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Academic Corner: Carbon sequestration of green roofs

by Matt Downs

In his latest article, Dr Tom Young of STRI Group and GRO Board Member, gives an overview of the carbon story of green roofs, and explains why care must be taken when just looking at the carbon footprint of green roofs as a measure of their success.

Dr Tom Young.

Green roofs are seen as, and quite rightly so, a key tool in mitigating the effects of climate change and improving liveability of urban areas. The carbon footprint of a development is a key metric in determining its sustainability credentials. As living systems, green roofs have the ability to store carbon in the long term. However, care must be taken when just looking at carbon footprint as a measure of success, as many other sustainability metrics should be considered to provide a full picture of environmental benefits. In this article I will give an overview of the carbon story of green roofs, and the key subjects that should be considered.

Carbon sequestration
As it grows, vegetation absorbs carbon dioxide from the atmosphere and stores it as carbon within plant tissue. As plants continue to grow this carbon is deposited in roots, leaves and stems. During the lifespan of plants, some of this carbon will be deposited or left in the soil/substrate. Once here, it will be decomposed and turned into humus aka soil, thus providing a longer-term carbon store. As vegetation grows throughout multiple years, this store of carbon in the soil/substrate will continue to increase and provide a net sequestration of carbon. However, the story isn’t quite as simple as this, as soils naturally emit carbon through the respiration of the microbial and fungal community that live within it. Therefore, to gain an accurate picture of the carbon balance of a green roof, long-term studies of carbon balances are needed.

Some early work on this subject focused on how much carbon was stored in above and below ground biomass. In two years of growth on a Sedum roof, approximately 0.38 kg C m-2 were sequestered as above and below ground biomass, as well as substrate organic matter (Getter et al. 2009). This was expanded by Whittinghill et al. 2014 who showed that more complex vegetation and deeper substrate can increase sequestration between 4.67-65.25 kg C m-2 over three years. Grass-dominated green roofs with irrigation have been shown to sequester 2.5 kg CO2 m-2 year (Kuronuma et al. 2018). Therefore, there is not a simple number when it comes to predicting how much carbon a green roof can sequester. 

Key items to consider when predicting carbon sequestration potential are;

• Vegetation type (woody vs non wood, fast vs slow growing)

• Substrate composition

• Substrate depth

• Irrigation

• Soil/substrate microbial community and soil respiration rates

• Climate

Carbon lifecycle 
It is also important to understand the initial carbon footprint of a green roof, as with all construction, the materials used to construct the green roof have a carbon cost (Bianchini & Hewage 2012). A green roof can only be considered to be a net carbon sequester once it has stored the same amount of carbon as it took to construct it in the first place (Kuronuma et al. 2018). Therefore, it is important to assess the different components of a green roof and reduce their carbon footprint if possible. Key components of a green roof system which are heavy contributors to its carbon footprint include;

• Substrate (production of artificial lightweight aggregates, transport to site)

• Drainage and filter layers (production of plastic materials)

• Maintenance (use of petrol driven tools, irrigation systems, water, pumps)

• Plants (pre-grown vegetation or seed will have required resources to produce)

Carbon savings 
It is also very important to consider the wider carbon savings on the host building and wider environment in order to calculate accurate carbon balances. Due to the many ecosystem services green roofs provide, carbon reductions are observed on multiple levels: 

• Insulation (heat and cooling) and reduced air condition use and energy use through cooling (up to 1.7-1.9 kg CO2 m2 year-2 Kuronuma et al. 2018)

• Stormwater reduction – reducing loads on combined sewage systems, thus reducing amount of water treated by water companies.

• Roof replacement – green roofs significantly improve a roof’s lifespan, reducing the amount of construction materials used during the lifespan of a building as the roof does not need to be replaced as often.

These, in addition to carbon directly absorbed by the roof, allow a green roof to generally save/absorb more carbon than was used to construct it after around 5-20 years (Kuronuma et al. 2018), and thereafter becoming a net carbon sequester.

In conclusion
The carbon balance and footprint of a green roof is a complex calculation and requires multiple inputs of data. Generally, green roofs become net carbon sequesters over their lifetime. However, the period over which this takes depends on the construction of the green roof, how it grows and where it is situated. It is also important to remember that not all green roof benefits must be directly related to carbon. For example, green roofs provide multiple ecosystem services such as air pollution absorption, improved and increased recreational space, which all still significantly benefit urban areas and improve their liveability, and thus are just as important to consider.

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