Editing IPCC:AR6/WGIII/Chapter-8 (section)

==== 8.4.3.2 Switching to Net-zero-emissions Materials and Supply Chains ====

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For the carbon embodied in supply chains to become net-zero, all key infrastructure and provisioning systems will need to be decarbonised, including electricity, mobility, food, water supply, and construction ( [[#Seto--2021|Seto et al. 2021]] ). The growth of global urban populations that is anticipated over the next several decades will create significant demand for buildings and infrastructure. As cities expand in size and density, there is an increase in the production of mineral-based structural materials and enclosure systems that are conventionally associated with mid- and high-rise urban construction morphologies, including concrete, steel, aluminium, and glass. This will create a significant spike in GHG emissions and discharge of CO 2 at the beginning of each building lifecycle, necessitating alternatives ( [[#Churkina--2020|Churkina et al. 2020]] ).

The initial carbon debt incurred in the production stage, even in sustainable buildings, can take decades to offset through operational stage energy efficiencies alone. Increased reduction in the energy demands and GHG emissions associated with the manufacture of mineral-based construction materials will be challenging, as these industries have already optimised their production processes. Among the category of primary structural materials, it is estimated that final energy demand for steel production can be reduced by nearly 30% compared to 2010 levels, with 12% efficiency improvement for cement ( [[#Lechtenböhmer--2016|Lechtenböhmer et al. 2016]] ). Even when industries are decarbonised, residual CO 2 emissions will remain from associated chemical reactions that take place in calcination and use of coke from coking coal to reduce iron oxide ( [[#Davis--2018|Davis et al. 2018]] ). Additionally, carbon sequestration by cement occurs over the course of the building lifecycle in quantities that would offset only a fraction of their production stage carbon spike ( [[#Xi--2016|Xi et al. 2016]] ; [[#Davis--2018|Davis et al. 2018]] ). Moreover, there are collateral effects on the carbon cycle related to modern construction and associated resource extraction. The production of cement, asphalt, and glass requires large amounts of sand extracted from beaches, rivers, and seafloors, disturbing aquatic ecosystems and reducing their capacity to absorb atmospheric carbon. The mining of ore can lead to extensive local deforestation and soil degradation ( [[#Sonter--2017|Sonter et al. 2017]] ). Deforestation significantly weakens the converted land as a carbon sink and in severe cases may even create a net emissions source.

A broad-based substitution of monolithic engineered timber systems for steel and concrete in mid-rise urban buildings offers the opportunity to transform cityscapes from their current status as net sources of GHG emissions into large-scale, human-made carbon sinks. The storage of photosynthetic forest carbon through the substitution of biomass-based structural materials for emissions-intensive steel and concrete is an opportunity for urban infrastructure. The construction of timber buildings for 2.3 billion new urban dwellers from 2020 to 2050 could store between 0.01 and 0.68 GtCO 2 per year depending on the scenario and the average floor area per capita. Over 30 years, wood-based construction can accumulate between 0.25 and 20 GtCO 2 and reduce cumulative emissions from 4 GtCO 2 (range of 7–20 GtCO 2 ) to 2 GtCO 2 (range of 0.3–10 GtCO 2 ) ( ''high confidence'' ) ( [[#Churkina--2020|Churkina et al. 2020]] ).

Figure 8.17 indicates that new and emerging structural assemblies in engineered timber rival the structural capacity of steel and reinforced concrete while offering the benefit of storing significant quantities of atmospheric carbon (see also Figure 8.22). ‘Mass timber’ refers to engineered wood products that are laminated from smaller boards or lamella into larger structural components such as glue-laminated (glulam) beams or cross-laminated timber (CLT) panels. Methods of mass-timber production that include finger-jointing, longitudinal and transverse lamination with both liquid adhesive and mechanical fasteners, have allowed for the reformulation of large structural timbers. The parallel-to-grain strength of mass (engineered) timber is similar to that of reinforced concrete ( [[#Ramage--2017|Ramage et al. 2017]] ). As much as half the weight of a given volume of wood is carbon, sequestered during forest growth as a by-product of photosynthesis ( [[#Martin--2018|Martin et al. 2018]] ). Mass timber is inflammable, but in large sections forms a self-protective charring layer when exposed to fire that will protect the remaining ‘cold wood’ core. This property, formed as massive structural sections, is recognised in the fire safety regulations of building codes in several countries, which allow mid- and high-rise buildings in timber. Ongoing studies have addressed associated concerns about the vulnerability of wood to decay and the capacity of structural timber systems to withstand seismic and storm-related stresses.

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'''Figure 8.17: Relative volume of a given weight, its carbon emissions, and carbon storage capacity of primary structural materials comparing one tonne of concrete, steel, and timber.''' Concrete and steel have substantial embodied carbon emissions with minimal carbon storage capacities, while timber stores a considerable quantity of carbon with a relatively small ratio of carbon emissions-to-material volume. The displayed carbon storage of concrete is the theoretical maximum value, which may be achieved after hundreds of years. Cement ratios of 10%, 15%, and 20% are assumed to estimate minimum, mean, and maximum carbon storage in concrete. Carbon storage of steel is not displayed as it is negligible (0.004 tonne C per tonne of steel). The middle-stacked bars represent the mean carbon emission or mean carbon storage values displayed in bold font and underlined. The darker and lighter coloured stacked bars depict the minimum and maximum values. Grey tones represent carbon emissions and green tones are given for storage capacity values. Construction materials have radically different volume-to-weight ratios, as well as material intensity (see representations of structural columns in the upper panel. These differences should be accounted for in the estimations of their carbon storage and emissions (see also Figure 8.22). Source: adapted with permission from [[#Churkina--2020|Churkina et al. (2020)]] .

Transitioning to biomass-based building materials, implemented through the adoption of engineered structural timber products and assemblies, will succeed as a mitigation strategy only if working forests are managed and harvested sustainably ( [[#Churkina--2020|Churkina et al. 2020]] ). Since future urban growth and the construction of timber cities may lead to increased timber demand in regions with low forest cover, it is necessary to systematically analyse timber demand, supply, trade, and potential competition for agricultural land in different regions ( [[#Pomponi--2020|Pomponi et al. 2020]] ). The widespread adoption of biomass-based urban construction materials and techniques will demand more robust forest and urban land governance and management policies, as well as internationally standardised carbon accounting methods to properly value and incentivise forest restoration, afforestation, and sustainable silviculture.

Expansion of agroforestry practices may help to reduce land-use conflicts between forestry and agriculture. Harvesting pressures on forests can be reduced through the reuse and recycling of wooden components from dismantled timber buildings. Potential synergies between the carbon sequestration capacity of forests and the associated carbon storage capacity of dense mid-rise cities built from engineered timber offer the opportunity to construct carbon sinks deployed at the scale of landscapes, sinks that are at least as durable as other buildings ( [[#Churkina--2020|Churkina et al. 2020]] ). Policies and practices promoting design for disassembly and material reuse will increase their durability.

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