To solve this challenge, the most prominent options are a switch to timber construction or the use of Carbon capture and storage for Zero carbon cement production, depending on your preferred structural material. However, forests cannot keep the pace of construction and scientists are already raising alarm on the intensive use of forest. In Global South urbanization and population growth are forcing high rates of new construction, therefore requiring structural material. An increased use of wood for structural applications, however, cannot be recommended since the world’s rain forests that are located in these regions are already dwindling. Construction wood, could be sourced from pine forests in the Northern Hemisphere. Yet, here the urbanization peaks were surpassed between the 1970s-1990s. Nowadays, new construction rates are low but stricter energy efficiency policies require the environmentally friendly retrofit of existing buildings. This means that in the regions where forests are capable of providing sufficient timber, structural material is not needed in high quantities but material for thermal insulation is. In the regions where structural material is needed, it cannot be sourced sustainably from forests.
Furthermore, forest takes time to grow so any transition to timber construction, whatever amount of capital invested, will take time before seeing the effect on the atmosphere as it requires new trees to grow. Before the growth of the new tree, building with timber is merely just a transfer of carbon from one stock, the forest, to another, the buildings. Therefore, timber is an answer, but not to the question of how to turn the global building stock into a carbon sink.
Concerning carbon capture and storage which would allow to deliver a carbon neutral cement, it is so far at pilot stage. It works from a technological point of view, but the consequences and the feasibility of a massive scale up are underestimated. The most recent report from the Carbon Capture Institute acknowledge that while we have reduced the carbon capture capacity by 30% over the last ten years, we need to increase it by 14’000% in the next 20 years… If we are realist, I think we can assume that there is a risk that the scale up will not be as fast as we would expect.
So we’re clearly gambling on the future. The construction industry fundamentally do not plan to reduce drastically emissions in the next ten years and is betting on the fact that the next generation will be able to implement CCS and take care of growing forests without cutting them while we will cut the forest to build our houses. How dare we? As would Greta Thunberg say.
On the contrary, around us, we have very fast growing plants. Bamboo, straw, hemp, flax… They are around us and adapted to the construction needs. In global South, where structural materials is urgently needed, bamboo is available. Beautiful buildings are already built with it. It can even be industrialised and can then be used exactly as timber… but the growth rate of bamboo is 5 to 7 years compared to the 30 to 60 years of tropical trees. In global North, where insulation materials is crucial to reduce heating demand on existing building stock, Straw is available. It can be blown in boxes or used directly as straw bale. Buildings with prefabricated strawbale façade are already build and we have enough extra supply to already implement such technology on all new and renovated buildings in Europe. Furthermore, in addition to storing carbon much faster than trees and therefore providing climate neutral building one year after the construction, straw also regulates indoor comfort without technical systems.
Even concrete can be interesting. During demolition of old buildings, concrete rubbles which would usually be able to carbonate and capture CO2 within geological time frame (10’000 years) can be fast carbonated and act as a carbon sink immediately after crushing. Furthermore, new concrete made out of highly substituted cement (CEM II/C or CEM III/B) carbonates at least there time faster than traditional concrete. It means that if these new concrete are positioned inside buildings where no corrosion risks happens, and are used in a design that would maximise as much as possible the exposed surface, they would reabsorb the CO2 released during cement production during the building life time. For instance, a 5cm 3d printed wall would be carbon neutral within 20 years.
To summarize, the built environment should not be perceived as a problem, but as a stepping-stone toward potential solutions. Rather than degrading the indoor air quality, it can actually improve it. it can heal the World and its inhabitants. We just need to drastically change our material diet. Like for our food where going towards vegetarism is better for planet, for personal health and for the well being of animals, in the buildings, we should reduce to the absolute minimum the materials which are fossil based and use as much as possible biobased materials or secondary materials such as excavation material, earth that can be circularized. This change in material diet is good for the planet as it allows to switch from buildings as carbon emitter to buildings as carbon sink.
In conclusion, the materials are there, in abundance. The technology is ready, tested and implemented in a multitude of buildings. What is lacking is a structured value chain. Positive change can be created quickly by creating both supply push (agriculture) and demand pull (public procurement and regulatory demand). Middle actors will need training provided by established construction associations or the vocational / professional education sector.