# Measuring Carbon in Cities and Their Buildings through Reverse Engineering of Life Cycle Assessment
## 1 Introduction  
In 2014, the European Council agreed on the 2030 climate and energy framework [1], followed by the approval of the related European Union (EU) climate and energy policies by the European Council and the European Parliament. The EU's ambition to reduce greenhouse gas emissions (GHG) to at least 40% by 2030 is consistent with the guidelines of the European Commission for nearly zero-energy buildings [2, 3, 4, 5, 6, 7], under EU reports for monitoring the countries' progress in the field. This is also coherent with the European Green Deal's longer-term objective [8, 9] to transform the EU into a competitive low-carbon economy by 2050, in the scenario of the recommended reductions of carbon emissions to be made by EU countries. This objective aligns with the Intergovernmental Panel on Climate Change (IPCC) goal of reducing its carbon emissions by 80-95% by 2050 (compared to 1990 levels), with milestones of 40% by 2030 and 60% by 2040 [1]. Setting out a plan to \"increase the EU (European Union) 2030 climate target to at least 50% and towards 55% in a responsible way\" [9]. According to the European Commission 2019 Global Status Report [3], the built environment accounts for around 40% of annual global CO\({}_{2}\) emissions. Reducing building sector carbon emissions is essential for achieving the carbon neutrality European Commission target by 2050. In buildings, therefore, in cities [10, 11]. EU climate action initiatives include three types of measures [1]: (1) cut GHG emissions; (2) expand the use of renewable energy and (3) meet energy efficiency targets. This research focuses on the first type of measure, the cut in GHG emissions. In particular, it focuses on low-carbon nature-based solutions [12; 13; 14] (bio-based materials for walls and roofs in buildings) for building design, since low-carbon construction materials [15; 16; 17; 18] have become critical in sustainable building design for the chosen climate action measure (1) above. For that purpose, it is vital to have a thorough understanding of the building's life cycle's carbon emission process [19; 20]. Life Cycle Assessment (LCA) is a modeling tool to evaluate environmental impacts related to a product, including carbon emissions, termed Whole Life Carbon (WLC), for the case of buildings [21; 22]. The Communication from the Commission to the Council and the European Parliament of Integrated Product Policy concluded that Life Cycle Assessment provides the best framework for embodied carbon [23]. Carbon emissions occur during the entire life cycle of a building. The European standard EN 15978 [24] sets the system boundaries of whole-building WBLCAs (Whole Building Life Cycle Assessment) and the Calculation method of the environmental performance of buildings, including carbon emissions. In this paper, the Reverse Engineering method application and the percentage reduction analysis enable minimizing the concerns about uncertainty and variability of WBLCA [25]. The system boundary specified by the standard consists of the following 16 sub-stages grouped into four life cycle stages: the Product stage, Construction Stage, Use Stage, and End of Life [24]. - Product Stage:  
A1. Raw material extraction A2. Transport to manufacturing site A3. Manufacturing - Construction Process Stage:  
A4. Transport to construction site A5. Construction Process - Use Stage:  
B1. Use B2. Maintenance B3. Repair B4. Replacement B5. Refurbishment B6. Operational energy use B7. Operational water use  
- End of Life Stage:  
C1. De-Construction demolition C2. Transport C3. Waste processing C4. Disposal - (Supplementary Information Beyond Building Life Cycle:  
D. Benefits and loads beyond system boundary: Reuse-Recovery-Recycling Potential). The carbon emissions released during each stage of the building life-cycle vary [26; 27]. Embodied carbon is the sum of GHG emissions during the stages: raw material extraction, transportation, manufacturing, construction, maintenance, renovation, and end-of-life for a product or a system.