Embodied carbon is an urgent issue because the emissions we release in the next 20 to 30 years are critical to keeping global temperatures at tolerable levels.
For building professionals, reducing carbon emissions that has typically meant increasing energy efficiency and pushing for renewable energy production, thus reducing the amount of carbon generated by the fossil fuels used to operate buildings.
While this is crucial it is also vital to think about the greenhouse gases that are emitted to construct our buildings in the first place—the embodied carbon.
The manufacture of building materials makes up 11% of total global greenhouse gas emissions, according to the latest data from the United Nations Environment Programme.
That 11% might sound small compared with the impact of operational energy (28%), but for new construction, embodied carbon matters just as much as energy efficiency and renewables. That’s because the emissions we produce between now and 2050 will determine whether we meet the goals of the 2015 Paris climate accord and prevent the worst effects of climate change.
The first step is to identify carbon “hot spots”—materials or systems that contribute the most to a building’s embodied greenhouse gas emissions.
The only way to get a really clear picture of how one material or system compares to another in the context of a building project is to use whole-building life-cycle assessment, or WBLCA. This process looks at multiple impacts of building materials, including global warming potential, over their entire life cycle—from extraction and manufacturing through the landfill or recycling plant.
The Carbon Leadership Forum, a network of experts on the carbon impacts of the building industry, has developed an LCA practice guide aimed at building professionals. Makers of WBLCA software tools also offer trainings to help users navigate the software and interpret results.
Not every project has a budget for a full-scale whole-building life-cycle assessment (although many firms are doing more limited LCA work on projects on their own time). Luckily, there are takeaways from this process that project teams can apply to their everyday work without additional expense or, in some cases, even without client buy-in or knowledge needed.
One of the most important takeaways from whole-building LCA is that structural systems almost always comprise the largest source of embodied carbon in the building—up to 80%, depending on the building type. So the first goal when looking to reduce the embodied carbon of a project is to target the structural system. Concrete, steel, and wood can all be optimized in different ways to reduce impacts.
If things don’t change with how we treat embodied carbon, impacts will total 90% of the carbon released from newly constructed buildings between 2015 and 2050.
Concrete has a large footprint because of the carbon-emitting process used to make one of its most important ingredients—the binder portland cement. By some estimates, production of portland cement is responsible for 5% of total global CO2 emissions. Replacing some cement with supplemental cementitious materials (SCMs) like fly ash or blast-furnace slag is a go-to way for project teams to reduce the embodied carbon of the concrete in their projects.
Reducing cement content can take many forms, he said, including simply using less by specifying higher-quality aggregate or reducing water content.
For the new Mexico City Airport project, Arup conducted extensive life-cycle assessment studies to reduce embodied carbon (the project is pursuing LEED v4 certification). Although the team spent most of the analysis time on modeling the enclosure correctly, in the end, according to Arup’s Frances Yang, S.E., it was the concrete mixes as well as the efficiency of the unique structural steel design that helped cut the total embodied carbon of the planned building by 10% compared with a benchmark building. Embodied carbon reductions totaled 130 million kilograms of CO2equivalent, she said—which is like taking 28,000 cars off the road for a year.
By weight, steel has a much higher embodied carbon footprint than concrete does—with one ton of steel representing approximately a ton of greenhouse gas emissions. According to the World Steel Association, steel production is responsible for 6.6% of greenhouse gas emissions globally—more than portland cement (see Better Steel, Lower Impacts).
A ton of steel represents about a ton of greenhouse gas emissions. Concrete buildings use a lot steel for reinforcement; this can be 90%–100% recycled steel if choosing North American products. North American steel generally has a lower carbon footprint than steel from overseas.
In North America, the industry has mostly switched over to Electric Arc Furnace technology—the process used to recycled steel. This, along with a cleaner electrical grid, has resulted in a 36% reduction in the industry’s carbon footprint since 1990
Structural systems bear the bulk of the embodied carbon footprint of buildings, but the enclosure is also significant, representing up to 15% of the global warming impact of a typical commercial office building,
Because of aluminum’s high embodied carbon, curtainwall systems have very large impacts. They also have high operational impacts, so it’s best to minimize their use.
Cover photo: By replacing portland cement and using other carbon-reducing strategies, the team was able to cut the embodied carbon of the new Mexico City Airport project by 130 million kilograms.