Planting trees is the most efficient and scalable nature-based solution to fighting climate change and helping to achieve the Paris Agreement targets of restricting global warming to 1.5°C. Forests have great potential to help absorb more of the excess carbon dioxide (CO2) in the atmosphere as trees absorb it and store as carbon, making forests the largest land based carbon sink. Preserving and restoring forests is internationally recognised as being essential to helping fight climate change, but there is a surprising way that trees could help store more carbon, by cutting them down.
Although all plant matter absorbs carbon dioxide from the atmosphere, trees sequester (absorb and store) some of the carbon in their trunk, potentially storing it for hundreds of years. Uniquely the wood itself continues to act as a carbon store after the tree is felled and the timber is used in construction or the manufacture of furniture. Using timber in construction also reduces the carbon footprint of buildings compared to other building materials, meaning that wood offers great potential to revolutionise the greenhouse gas emissions of the construction industry.
Trees absorb carbon dioxide from the atmosphere and use photosynthesis to convert it into glucose, which is then circulated to the cells where it is needed and transformed into energy via respiration. Any unused glucose is stored as starch in the living cells of the trunk and roots. Over time the living layer of the tree trunk is turned into non-living heartwood, of which carbon is a key component. The carbon remains locked in the heartwood until it decays or is burned. For a more detailed description of how trees sequester carbon see our blog post How Do Trees Store Carbon.
This means that if trees are felled and cut into timber or pulped, the carbon stays in the wood or paper that is produced, again until it decays or is burned. Wood has been used as a construction material for thousands of years, but modern processing techniques are broadening the horizons of where and how wood can be used to construct buildings.
There is pressure on the construction industry to find alternative building materials because cement and concrete production account for 8% of global greenhouse gas emissions and the global iron and steel industry account for another 5%. It is estimated that half a tonne of CO2 is emitted to manufacture 1 tonne of concrete, and 2 tonnes of CO2 are emitted in the manufacture of 1 tonne of steel. Wood is increasingly being considered as a viable alternative in both residential and larger building projects as it is the ultimate renewable material with multiple benefits for reducing carbon.
Firstly, the use of timber reduces the embodied carbon in a building, i.e. the carbon footprint of the building’s creation, because there are lower emissions associated with its production. Timber also acts as a carbon sink because it preserves the carbon that is stored in the wood, preventing its release back into the atmosphere as carbon dioxide. Sustainable forestry practices can ensure that felled trees are replaced and that new growing trees will continue to absorb carbon dioxide, increasing the carbon sequestration potential. If buildings are designed so that the disassembly and recycling of materials is incorporated into the initial plan, then the timber can be further reused. Any timber has these benefits that combine to make it a renewable source of material, but engineered wood products extend the capabilities and carbon sequestration potential of timber even further.
Timber has been used to build houses for thousands of years, but more recently the development of engineered structural timber or ‘mass timber’ has allowed wood to be used in major construction projects to create ‘plyscrapers’. Mass timber involves attaching pieces of either softwood (e.g. pine, spruce or fir) or hardwood (e.g. birch, ash or beech) together to form panels or structural elements such as beams. This results in timber that is strong, fire-resistant, lightweight, versatile and aesthetically pleasing. There are two primary forms of mass timber that have different applications.
Cross-laminated timber (CLT) is the most popular form of mass timber and the one that holds the most promise for architects. It is formed by gluing dried lumber boards cut from a single log in layers (typically 3, 5 or 7), with the grain in each layer lying perpendicular to the previous layer.
The depth of the wood panel can be adjusted (up to 30cm thick) by changing the number of layers and multiple panels can be joined together to cover larger expanses. The structure of CLT provides its strength and it can match or exceed the performance of concrete and steel, making it suitable for floors, walls, ceilings or even entire buildings.
Glue-laminated timber (glulam) is similar to CLT but the lumber boards are arranged with the grain running in the same direction. It is commonly used for beams and columns and can be readily formed into curved shapes. Glulam allows for larger and longer expanses than CLT and has been used for centuries; the earliest known example that is still standing is the 1866 assembly room of King Edward VI college in Southampton.
One of the advantages of mass timber is that it can use weaker or younger growth trees with smaller diameters (as small as 11.5cm) than traditional timber requires. This has benefits for forest biodiversity because stands of older trees can be left intact and younger regenerative growth or logs from thinning can be used to provide effective timber products. Using wood in construction projects turns urban spaces into a carbon sink and avoids the high emissions of alternative building materials. Each cubic metre of cross-laminated timber is estimated to contain 1 tonne of sequestered CO2, representing a significant carbon store in every building in which it is used, although this will vary dependent on forestry practices. This equates to a potential 26.5% reduction in global warming potential of a CLT hybrid building compared to a concrete building.
Contrary to what might be expected, mass timber performs well in fire and indeed in some construction projects steel frames are given a mass timber cladding to protect them in the case of fire. Mass timber panels and beams are also lightweight compared to steel and concrete, allowing larger structures such as Dalston Works to be built with more shallow foundations on sites where there is infrastructure such as tunnels below. Wooden structures also have good resilience to earthquakes and can be repaired afterwards, unlike concrete which cracks and needs to be replaced.
Brock Commons, an 18 storey hybrid mass timber residence in the University of British Columbia.
Using mass timber in construction allows buildings to be completed more quickly as panels are manufactured to fit and shipped to site, resulting in lower labour costs. Mass timber buildings can reduce construction time by 25% to 30% and reduce onsite traffic by 90% and there is less waste because the panels can be precision designed and prefabricated to fit specific spaces. The buildings that result are undeniably aesthetically appealing and there can be health and wellbeing benefits associated with wooden buildings as they lower heart rate and blood pressure and improve air quality inside. Finally, the increased use of timber creates rural jobs in forestry and incentivises the creation of sustainable forests.
It is only recently that the carbon sequestered in harvested wood products has been included as part of national greenhouse gas inventories, with IPCC guidelines being published in 2006. Calculating how much carbon is currently stored in timber is a complex process, partly due to balancing the carbon losses and gains caused by international timber movements. Extrapolating the global carbon sink potential for timber used in construction is even more complicated as there are multiple facets: estimating the future demand for new buildings, assessing whether there is scope to increase forest cover to meet that demand, accounting for the emissions reductions from using timber, and accounting for the carbon sequestered in the timber. There is also a significant question about how to factor in the fate of the timber products when the building is replaced, although wood can be recycled to avoid the carbon being returned to the atmosphere.
It is estimated by one study that there is currently around 0.09 billion tonnes of CO2 stored annually in harvested wood products across the world. This represents less than 1% of the current carbon budget of 36.6 billion tonnes of CO2 produced by human activity. In the UK the figure for carbon sequestered within timber frame houses and engineered wood in new builds is estimated to be 1 million tonnes of CO2 annually, again less than 1% of the current 354 million tonnes of CO2 emissions produced in the UK.
The projections of how extensive this storage capacity could be depends on the percentage of timber used in construction. One recent study calculates this based on timber being used in 10%, 50% and 90% of new builds over the next 30 years, with global annual carbon storage figures of up to 0.08, 0.37 and 0.67 billion tonnes of CO2 respectively. In the 90% timber scenario this pushes the figure as a percentage of global carbon emissions to 2% at current figures, although this proportion would increase if global emissions drop. Another study estimated that the increase would be up to 0.44 billion tonnes of CO2 sequestered annually if the proportion of timber used remained the same. In addition, there would be a significant reduction in global emissions, potentially of 14 to 31%, as the use of concrete, cement and steel would be reduced.
The most striking explanation of how extensive this carbon store could be is best expressed by the fact that a building constructed from mass timber stores more carbon per square metre than the equivalent area of living forest, leading one article to conclude that we should chop trees down to sequester carbon and replant with younger trees that absorb carbon faster. The demand for new buildings is expected to soar, with the current stock doubling in the next 40 years, so it is essential that more sustainable building materials are used.
One of the key questions is whether there is scope within existing or potential forest cover to produce enough timber to meet the potential demand, without endangering old growth forests. A recent study estimated that there is sufficient surplus in current global forestry operations to allow timber to replace up to 90% of construction materials if some of the timber used for fuel is reallocated. A subsequent study examined this at a more national level and concluded that there are significant deficits within individual countries, where forest cover would need to be increased. As an example, the UK currently imports 81% of its wood products, potentially from forests that are not sustainably managed, although there is capacity to increase our timber extraction sustainably within the UK. Due to the time it would take new trees planted now to grow to an appropriate size, any discrepancy between demand and supply would require logging existing forest, which could lead to further deforestation in sensitive areas. Replacing the trees as they are felled is a solution and highlights that it is critical that any increase in the use of timber in construction is linked to high sustainability in forest operations.
Deforestation for timber harvesting and land clearance is one of the key drivers of climate change and biodiversity loss, and it is globally acknowledged that there is an urgent need to stop deforestation of primary forest. Increasing timber demand appears to conflict with the need to restore forests high in biodiversity, but incorporating sustainable practices into forestry, such as those stipulated in the international Forest Stewardship Council (FSC) voluntary accreditation, can ensure that timber is a truly renewable product. FSC commitments include replacing trees that are felled, protecting sensitive forest areas from harvesting, preserving biodiversity, and ensuring that local workers are employed in forestry operations, with full traceability throughout the supply chain. FSC requirements are linked into national schemes such as the UK Woodland Assurance Standard and the UK Forestry Standard, which stipulates that every new forestry project must include at least 5% planting of native broadleaves to increase native biodiversity. The UK has a good record of sustainable forestry and 80% of the wood harvested in the UK is grown to FSC standards, although as previously mentioned this is homegrown wood is only a small fraction of the timber that we use.
The need to increase biodiversity in habitats globally has led to criticism of non-native forest plantations as they do not tend to increase habitat for native species. In the UK, however, the non-native plantations of species such as Sitka Spruce have adapted to our climate and biodiversity can be high across many taxonomic groups. Effective forest management can improve biodiversity even in ancient woodlands and forests high in biodiversity also have higher carbon sequestration. Forestry operations such as thinning and creating clearings restore biodiversity and generate logs that can be used for mass timber. It is essential that we manage our existing forests effectively and increase forest cover to improve biodiversity, but rather than forestry being an additional pressure on maintaining forest cover, sustainable timber extraction can form part of the restoration process.
Organisations such as Wood for Good in the UK and Think Wood in the US are highlighting how the use of mass timber in construction can play a role in helping to fight climate change. Combining the carbon storage potential of wood products with sustainable timber harvesting and extensive afforestation offers a compelling vision of a more sustainable urban future, helping to reduce emissions, improve wellbeing and restore biodiversity, without limiting progress. Trees really could help save the planet, in more ways than we initially appreciated.