Climate change exists. Hurricane Sandy, the unprecedented summer heat waves and droughts, and the alarming seasonal reduction of the extent and thickness of polar ice are harbingers of what’s ahead. The impacts of climate change are here, and if we don’t act expeditiously, the effects we’ve already endured will be significantly more severe. We must adapt to the changes ahead to protect infrastructure and populated areas, but adaptation without aggressive climate mitigation would be extremely costly, both financially and culturally. Serious mitigation means reducing carbon dioxide emissions to bring the concentration of CO2 in the atmosphere back down to the 1990 level of 350 parts per million.
It is generally understood that CO2 is a greenhouse gas (GHG) and contributes to global warming. CO2 is also the most abundant GHG in the atmosphere. Scientists have established that CO2 persists in the atmosphere for many years. For 100 pounds of CO2 emitted today, 33 lbs. will still be in the atmosphere in 100 years, and 19 lbs. after 1,000 years. The implication is widely accepted by climate specialists but less well known by the general public: We need to stop putting carbon in the atmosphere now. We can begin to do this by understanding the impacts of our building materials and making informed choices. To this end, the Carbon Working Group of the Structural Engineering Institute’s Sustainability Committee has published a white paper collecting and parsing CO2 data on structural materials.
Structural engineers have a proactive role to play in addressing the very real threat of climate change, since structural materials contribute substantially to the lifecycle CO2 emissions of buildings and other structures. The "embodied carbon" of building structure is the sum of CO2 emissions associated with the extraction of raw materials, manufacture of products, and construction activities. It is estimated that on average half of the embodied carbon in buildings is due to structure. This is not a surprise considering the contribution of the structure to the overall mass of buildings. Once we understand the CO2 impacts of the materials we specify, we can work to reduce those impacts and make a meaningful contribution to addressing the climate crisis.
The white paper provides the basic facts we need to get started. The authors combed through publically available CO2 emissions data per unit weight of product for the major structural materials – concrete, steel, masonry, wood, and fiber-reinforced plastics. The white paper explains where CO2 is emitted in the material lifecycle and offers strategies for reducing the carbon footprint of each material.
Concrete, for instance, emits about 1 ton of CO2 for every ton used, primarily due to the manufacture of cement. These emissions can be reduced by cutting the cement content in concrete, for instance by substituting fly ash and other supplementary cementitious materials. Engineers can do their part by specifying "reduced carbon" mixes and products. They can also take advantage of new technologies, such as forced carbonation, which sequesters CO2 in precast concrete and CMU during curing. Another source of emissions that applies to all structural materials is transportation. If materials are sourced a great distance from the project site, transporting them, especially if by truck, can contribute significantly to the carbon footprint of those materials. On the other hand, designers need to weigh the trade-offs if local producers use outmoded manufacturing processes that generate higher emissions than more distant producers.
The best way to compare the carbon footprint of two alternative materials is to compare quantities that are functionally equivalent, since, for example, a pound of steel does not provide the same structural function as a pound of wood. To provide an example of the carbon footprint for different materials providing like function, the white paper compares three different framing solutions for the same commercial office floor plan. A schematic material takeoff for each scheme is itemized along with the CO2 impacts from each component. The results reveal which elements of framing have the most significant impact, while also bringing to light the limitations of the available emissions data.
Engineers can use the CO2 data and manufacturing background provided in the white paper to analyze the CO2 footprints of their structural designs. While factors such as the choice of materials play a role, often the building solution that uses the least material also has the least embodied CO2 emissions. Since structural engineers already design for material economy, we are well-positioned to make informed decisions to reduce the carbon impact of structures. Just as we uphold an obligation to design structures that are safe for their occupants, we also need to provide engineering solutions that protect our planet for generations to come.
The SEI Sustainability Committee carbon white paper, "Structure and Carbon: How Materials Affect the Climate," is available as a free download at http://tiny.cc/SEIcarbon. More information can be found on the committee’s web page at www.seisustainability.org
Helena Meryman, LEED AP BD+C, is an independent sustainability consultant in San Francisco. She can be reached at firstname.lastname@example.org. Adam Slivers, S.E., is an Associate at KPFF Consulting Engineers. He can be reached at Adam.Slivers@kpff.com. Mark D. Webster, P.E., LEED AP BD+C, is a project manager with Simpson Gumpertz & Heger Inc. in Waltham, MA. He can be reached at email@example.com. All three are members of the SEI Sustainability Committee. The committee’s website is www.seisustainability.org