Carbon capture, utilization, and storage (CCUS) in landscape architecture (part 1) - Types of carbon cycles, carbon storage lifespans, and life-cycle carbon assessments

By Chi-Ru Chang, Professor of Landscape Ecology, Department of Landscape Architecture, Chinese Culture University, Taiwan

The IPCC’s sixth assessment report continues to identify the vulnerability of human and natural systems to increasingly severe, interconnected and often irreversible impacts of climate change as an issue requiring immediate global attention.  Landscape Architects worldwide are responding to this challenge by actively discussing and finding ways to prepare cities and communities for resilience under climate change. As a creative discipline working with biological elements, however, Landscape Architecture has also the potential to actually reduce atmospheric carbon dioxide and take a more active role in alleviating the cause of climate change.

This article is the first of a series of four articles that will re-examine the multidimensional aspects of carbon cycles. In this first article, we shall discuss how carbon cycles determine carbon balance, carbon storage lifespan, and life-cycle carbon assessments. In the second and third articles, we shall discuss in more detail the factors that drive carbon release and carbon storage, and how we may creatively make use of carbon cycles for the benefit of humans. In each of the first three articles, we shall end with a list of corresponding ways Landscape Architects may contribute to a healthier carbon cycle. In the fourth and last installment, the IFLA APR climate-change working group will try to lay out a draft carbon-healthy framework for landscape planning, design, management, and what Landscape Associations can do. The effort will be by no means complete, but instead hope to inspire more discussions on how Landscape Architects can lead the way in solving the word’s problems by creating innovative solutions for carbon capture, utilization, and storage.

How carbon cycles determine carbon balance

The carbon cycle starts with carbon capture, in which atmospheric CO2 is taken out of the air and the carbon is transformed into organic material and thus fixed as non-gaseous form. Carbon is captured naturally through photosynthesis by terrestrial vascular plants (e.g. grasses, forbs, shrubs, trees, and vines) and non-vascular plants (e.g. mosses), aquatic plants, macro-algae (including kelps and other seaweeds), micro-algae, and other non-algae phytoplankton. CO2 may also be taken from the air using technology called “artificial photosynthesis,” where they are stored in either solvents or solids to produce usable material such as plastics or fuels.

After carbon is captured, it is stored in organic form until it is finally released back into the air as CO2. The concentration of atmospheric CO2 is a result of the global balance between this removal and addition of CO2 to the air.

• A carbon positive cycle is when the emission of CO2 is larger than the removal of CO2 from the air. This is the current state of the world where global atmospheric CO2 is continually increasing and causing world problems.
• A net-zero or neutral carbon cycle is when the CO2 emissions are balanced by the removals. This results in a non-increasing, but also non-decreasing atmospheric CO2 level. This may not stop the world’s problems, but may at least stop it from getting worse.
• A carbon negative cycle is when the emission of CO2 is smaller than the removal of CO2 from the air. This is what we need to aim for globally to decrease atmospheric CO2 and relieve the world from problems associated with climate change.

Carbon positive cycles (left), Net-zero or neutral carbon cycles (center),
and Carbon negative cycles (right)

Presumably, when all sites conform to a net-zero carbon standard, we may achieve a global net-zero carbon cycle. In reality, there will always be areas that have difficulty achieving this target. This is why we need certain areas to achieve carbon negative cycles locally to counter-balance the emissions from other landuse zones. The zero-carbon target set by many countries has activated carbon markets in which forest carbon offset is a common means to counter-balance carbon emissions. This can be an opportunity for Landscape Architects.

Carbon storage lifespan is linked to organism and organic material lifespans

Carbon stored in organic matter is released back into the air through burning or decomposition. The latter starts when various portions of living organisms are detached from the organism (e.g. shed leaves, detached twigs) or after the organism dies. Because different plant-forms have different life-spans, carbon is stored for different lengths of time. Carbon stored in an annual herb, for example, is released back into the air faster than carbon stored in a perennial herb, which is in turn faster than that in a perennial tree. Parts of a plant that is removed through pruning, clipping, or mowing also shortens its carbon storage lifespan.

Life-cycle carbon assessments

Life-cycle assessments evaluate the environmental impacts associated with all life-cycle stages of a product, process, or service. These life-cycle stages include the extraction of raw material, material processing, production, transport, distribution, use, maintenance, and disposal or recycling. For example, although turfgrass captures carbon through photosynthesis, it requires fertilization, mowing (often fuel-powered), and irrigation during maintenance. Given the little amount of carbon turfgrass captures, its short lifespan, and even shorter carbon-storage lifespan for mowed grasses, life-cycle carbon assessments of lawns have determined them as net carbon emitters, emitting over 1 kg of carbon per m2 per year.

Application by Landscape Architects

• Landscape Architects need to aim not only for net-zero carbon target for our sites, but rather negative carbon targets to help counter-balance the massive carbon emissions from human activities occurring in other landuse zones. The more carbon our sites capture and store, the higher the chance that costs associated with landscape projects and maintenance may be covered by carbon trade offsets.
• Short-lived organisms such as herbaceous plants store carbon for short periods of time and generally do not contribute to carbon sequestration. When such plantings are specified, they should at least form net-zero carbon cycles through continuous regrowth.
• When plants are trimmed or pruned regularly, they also return CO2 back to the air at a rate that is too fast to help mitigate global carbon balance. Lawns, in particular, are life-cycle carbon emitters because they retain too little carbon and only for too short periods of time to compensate for their carbon emissions through fertilizer use and maintenance-associated energy use. Landscape projects can help reduce carbon emissions by:

1. Specifying minimum-maintenance landscape management plans (LMP) that use minimum irrigation, mowing and pruning.
2. Identifying and specifying plants in designs that require minimal maintenance not only cuts down maintenance costs, but also stores more carbon.
3. Specifying areas that do not require as much mowing and pruning in the landscape design.
4. Identifying and specifying turfgrass that is suitable for your environment, reducing the need for fertilizers and irrigation.
5. Specifying the recycling of grass clippings to reduce the need for fertilization lowers the life-cycle carbon emission from fertilizer production.
6. Reducing the use of leaf blowers and fuel-powered lawn-mowers.

• Longer-living organisms like trees store carbon and keep it out of the atmosphere for longer periods of time, resulting in carbon sequestration. These plants are the ones that have true potential for balancing out atmospheric carbon emissions from human activities, and should be specified in designs wherever possible.
• To prepare for forthcoming carbon assessment needs, each climate region should start drawing up a list of how much carbon different types of trees sequester. To maintain biodiversity at the same time, this list should cover as many types of trees as possible to avoid the homogenization of planted trees as an unintended result of a short list.