Methodology for evaluating the uncertainty of embodied carbon assessments in construction projects across project phases

There is increasing attention toward reducing greenhouse gas emissions on construction projects in light of their significant contribution to global warming. One of the scrutinized aspects is the measurement and reduction of embodied carbon (EC), including the materials and activities involved in the project execution. However, the assessment of this environmental impact generally involves uncertainty due to the lack of precision in emission factors and the inherent unpredictability in prospective project parameters. Since data quality and accuracy for embodied carbon assessments vary significantly across project phases, it is crucial to evaluate how the uncertainty in the results changes when the assessments are performed at different times of the project development. This study presents a new framework using Monte Carlo (MC) simulation to systematically evaluate and compare uncertainty in carbon assessments across project phases. Three case studies are used to validate this framework and statistically model how the uncertainty varies from one phase to the other. The individual assessment of the case studies reveals that the change in uncertainty for different project phases can range up to 20%-31% based on the differences in the coefficient of variation (COV). Similarly, the mean value reported can vary from 6% to 14%. The uncertainty tends to be reduced for later project phases showing confidence intervals for the coefficient of variation of 0.33-0.39, 0.12-0.28, and 0.04-0.20 for concept, engineering, and construction stages, respectively. However, the specific values depend also on the particularities of the project and the data that each project has available. This work demonstrates that an uncertainty analysis should be included with all embodied carbon assessments to increase the transparency of the emissions reported and the comparison of alternatives. By increasing the reliability of assessments, this study supports the reduction of the net contribution of the construction industry to global carbon dioxide emissions.