First, let me say that this is a huge topic that is difficult to respond to completely. What follows is my attempt at an answer that is based on our efforts to understand what targets we should be setting for embodied carbon reductions.
Yes this is an important topic that I think most in our line of work have come across. I have looked at baseline (or benchmark) studies from CLF (2017), deQo (2017), Carbon Heros One Click LCA (current), and various studies including “Towards embodied carbon benchmarks in Europe” by Ramboll (2022), AIA COTE, MDPI Sustainability Journal (2018), “Embodied Carbon Benchmarks for European Buildngs” by One Click LCA (2021), benchmarks set by RIBA, LETI and WBCSD, and many case studies too numerous to mention.
What we have learned is that the life cycle stages considered, the scope and completeness of building systems included, and the timing and accuracy of the BIM that populates the amounts of materials considered are the biggest factors for the wide range of baseline values out there today.
The consistent reporting of life cycle stages, and the scope and completeness of building systems included are starting to be addressed with WBLCA reporting templates and guidelines such as those by CLF, RICS, LETI, and WBCSD. The timing and accuracy of the BIM work by the wide range of those who perform WBLCAs is harder to control.
For setting targets within our collaborative for use until better databases are developed by national and international organizations, my opinion is that baselines are best based on LCAs for each building system as determined by the disciplines responsible for each system. Realizing that this is an ideal that is difficult to attain, our current focus is the development of ranges of embodied carbon for each building system as a way to validate the embodied carbon values provided for each system, or to supplement those values that are not provided. These ranges can also be used to estimate the WBLCA early in design.
Life cycle stages are key to this conversation as well, and my understanding of most baselines is that they generally include stages A1-A5 and that some extend through stage D. The embodied carbon for stages A1-A5, for a short time frame such as now to 2030, is the highest value embodied carbon regarding staying below a 1.5 C target. Ideally all life cycle stages would be included in a baseline studies as this would include benefits such as the recycling of steel, the reuse of building components, and the sequestration of carbon in wood products.
In using the building system approach to determine ranges of embodied carbon, there are several examples of progress including the CLF studies for interiors (including FF&E) and MEP, and the SE 2050 Challenge which is collecting data from over 100 structural engineering firms for the structural system embodied carbon amounts. Using these studies and the other resources mentioned above, our organization, the Colorado Embodied Carbon Collaborative, has started developing ranges for each building system to use until more thorough baselines are developed.
We are anticipating that CLF will release an updated baseline study later this year, and we have the expectation that it will be a good resource. We are also watching England for the work being performed by the Built Environmental Carbon Database. A third benchmarking effort in the works is by Athena in Canada. There are more out there, I am sure.