Facility Thought Modeling

Turning Cities into Carbon Sinks

By: Sydney Covey and Mike Castle

Feb 10, 2021

More and more, sustainable building practices are becoming a priority for owners and contractors alike.  One of the most exciting green building trends in recent years is the use of mass timber, specifically cross-laminated timber (CLT), for multi-story buildings. This product type offers construction projects several benefits such as an impressive strength-to-weight ratio that makes it an ideal alternative to concrete and steel, as well as enhanced precision and ease of assembly because it is prefabricated in a factory. These qualities have led to this material’s implementation on large-scale projects across the country. And while these building benefits are great for project teams, mass timber and CLT offer sustainable solutions that can benefit everyone.

An Environmental Issue

Historically, conversations about environmental impact have focused on the use stage of a building and its energy consumption. Rightfully so, as the Global Alliance for Buildings and Construction estimates that building operations account for about 28% of greenhouse gas (GHG) emissions. However, the emphasis on reducing lifecycle emissions during the use stage, in additional to a rise in clean energy, has already yielded more efficient buildings. As a result, materials and construction will begin to represent a growing fraction of a buildings’ carbon impact.

The Global Alliance for Buildings and Construction estimates that about 11% of GHG emissions come from building materials and construction. Concrete alone is responsible for 4-8% of the world’s CO2 emissions, as it is the second most widely used substance on earth, second only to water. Concrete also accounts for roughly 85% of all mining and is linked to an alarming depletion of the world’s sand. Taking this into consideration, it seems logical that alternative building materials are critical to reducing GHG emissions.

Hiding CO2 in Buildings

Carbon sequestration, the process of capturing and storing atmospheric carbon dioxide, is one method of reducing the level of CO2 in the atmosphere. With a historic rise in carbon levels, and countries around the world committing to reducing greenhouse gas (GHG) emissions, carbon sequestration is a valuable process to take advantage of wherever possible. Mass timber and CLT can have a huge impact here.

Laminated timber products are made up of layers of softwood species such as spruce. The trees used in the manufacturing process efficiently captured carbon as they grew. However, once a tree has matured, its carbon-capturing abilities decrease significantly. Towards the end of the tree’s life, it will begin to release the carbon it has been storing for years, either by decaying on the forest floor or through its use in consumer products.

According to Galina Churkina of the Yale School of Forestry and Environmental Studies, if this older wood is used in a building, the CO2 it would otherwise emit has the potential to be locked away for decades, or even hundreds of years. Such timber buildings create carbon sinks — a place that keeps CO2 from getting into the air — in addition to reducing the production of cement and steel.

Cities of the Future

Estimates show that 2.3 billion more people will be living in urban areas by 2050, translating into mass increases in both multifamily and commercial building development. This also translates to an exponential increase in GHG emissions unless cities can alter the current status quo of building development.

Due to mass timber’s ability to sequester large amounts of carbon, taking advantage of this material provides an opportunity to turn entire cities into massive carbon sinks as they develop and redevelop. It also offers a sustainable alternative to demolishing worn wooden structures by replacing pieces of wood structures with new timber pieces.

According to Wood for Good, a global wood promotion campaign, a timber building decreases its overall carbon footprint by up to 75% because of its wood sequestrated CO2. To put this into context, a typical timber house can sequester 19 tons of CO2, meaning building 200,000 timber homes per year would result in over 3.8 million tons of captured CO2. For further context, a single five-story timber building has the potential to cut GHG emissions by levels equivalent to removing as many as 600 cars from the road for a year.


Generally, there are two main ways to mitigate climate change and protect the environment. One is to produce less carbon, while the other is to capture it. Timber buildings expel fewer emissions over their lifecycle by emitting less carbon during manufacturing and storing carbon during its use phase. When the trees come from sustainable forests – meaning they are replanted after being harvested – and as long as the wood is recycled when the building is deconstructed, mass timber could prove to be a powerful carbon-negative solution. By enabling cities to subtract more GHGs from the atmosphere than they emit, cities implementing widespread timber development could turn current GHG emissions paradigms on their head. Mass timber brings carbon neutrality closer to reality.

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