Figure 2.1 Embodied and operational carbon emissions
A building’s carbon footprint over its lifespan is the sum of its embodied plus operational emissions.
Transitioning to low-carbon built environments requires the design of material strategies that have multiple benefits and that take a “whole life-cycle” approach, in line with the principles of a circular economy. To appreciate the value of such an approach, it is important to first understand how and where most of the greenhouse gas emissions from the built environment are generated. These emissions are broadly split across two categories: embodied emissions and operational emissions (see Figure 2.1). Understanding the difference is key to decarbonizing the built environment sector:
Embodied emissions are all the emissions associated with the construction (and deconstruction) of a building. They are generated during the extraction, manufacturing, transport and on-site construction of building materials (new buildings as well as renovations) and at “end-of-life” demolition, or, preferably, re-use for new buildings (GlobalABC and UNEP 2021).
Operational emissions are the emissions generated through the function and maintenance of the building. They are released while maintaining the building’s indoor “comfort levels,” including by heating, cooling, lighting and electrical appliances. The initial design choices for a building (such as the building materials used) as well as upgrading materials during renovations, have significant impacts on the amount of operational carbon and on opportunities for recycling.
Figure 2.2 Projected contributions from embodied and
operational carbon within the building sector
Under business as usual, embodied emissions will contribute nearly half of all building emissions by mid-century.
Within the total share of emissions from building and construction (37 per cent), the majority (11 per cent) are indirect operational emissions from residential buildings (see Figure 1.1). However, at least 6 per cent are embodied emissions from the most commonly used building materials: concrete, steel and aluminium.
In recent years, considerable attention has been focused on how to reduce operational carbon in the built environment, as it currently contributes the lion’s share of emissions from the sector (75 per cent) (see Figure 2.2). However, the share of embodied carbon of materials is projected to surge from 25 per cent to nearly half (49 per cent) by mid-century (OECD 2019). Meanwhile, the share of operational carbon will shrink as electricity grids increasingly transition to renewable energy and as building operations become more efficient (Architecture 2030 2022).
Figure 2.3 Embodied and operational carbon emissions over the building lifespan
Operational carbon will continue to decrease with grid decarbonisation, while embodied carbon is set to remain high without meaningful action.
Figure 2.3 illustrates how, over a building’s lifespan, annual emissions from operational carbon (blue bars) will continue to decrease as the grid is decarbonised by 2050. Meanwhile, embodied carbon (green bars) will remain high, if meaningful action is not taken to reduce it.
Figure 2.4 Impact of material selection on urban surface temperatures and the urban heat island effect
Building materials literally “change the climate” and are directly responsible for rising temperatures in urban areas.
The choice of construction materials impacts every aspect of a building’s life-cycle emissions. Material selection has a huge impact on operational emissions because of the way it affects energy demand. Material choices can literally “change the (micro)climate” by contributing to the urban heat island effect (Narumi, Levinson and Shimoda 2021) (see Figure 2.4). A material can either absorb the heat from the sun (as with concrete and brick), reflect solar heat gain (as with light-coloured surfaces) or transform solar energy (through on-site power generation and/or living materials such as green roofs).
The use of heat-absorbent materials such as concrete increases urban temperatures and the energy demand for cooling in buildings using mechanical air-conditioning (Davis and Gertler 2015; Deroubaix et al. 2021). Impervious surfaces such as concrete also cause excess water run-off and add to the carbon costs of pumping and treating stormwater. In certain climates, when designed properly, high-mass materials within buildings could support passive thermal effects and reduce requirements for heating and/or cooling (Pérez-Lombard, Ortiz and Pout 2008).
Given the huge impacts that building materials such as concrete have on both embodied and operational energy, the management of building material processes accounts for nearly one-fifth of global embodied carbon emissions, across the entire life cycle (OECD 2019).