The housing of the world contains a lot of carbon, mostly in wood, so it is interesting to ask if this carbon stock is large enough to represent a significant removal of carbon dioxide from the atmosphere by the process of photosynthesis that produced the wood. A “sink” for carbon in wooden structures is often touted by the forest products industry as its contribution to solving climate change—harvest trees to put the carbon into saw-timber and grow new trees to take more carbon dioxide from the atmosphere.
The proliferation of large wooden buildings, especially apartment complexes, built from wood avoids some of the emissions of carbon dioxide from the production of steel and cement, about which I have blogged in the past.
Unfortunately, the pool of carbon in wooden structures is not huge. For the United States, the carbon content of buildings, about 1 x 1015 g, compares to about 18 x 1015 gC in all U.S. forests. Worldwide, the annual incorporation of carbon into new structures—about 0.10 x 1015 g/yr—is about 1% of the carbon dioxide emissions to the atmosphere from human activities, largely burning fossil fuels. And typically, the carbon contained in wood products is only about 20% of what is released during the harvest of trees; nearly 60% is returned to the atmosphere by the decomposition and burning of logging wastes and the emissions from logging operations. (The rest is in landfills).
We all need a place to live, but to look at new housing as a solution to the carbon storage and the climate change problem is misguided.
Another frequently asked question is whether old buildings are worth preserving for their carbon stock, despite the fact that they may use a lot of energy to heat, due to poor insulation. On the other hand, most new structures incorporate air conditioning, which consumes energy. The energy used to construct a new structure is known as embodied energy—energy used to fabricate construction materials and ship them to the site. If we tear down an old building and replace it, we incur new costs of embodied energy in construction materials, which must be used to “discount” the energy savings in its subsequent operation. One study in China found that, despite energy savings in operation, it took 18 to 41 years to recover the embodied energy in new construction. Retrofits of existing structures seem well advised.
References
Ding, G. and X. Ying. 2019. Embodied and operating energy assessment of existing buildings—demolish or rebuild. Energy 182: 623-631.
Dixit, M.K. 2017. Embodied energy analysis of building materials: An improved IO-based hybrid method using sectoral disaggregation. Energy 124: 45-58
Hudiburg, T.W, B.E. Law, W.R. Moomaw, M.E. Harmon, J.E. Stenzel. 2019. Meeting GHG reduction targets requires accounting for all forest sector emissions. Environmental Research Letters 14: 095005.
Johnston, C. M. T., and V. C. Radeloff. 2019. Global mitigation potential of carbon stored in harvested wood products. Proceedings of the National Academy of Sciences 116: 14526–31.
Schlesinger, W.H. 2017. Embodied Energy.
