# The Environmental Stake of Bitcoin Mining: Present and Future Challenges
## 2 Materials and Methods
### Life Cycle Inventory
#### 2.2.2 Electricity  
The carbon intensity (i.e., kgCO\({}_{2}\) eq/kWh) of electricity principally depends on the energy sources and process technology employed in power generation [50; 51; 52]. In order to simplify the description of the results, electricity will be divided into two macro-categories according to its derivation, i.e., high-carbon electricity (HCE), constituted by oil, coal and natural gas sources; and LCE, constituted by the electricity generated by nuclear and renewable resources. The electricity mixes, specific to each country involved in the mining, were accurately modelled in the SimaPro software (v. 9.6). In the case of the baseline scenario (i.e., 2022), electricity mixes were modelled by referring to the most updated information reported in IEA (2022) [53], while the environmental information associated with the electricity production and supply was drawn by the ecoinvent database [54]. The use of the ecoinvent database and the SimaPro software allows for the estimation of GHGs by considering all phases of the life cycle of the electricity consumed during the mining phase. This includes the impacts associated with the extraction and procurement of raw materials needed for electricity production, the infrastructure burdens, whether at the level of the production plant or the electricity distribution network, the electricity grid losses, as well as emissions during the production phase at the plant (e.g., combustion gas emissions in the case of thermoelectric power plants). When ecoinvent contained multiple technologies for producing electricity from the same source, the production from that source was distributed among the available technologies according to the proportions indicated in the ecoinvent proxy [54]. For instance, if in the United States, nuclear energy production accounted for 6.4% of the country's power generation in 2022 [54], and both boiling water reactor (32%) and pressurized water reactor (68%) technologies were operating in the US, it was assumed, for that country, that 2.0% of the nuclear power generation is produced using the boiling water reactor technology and 4.4% by using the pressurized water reactor technology.  
According to Section 2.2.1, 'Other' countries accounted for 9% of the Bitcoin mining and, to simplify the calculation, by limiting the number of countries and their data, the corresponding electricity mix was not modelled, and the 91% covered by the eight states mentioned above was proportionally scaled to 100%.  
Future projections associated with country energy mixes are reported in Table S2 and were drawn by different resources, often specific to the analyzed country. The first projection scenario is modelled by referring to the best available data (BAD) found in the literature and on the web for each involved country. Two more scenarios were created according to different estimations provided by, respectively, the International Renewable Energy Agency (IRENA) [55] and EMBER [56], which report percentages of LCE shares referring to global averages. More details and references about the BAD, IRENA, and EMBER scenarios are reported in Table S3 of the ESI. The authors are aware that future projections are not absolute predictions, so the explored shares may not be easily achievable for all the involved countries. Conversely, for some countries (e.g., Canada and Germany), these objectives have already been met. Nevertheless, it was decided to maintain the BAD as a reference scenario and to provide these two alternative perspectives to understand how the conversion of energy mixes could impact the carbon intensity of Bitcoin mining. In the cases where the set objectives had already been achieved, a stationary evolution to the future was assumed (e.g., Canada had already achieved a percentage of 82.5% by 2022, while the IRENA scenario indicates a global percentage of 40% by 2030, so that, for 2030, it was decided to maintain the 82.5% for Canada, and so on).