Planting trees is currently the most effective and scalable nature-based solution for combatting climate change and limiting global warming to 1.5°C. Forests serve as a crucial mechanism for absorbing excess carbon dioxide (CO2) from the atmosphere, as trees capture and store carbon, acting as the largest terrestrial carbon sink. The preservation and restoration of forests is universally acknowledged as being imperative in the battle against climate change, but surprisingly trees could store even more carbon if they are cut down.
While all plant matter absorbs carbon dioxide from the atmosphere, trees uniquely sequester (absorb and store) a significant portion of carbon within their trunks, potentially locking it away for centuries. Remarkably, even after trees are felled and the timber is used in construction or furniture manufacturing, the wood itself continues to function as a carbon store. Incorporating timber in construction processes also diminishes the carbon footprint of buildings compared to alternative materials, highlighting the transformative potential of wood in reducing greenhouse gas emissions within the construction sector.
How do trees store carbon?
Trees absorb carbon dioxide from the atmosphere and convert it into glucose using photosynthesis, which is then transported to cells for energy production through respiration. Any surplus glucose is stored as starch in the living cells of the trunk and roots. The living layer of the tree trunk gradually transforms into non-living heartwood, which is high in carbon. This carbon remains sequestered within the heartwood until decomposition or combustion occurs. For a more comprehensive understanding of how trees sequester carbon, refer to our blog post How Do Trees Store Carbon.
Consequently, when trees are harvested and processed into timber or pulp, the carbon remains trapped within the resulting wood or paper until it decomposes or is burned. Although wood has served as a construction material for millennia, modern processing techniques are expanding the possibilities of where and how wood can be employed in building construction.
Can timber be used sustainably in construction?
The construction industry faces mounting pressure to explore alternative building materials due to the substantial greenhouse gas emissions attributed to cement and concrete production, which contribute to 8% of global emissions, and the iron and steel industry, which accounts for another 5%. Not only is this a significant proportion of global emissions, but these emissions are still rising rather than falling. Manufacturing 1 tonne of concrete emits an estimated half a tonne of CO2, while 1 tonne of steel production releases about 2 tonnes of CO2. Consequently, wood is increasingly being recognized as a viable alternative in both residential and larger-scale building projects, as it is the ultimate renewable material and has many benefits in reducing carbon.
The utilisation of timber reduces the embodied carbon in a building (i.e. the carbon footprint associated with its creation) due to lower emissions linked to its production. Additionally, the heartwood in timber serves as a carbon sink, preventing its release into the atmosphere as carbon dioxide. Sustainable forestry practices play a crucial role in ensuring that harvested trees are replaced, allowing new growth to continue absorbing carbon dioxide and enhancing carbon sequestration potential. Incorporating disassembly and recycling into the initial design of buildings further extends the reuse potential of timber. These inherent benefits of timber collectively make it a renewable material source, but engineered wood products extend the capabilities and carbon sequestration potential of timber beyond even these advantages.
Mjøstårnet, currently the world’s tallest timber skyscraper in Norway. Photo credit: Øyvind Holmstad https://commons.wikimedia.org/wiki/File:Narsot%C3%A5rnet_ved_Mj%C3%B8sa_02.jpg
What is mass timber?
Mass timber is a relatively recent advancement in engineering structural timber which has facilitated its use in significant construction projects such as the building of ‘plyscrapers’. Mass timber involves assembling pieces of either softwood (such as pine, spruce, or fir) or hardwood (such as birch, ash, or beech) to create panels or structural components like beams. This results in timber that is robust, fire-resistant, lightweight, versatile, and visually appealing. Two primary forms of mass timber exist, each with distinct applications.
Cross-laminated timber (CLT) is the most common form of mass timber and holds considerable promise for architects. It is made by bonding dried lumber boards, obtained from a single log, in layers (typically 3, 5, or 7), with the grain orientation in each layer perpendicular to the preceding one. The thickness of the wood panel can be adjusted (up to 30cm thick) by varying the number of layers, and multiple panels can be interconnected to cover larger areas. The structural integrity of CLT contributes to its strength, enabling it to rival or surpass the performance of concrete and steel, making it suitable for floors, walls, ceilings, or entire buildings.
Cross-laminated timber blocks. Photo credit Oregon Department of Forestry (https://www.flickr.com)
Glue-laminated timber (glulam) shares similarities with CLT, but the lumber boards are arranged with the grain running in the same direction. It is commonly employed for beams and columns and can be easily shaped into curved forms. Glulam enables the creation of larger and longer spans compared to CLT and has been utilised for centuries. One of the earliest surviving examples is the 1866 assembly room of King Edward VI College in Southampton.
Glulam arches in the Sheffield Winter Gardens. Photo credit: Dave Pickersgill https://commons.wikimedia.org/wiki/File:Winter_Gardens,_Sheffield_-_geograph.org.uk_-_3395287.jpg
What are the benefits of using mass timber?
One of the key advantages of mass timber lies in its ability to make use of weaker or younger growth trees with smaller diameters, sometimes as small as 11.5cm, compared to what traditional timber requires. This approach offers significant benefits for forest biodiversity since stands of older trees can remain undisturbed while younger regenerative growth or logs from thinning can be utilized to produce effective timber products. Incorporating wood into construction projects transforms urban areas into carbon sinks and mitigates the high emissions associated with alternative building materials. It is estimated that each cubic metre of cross-laminated timber contains approximately 1 tonne of sequestered CO2, representing a substantial carbon reservoir in every building where it is deploed, although this may vary depending on forestry practices. This translates to a potential 26.5% reduction in the global warming potential of a CLT hybrid building compared to one made of concrete.
Contrary to expectations, mass timber demonstrates excellent performance in fire resistance. In fact, in some construction projects, steel frames are cladded with mass timber to shield them in the event of a fire. Additionally, mass timber panels and beams are lightweight compared to steel and concrete, enabling the construction of larger structures such as Dalston Works with shallower foundations, even in areas with underlying infrastructure such as tunnels. Wooden structures also exhibit good resistance to earthquakes and can be repaired afterwards, unlike concrete which tends to crack and needs replacement.
The use of mass timber in construction facilitates quicker project completion as panels are manufactured to precise specifications and transported to the site, resulting in reduced labour costs. Mass timber buildings can decrease construction time by 25% to 30% and cut onsite traffic by 90%, with less waste generated due to precision design and prefabrication of panels tailored to specific spaces. Moreover, the resulting buildings boast undeniable aesthetic appeal, and there are potential health and wellbeing benefits associated with wooden structures, such as lowered heart rate and blood pressure, and improved indoor air quality. Lastly, the increased utilisation of timber creates rural job opportunities in forestry and incentivizes the establishment of sustainable forests.
What is the global potential of timber as a carbon store?
Only recently has the carbon sequestered in harvested wood products been incorporated into national greenhouse gas inventories, in the IPCC guidelines published in 2006. Assessing the current carbon storage in timber is a complex undertaking, partly due to the need to balance carbon losses and gains resulting from international timber movements. Predicting the global carbon sink potential for timber used in construction is even more complex. It requires the consideration of various factors such as estimating future demand for new buildings, evaluating the potential to expand forest cover to meet that demand, accounting for emissions reductions from timber use, and quantifying the carbon sequestered in the timber. Additionally, there is a significant question about how to account for the fate of timber products when buildings are replaced, although recycling wood can prevent carbon from re-entering the atmosphere.
One study estimates that approximately 0.09 billion tonnes of CO2 are stored annually in harvested wood products worldwide, representing less than 1% of the current carbon budget of 36.6 billion tonnes of CO2 produced by human activity. In the UK, the estimated carbon sequestered within timber frame houses and engineered wood in new builds annually is around 1 million tonnes of CO2, again less than 1% of the current 354 million tonnes of CO2 emissions produced in the UK.
Projections regarding the extent of this storage capacity depend on the percentage of timber used in construction. One recent study calculates potential scenarios based on timber usage in 10%, 50%, and 90% of new builds over the next 30 years, with global annual carbon storage ranging from up to 0.08 to 0.67 billion tonnes of CO2. In the 90% timber scenario, this represents 2% of global carbon emissions at current figures, though this proportion would shift if global emissions decrease. Another study estimated a potential increase of up to 0.44 billion tonnes of CO2 sequestered annually if the proportion of timber used remains constant. Moreover, there would be a notable reduction in global emissions, potentially ranging from 14% to 31%, as the use of concrete, cement, and steel decreases.
The scope of the carbon storage potential in construction timber is best illustrated by the fact that a mass timber building stores more carbon per square meter than an equivalent area of living forest. This has prompted discussions on whether harvesting trees to sequester carbon and replacing them with younger, faster-growing trees would be the optimal carbon sequestration strategy. With the expected surge in demand for new buildings, doubling the current stock within the next 40 years, it's imperative to prioritize the use of more sustainable building materials.
A key question arises regarding whether existing or potential forest cover can produce enough timber to meet potential demand without jeopardizing old-growth forests. A recent study suggests there is sufficient surplus in current global forestry operations to allow timber to replace up to 90% of construction materials if some timber allocated for fuel is redirected. However, another study at a national level reveals significant deficits within individual countries, necessitating an increase in forest cover. For instance, the UK presently imports 81% of its wood products, potentially from forests lacking sustainable management practices, although there is potential to sustainably increase timber extraction within the UK. The World Bank is already predicting that demand for timber will quadruple by 2050, which recent research argues we do not have the capacity to sustain, potentially leading to a timber shortage crisis. Any disparity between demand and supply would require logging existing forests, potentially leading to further deforestation in sensitive areas. Replacing felled trees is essential, emphasizing the critical need for any increase in timber use in construction to be tied to high sustainability standards in forest operations.
Can forestry be sustainable?
Deforestation is one of the primary contributing factors to climate change and biodiversity loss, whether for land clearance or timber extraction, and there is a global consensus on the urgent necessity to halt deforestation of primary forests. The escalating demand for timber seems to be incompatible with the need to preserve biodiverse forests. However, integrating sustainable practices into forestry operations, as outlined in voluntary accreditations like the international Forest Stewardship Council (FSC), can ensure that timber remains a genuinely renewable resource. FSC accreditation requires the replacement of felled trees, the safeguarding of sensitive forest areas from logging, the preservation of biodiversity, and the employment of local workers in forestry activities, with full supply chain traceability. These requirements are also integrated into national schemes such as the UK Woodland Assurance Standard and the UK Forestry Standard, which mandate at least 5% planting of native broadleaves in every new forestry project to enhance native biodiversity. The UK boasts a commendable record of sustainable forestry, with 80% of harvested wood grown to FSC standards, although domestically sourced timber constitutes only a fraction of our timber consumption.
The global need to enhance biodiversity globally has led to stigmatisation of non-native forest plantations due to their limited contribution to native habitats. However, in the UK, non-native species like sitka spruce have adapted to local climate conditions, and biodiversity can thrive in these plantations across various taxonomic groups. Effective forest management can bolster biodiversity even in ancient woodlands, with biodiverse forests exhibiting enhanced carbon sequestration. Practices such as thinning and creating clearings in forests not only restore biodiversity but also yield logs suitable for mass timber production. It is crucial to manage existing forests efficiently and expand forest cover to enhance biodiversity but rather than posing an additional threat to forest cover, sustainable timber extraction can be integrated into the restoration process.
Organizations like Wood for Good in the UK and Think Wood in the US advocate for the role of mass timber in construction as a means to combat climate change. Combining the carbon storage potential of wood products with sustainable timber harvesting and extensive afforestation presents a compelling vision of a more sustainable urban future. This approach aids in emissions reduction, improves wellbeing, and promotes biodiversity restoration without impeding progress. Trees do indeed hold the potential to contribute significantly to saving the planet in ways that were previously underestimated.
