This is a companion article to "Reducing Carbon," by Craig Savage.

The atomic element carbon is the main component of the earth’s biological compounds and many of its minerals, including limestone. Limestone is significant because it’s the raw material for most cement. To put that in perspective, global CO₂ emissions from cement production add up to approximately 829 million metric tons, accounting for about 3.4% of global CO₂ emissions from fossil-fuel combustion and cement production.

As buildings become more efficient, the amount of energy needed to operate them falls. The more efficient buildings become, the more operational energy approaches the energy embodied in the manufacturing of the building materials. Energy is often seen as a proxy for carbon emissions—a comparison that is fairly accurate when the majority of energy comes from fossil fuels. As more energy is decarbonized (generated from non-fossil-fuel sources), the reduction of operational carbon will accelerate, and embodied carbon will become the dominant source of carbon emissions from buildings. This is what Canadian architect and educator Lloyd Alter calls “the ironclad rule of carbon.” (Chart data by John Ochsendorf/MIT)
As buildings become more efficient, the amount of energy needed to operate them falls. The more efficient buildings become, the more operational energy approaches the energy embodied in the manufacturing of the building materials. Energy is often seen as a proxy for carbon emissions—a comparison that is fairly accurate when the majority of energy comes from fossil fuels. As more energy is decarbonized (generated from non-fossil-fuel sources), the reduction of operational carbon will accelerate, and embodied carbon will become the dominant source of carbon emissions from buildings. This is what Canadian architect and educator Lloyd Alter calls “the ironclad rule of carbon.” (Chart data by John Ochsendorf/MIT)

The carbon cycle is the sequence of events describing the movement of carbon as it is continually cycled throughout earth’s biosphere and includes the process of carbon storing (sequestration) in “carbon sinks” and the subsequent release of that carbon as the cycle repeats.

Here’s how carbon cycles, in the form of CO₂, when first captured in a tree, then turned into lumber, and finally back to CO₂ in the atmosphere: Once a tree seed germinates, it begins to capture and process the CO₂ in our atmosphere. Using the energy of the sun, along with water and other minerals and elements, the plant assembles various proteins into the parts of a tree—roots, bark, leaves, branches, trunk, and so forth. Through the years, the tree captures and stores (sequesters) significant amounts of CO₂ from the air. According to the U.S. Energy Information Administration, one silver maple tree will sequester about 400 pounds of CO₂ in 25 years.

If the tree is burned, perhaps as pellets to heat a house, the CO₂ is returned to the atmosphere immediately—or its stored CO₂ could be returned over time if the tree dies and rots. Alternatively, if we process the tree into lumber and account for the energy expended in cutting, hauling, and milling (its embodied energy), much of the CO₂ remains captured in the lumber (scientists are still trying to quantify the CO₂ left behind in roots and slash). That CO₂ is released only when the building is burned, demolished, put in a landfill, or simply left to deteriorate. You probably notice that stored CO₂ in the lumber (or hemp, bamboo, rice stalks, cellulose, and the like) is only for the life of the structure; ultimately, the carbon is released and the cycle repeats—as it has for millions of years.

So why do we care about stored carbon? For years, buildings were leaky and energy was dirty, resulting in massive amounts of CO₂ being released into the atmosphere to heat, cool, and operate them. As we tighten up and insulate the building envelope, and heat, cool, and run buildings with clean, renewable energy (decarbonized energy), the amount of operational energy gets much smaller relative to the upfront, or embodied, energy, which becomes significantly more important (see chart, above).

Our goal is to put embodied carbon into storage, not into the atmosphere, even if only for the life of a building (or longer if we can reuse, recycle, or otherwise extend the building life). This helps to lower the CO₂ going into the atmosphere and to keep the resulting heat from CO₂’s greenhouse effect within survivable human limits, hopefully until other mitigating efforts can come into play.

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