Carbon Denominators

Mitigating climate change demands rapid reductions of greenhouse gas emissions from the construction and operation of buildings. As the design and construction industry improves tools and techniques for adding up buildings’ contributions to greenhouse gas emissions it must also consider and critique the methods used to normalize these data for analysis: how to divide them. Using Life Cycle Assessment methods, we accounted for the lifetime global warming potential of four case study buildings, each endemic of a primary structural material: steel, concrete, masonry, and mass timber. To improve the critical understanding of these denominators role in comparisons and decisions, we normalized the absolute totals using spatial (kgCO2eq/m2), temporal (kgCO2eq/year), and human (kgCO2eq/person) dimensions. The expanded analysis and visualization of lifetime carbon using novel metrics more closely associates these impacts with buildings’ purpose to shelter people over time. Attributing emissions to people, rather than buildings offers a meaningful and nuanced basis for comparison, for example, normalizing based on occupants shows that as the density increases, carbon intensity per person declines. Attending to the spatial demands of use, dividing emissions by net rather than gross area means emissions intensity decreases as building systems become more spatially efficient, while simultaneously increasing the potential occupant density. In long-lived buildings, the temporal carbon intensity (per year, or per generation) declines with age, and the time value of carbon suggests that future emissions reductions may be worth less than the present emissions to achieve them compared to even the least carbon-intensive new construction, thus emphasizing the urgent need for adaptation of existing buildings. A critical reassessment of the denominators used to normalize emissions complicates short-term considerations of life cycle emissions and militates for an architecture of persistence: designed for human use and reuse, for adaptation and maintenance.