The idea of storing carbon in wood-based building products has been gaining traction in the forestry and construction business for several years. On the surface, it certainly seems to make sense.
As trees grow they breath in carbon dioxide, storing it as carbon in their roots, trunks, branches and leaves. If we can harvest this wood biomass - which is about 50% carbon after you subtract out the moisture - and use wood products as structural components in buildings, it seems that we should be able to use our homes and offices as carbon sinks.
Trees are after all a renewable resources. According to Natural Resources Canada the rate of deforestation in 2010 was a mere 0.02%, meaning that for every tree that is harvested, a new one is planted in it's place. So why is there a growing body of evidence that building with wood is neither environmentally friendly or low-carbon?
Efficiency: Unfortunately when we transfer wood from the forest ecosystem to the built environment, the majority of the carbon initially locked inside the tree is lost to harvesting slash, mill residues and construction waste. It's estimated that only about 15-33% of the carbon that was stored in the initial living tree ends up stored in long-lived wood building products. The majority of the carbon in trees, which took decades to be absorbed from atmosphere, is emitted very quickly after a harvest.
Damage to Carbon Pools: Most of the carbon in a forest ecosystem is actually contained in the soil. For instance in Canada's Boreal forests, 85% of the total ecosystem carbon is stored in the soil. What happens to this carbon when we start logging trees? A recent meta-analysis of harvesting operations found that on average, soil carbon stocks decline by 11.2% after a harvest. Unfortunately it looks like when we try to transfer some of the above-ground carbon to building products, we seriously disrupt and damage the most important carbon pool in the ecosystem.
Not all wood is renewable: Different species of trees in different climate zones grow at different rates. Some forests are young (30 years old) and planted specifically for commercial harvest, while others are ancient (250 years old) and are home to a rich and diverse ecosystem. When we harvest one of these "old-growth" forests, we're effectively vaporizing an environmental treasure, a rich and dense carbon store that can't be replaced. Centuries old carbon sinks take centuries to regenerate, they're not renewable.
Let's get serious about DESIGN: 80-90% of a building's lifetime emissions are incurred during the use-phase of the building (ie. the emissions associated with the electricity use, heating and cooling of the building). That means that as much as 90% of the carbon footprint of the built environment has little or nothing to do with what materials are used in construction. It's all about design and operating efficiency.
The idea of using wood-based building products to sequester carbon in the built environment was innovative, and appeared promising, but the evidence is starting to come in that the high hopes for this strategy are probably misplaced.
The challenge of reducing the carbon footprint of the built environment is a matter of designing efficient and resilient buildings. Designing for minimal heat loss, low-maintenance costs, resilience in the face of extreme weather and climate, and access to clean energy sources, such as wind, solar, geothermal and others. There are no quick fixes, let's leave our forest carbon sinks intact and challenge ourselves to build better buildings.