Director Callum Hill SBE0021
Written evidence from Callum Hill, Director, JCH Industrial Ecology Ltd
Decarbonisation and the built environment
Callum Hill JCH Industrial Ecology Ltd
According to the World Green Building Council (WGBC), the built environment is responsible for nearly 40% of global carbon emissions, 70% of which comes from the operational carbon and 30% from the embodied carbon of construction materials (up-front carbon). To keep the global temperature increase below 2oC, all sectors of the economy must rapidly decarbonise. The vision of the WGBC is that by 2030 all new buildings, infrastructure and renovations will have at least 40% less embodied carbon and that by 2050 the embodied carbon should be net zero. These are ambitious targets.
Embodied carbon is the total of the greenhouse gas emissions that are associated with the extraction, processing, manufacture and transport of a product from the point of the raw resource in nature to the product leaving the factory gate. This is known as cradle to gate (modules A1 to A3, BS EN 15804). In some cases, the transport to the building site and the emissions associated with installation are also included. When examining the full impact of different materials used in construction maintenance, replacement, disposal, re-use, plus the operating carbon emissions associated with building use must all be included. This article will concentrate on the up-front carbon.
Because different greenhouse gases have different impacts, the total emissions are converted into a common factor, known as the global warming potential (GWP), which is reported in carbon dioxide (CO2) equivalents (e.g., kg CO2e). Sometimes, these are direct emissions; for example, the release of fossil carbon as CO2 during the conversion of limestone to clinker in cement manufacture. Quite often, these emissions are indirect, such as the release of CO2 by power stations in the generation of electricity. Is it really possible to get all these emissions down to zero?
Although emissions, such as those associated with electricity, are decreasing as the grid decarbonises; others, such as transport and process emissions are more difficult to reduce. Indeed, some industries will not be able to achieve net zero without disuptive infrastructural changes and the use of carbon capture and storage.
The use of timber confers significant advantages in this respect because it is associated with lower ‘up-front’ emissions, but additionally – a timber building can act as a carbon store during its life. This is illustrated in Figure 1, where examples (from the UK Climate Change Committee study) for timber-rich and conventional buildings are given. All buildings contain some timber, which is why conventional constructions store some atmospheric carbon. By choosing timber-rich construction it is possible to store more carbon in the building, even more than the up-front carbon emissions.
Figure 1: The GHG emissions and stored atmospheric carbon associated with conventional building types and timber-rich buildings.
However, at the end of life of the building, the timber may be disposed of by incineration, thereby releasing the stored carbon back to the atmosphere. This has prompted some industries to claim that carbon storage should be ignored and depending on the assumptions used can actually make timber appear to be worse than other materials. However, carbon storage undoubtedly has a mitigation benefit and this can be illustrated by using the stocks and flows approach.
Atmospheric CO2 is sequestered by plants during photosynthesis and remains stored until the carbon is oxidised at the end of life. The stored atmospheric carbon in the harvested wood products made from the timber can be used as a way of mitigating climate change, but this crucially depends on the length of time that the carbon is stored.
The Intergovernmental Panel on Climate Change approach to reporting the carbon storage benefits of using biogenic materials in the built environment is to calculate the carbon storage in a product pool, which is an economic approach to the problem. If the harvested wood products (HWP) pool is considered from a carbon storage point of view, there is a flow of biogenic carbon into the pool from the forest and there is a flow out of the pool as the timber is oxidised and the carbon is returned to the atmosphere. An increase in the HWP pool therefore provides a benefit during the time that the size of the pool is increasing; thereafter the pool will be in equilibrium with the environment. In other words, there is a mitigation benefit for as long as the carbon flow into the pool exceeds the carbon flow out of the pool (Figure 2).
Figure 2: Stocks and flows of carbon. If the outflow exceeds the inflow, the HWP pool will shrink (a); if the flows of carbon into and out of the HWP pool is equal, the pool is stable (b); and if the inflow exceeds the outflow, the HWP pool grows (c). In (b), the HWP pool is in equilibrium with the atmosphere and in (c) the HWP pool is acting as a carbon sink.
The most common way of modelling this carbon flow is to use national data for measuring the inflows into the pool and a mathematical function to represent the outflow. An example is shown in Figure 3, where it can be seen that the amount of carbon stored increases for a considerable period of time.
Figure 3: The carbon storage potential of timber in the built environment - the longer the lifetime, the greater the storage.
The amount of carbon stored is related to the lifetime of the HWP in the pool and so is the time to equilibrium. Until equilibrium is reached, the carbon stored is increasing and the HWP pool as therefore acting as a carbon sink. The longer the product lifetime, the more extended the mitigation period. The challenge now is to find a way to show the carbon storage benefit at a product and building level to incentivise the correct behaviour.
Professor Callum Hill
JCH Industrial Ecology Ltd
 BS EN 15804:2012+A2:2019 Sustainability of construction works. Environmental product declarations. Core rules for the product category of construction products, BSI
 Spear, M., Hill, C., Norton, A., Price, C. (2019) Wood in Construction in the UK: An Analysis of the Carbon Abatement Potential. Report BC-1383-2018-ES, The BioComposites Centre, Bangor University, Bangor, LL57 2UW, UK.