When talking about climate change, conversations typically focus on our emissions. In the built environment, we talk about embedded emissions from materials and the construction process, and we also talk about emissions from building operations.
However, if we go back to the first principles of climate change, the only metric that matters is the greenhouse gas concentrations in our atmosphere. At the time of writing, the concentration of CO2 in our atmosphere was 419.05 parts per million.
This metric is the net result of two other metrics:
- the amount of CO2 emitted into the atmosphere from human activity,
- the amount of CO2 removed from the atmosphere via a process known as carbon sequestration.
Given that we are in the middle of a climate crisis, I believe we must fully understand both of these two metrics and how our work in the built environment can contribute to both. This article will focus on the latter point: carbon removal from the atmosphere within a built environment context.
We will explore the different types of carbon sequestration and how our built environments can be adapted to play an essential part in carbon removal from our atmosphere.
What is carbon sequestration?
Carbon sequestration is the process of removing carbon atmospheric carbon dioxide from the air or preventing it from being emitted into the atmosphere in the first place. The carbon dioxide is then stored in a medium; which medium depends on the type of carbon sequestration.
Three main types will be discussed at length in this article: biological, geological, and technological carbon sequestration.
In biological sequestration, carbon is stored in oceans, soils, and vegetation (especially forests).
In geological sequestration, carbon is stored in geologic formations.
Carbon sequestration is highly related to carbon capture and storage (CCS), and although there are subtle differences in these two terms, we will use them interchangeably here.
Types of Carbon Sequestration (and carbon capture)
- Biological Carbon Sequestration:
- Geological Carbon Sequestration.
- Technological Carbon Sequestration.
In the following section, we will explain each of these three main methods for carbon sequestration.
Biological carbon sequestration by organic matter
In biological sequestration, plants, trees, soil, peat, marshlands, aquatic ecosystems (any organic matter) are used to draw in carbon dioxide through photosynthesis. This includes soil carbon sequestration, forest carbon sequestration. This process of photosynthesis takes carbon dioxide as input, uses the CO2 to grow, and emits clean oxygen as a result.
The soil carbon stock (i.e., how much carbon is currently stored in soils around the world) makes up a substantial proportion of our total carbon stocks in all our terrestrial ecosystems (nearly 80%, or 3170 giga tons of carbon). The removal of carbon from the air and storage of carbon in soils depends on soil fertility and soil health. The primary way carbon is stored in soil is soil organic matter (SOM).
Biological sequestration is the primary method for so-called carbon offsets', whereby an individual or company will pay an intermediary to 'offset' the carbon emissions that they have produced (or will produce in the future). Typically a tree or forest will be used as a carbon sink.
Biological sequestration is an efficient sequestration method but can take longer and can require large portions of land.
Geological carbon sequestration
In geological sequestration, carbon dioxide is typically captured from a source of carbon (i.e., a power plant or from industrial processes whereby fossil fuels are burnt) and drilled deep underground into geological rock formations under the Earth's surface. The geologic formations consist of porous rocks that trap CO2. An upper layer of rock overlying these formations is impermeable and non-porous so that the CO2 cannot escape.
This is instead of the carbon source emitting greenhouse gases directly into the atmosphere. Deep underground, the carbon dioxide does not contribute to carbon dioxide concentrations in the atmosphere.
The roundtrip in the carbon cycle from extraction (from geological rock formations), to burning, and to storage of carbon in geological rock formations is not 100% efficient. So the best course of action is to minimize the amounts of carbon emitted into the atmosphere by leaving fossil fuels in the ground - not burning them in the first place.
Technological carbon sequestration
Finally, there is a third method: technological carbon sequestration. In this high-technology-based solution, the air is 'sucked' out of the air via a technique such as Direct Air Capture (DAC). As the name suggests, in DAC, an industrial scale hoover is used to suck air from the atmosphere. The technology inside the machine cleans the air, stores the carbon, and emits clean air.
There are limitations to the amount of carbon dioxide emissions that this process can currently sequester (also known as its carbon sequestration capacity) due to the technical challenge.
Currently, this technology is costly (over $500 per tonne of CO2 removed). Still, research is underway worldwide to improve efficiency, overcome the technical challenge and improve the cost/ benefit ratio.
Carbon sequestration in the built environment
Now, with a greater understanding of the concept of sequestration of carbon, we can begin to look for ways in which our built environment can be adapted to sequester more carbon and therefore improve into carbon footprint.
In the following section, I will explore three ways in particular, but of course, this list could be endless. Feel free to message me with your other ideas for carbon sequestration in the built environment.
Carbon sequestration and timber buildings
In biological sequestration, we discovered that forests, living trees, and other natural species can sequester carbon through the process of carbon dioxide. Therefore, a logical place to start talking about sequestration in the built environment would be timber buildings.
Timber buildings are an incredibly sustainable method of building (compared with steel and concrete) as the carbon used to create them is already part of the natural carbon cycle; we are not added extra carbon to the atmosphere.
Talking about carbon sequestration specifically, trees sequester carbon when they are living (through photosynthesis). Once you chop down a tree to be used in construction, this process stops (and even reverses a little). So the idea that timber buildings suck carbon from the air once constructed is not entirely accurate.
Carbon sequestration and concrete
Concrete is a massive contributor to climate change in general, not just in the built environment, and as such needs to be seriously considered in any climate change mitigation strategy. We need to act fast to remove this material (in its current form) from our supply chains. But this will be tricky because, globally, we use so much of it! However, innovation is happening, and I want to highlight two specific methods for sequestering carbon with carbon. Firstly is the capture of carbon dioxide from the concrete production process. This carbon can then be sequestered either geologically (as discussed above) or using innovative other techniques such as injection into the concrete once it is poured. This process is being perfected by startup Carbon Cure (backed by Bill Gates). Another way we can sequester carbon from our existing stock of concrete (and there is a lot of it) is to play on a characteristic of concrete. A slight breaking down of the outer layer of concrete forces a reaction with the air. Carbon dioxide is drawn in from the air via carbonation. This is an idea that could be used at scale in the industry but has not yet been fully explored (to my knowledge).
Carbon sequestration and soil/ landscape design
Finally, our built environments can be designed and adapted to include much more nature-based solutions and sustainable land management practices to maximize the potential for biological carbon sequestration and minimize climate impact. This means not just more plants, but thinking deeply about how soil is managed throughout the construction process, and how your elements of your design can be used for long-term storage of sources of carbon.
Construction activity typically involves the movement, or removal of soil, but keeping soil in place has profound effects on the soil carbon sequestration potential of your land. It must be included in any strategy to reduce greenhouse gas emissions.
Final notes (and a warning)
While the potential to increase the amount of carbon that our built environments sequester is huge, just focusing on sequestration as a way of balancing continued fossil fuel use is not an option. Carbon removal is not a replacement for stopping the use of fossil fuel-based materials and associated human activities; we must do both. We are at a point in the climate crisis whereby every possible intervention must be brought online rapidly to minimize the destruction of our planet, maximize food security, and prevent the collapse of our societies. This includes keeping as much fossil fuels in the ground, shifting our reliance on fossil fuels based materials in the built environment, and thinking deeply about how we can optimize our built environments to sequester as much carbon dioxide as possible.