Every building locks in "embodied" CO2 the moment its structural materials are produced. Mass timber stores biogenic carbon absorbed by the tree; concrete and steel only emit.
net_kgCO2e_per_m2 = production − biogenic_sequestration
timber: 350 (production) − 550 (stored) = −200 kgCO2e/m²
concrete/steel: 600 (production) − 0 = +600 kgCO2e/m²
building_total = net_per_m2 × GFA (GFA = footprint × floors)
- City grid size — number of building plots in the modelled skyline.
- Mass-timber share — fraction of new plots built with a CLT/glulam structural frame instead of concrete/steel.
- Building density — average floor count per plot, which scales gross floor area (GFA) and total material demand.
- Retrofit rate — how fast existing concrete/steel buildings are deep-retrofitted rather than demolished and rebuilt; retrofit avoids roughly 50–75% of the embodied carbon a full rebuild would cost (Carbon Leadership Forum / Preservation Green Lab estimates), so it pulls the conventional stock's footprint down over time.
Timber is ~50% carbon by dry mass, so 1 kg of dry wood locks up 44/12 × 0.5 ≈ 1.83 kg CO2. A building is genuinely "climate-positive" only when that stored carbon exceeds everything emitted to produce, transport and assemble its materials — the green cells above.
Figures are order-of-magnitude approximations from published embodied-carbon benchmarks (ICE database, LETI/RICS whole-building carbon guidance), not a specific building's certified LCA.