4}{*}{Data center} & Construction cost & 15.80 \(\times\) 10\({}^{9}\) & JPY/facility & [36] \\ \cline{2-5}  & Legally decided service lifetime & 50 & Years & [37] \\ \cline{2-5}  & CO2 emission intensity for & 0.30 \(\times\) 10\({}^{-3}\) & kg-CO2/JPY/facility & [38] \\ \cline{2-5}  & \multicolumn{1}{l}{Average quantity of server rack} & \multirow{2}{*}{1300} & \multirow{2}{*}{units/facility} & [36] \\  & per data center & & & \\ \hline \multirow{4}{*}{Serves} & Weight of 30 U server rack & 121 & kg/units & Adopted basing on \\  & & & & information of servers \\ \cline{2-5}  & Legally decided service lifetime & 4 & Years & [37] \\ \cline{2-5}  & CO2 emission intensity for & 143 & kg-CO2/kg/units & [39] \\ \cline{2-5}  & \multicolumn{1}{l}{Storage capacity per 1U} & \multicolumn{1}{l}{\(36\times 10^{6}\)} & MB/U & Adopted basing on \\ \cline{2-5}  & U size per server rack & 30 & U/units & information of servers \\ \cline{2-5}  & Storage capacity & 1080 \(\times\) 10\({}^{6}\) & MB/units & manufactures and suppliers \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Utilized data for construction and manufacture process. \begin{table}
\begin{tabular}{l c c c} \hline \hline \multicolumn{1}{c}{**Item**} & \multicolumn{1}{c}{**Unit**} & **Source** \\ \hline Average total floor area of data center & 11,800 & m\({}^{2}\)/facility & [36] \\ CO2 emission intensity of data center & 652 & kg-CO2/m\({}^{2}\)/facility & [40] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Utilized data for usage process: usage of data center. be smaller due to the energy savings by the network equipment and expansion of data streaming volume. Therefore, using this result might overestimate the life cycle CO2 emissions. Thus, a regression analysis, with CO2 emissions and year as the explained and explanatory variables, respectively, was conducted based on the published results for CO2 emissions from 2000 to 2003. The created equation is as follows:  
\[lnCO_{2}=-1738.7 lnY+13212 \tag{1}\]  
where CO2: CO2 emissions derived from the usage of communication networks (kg-CO2/MB) and Y: year. The correlation coefficient of the regression equation was 0.99. Therefore, this equation showed that the fit was good. CO2 emission intensity in 2019 was estimated using this equation. Table 3 shows the data utilized by the users for listening and/or watching through online streaming. CO2 emissions derived from this were calculated by multiplying the power consumption of the PCs or smartphones, annual usage time of video and/or music streaming, and CO2 emission intensity. The power consumption of the PCs was collected from kakaku.com [42], which listed the best-selling home appliances and power consumption of 30 popular desktops and laptops as of January 2021. Afterward, a median of 25% power consumption was integrated.