Structure and patterns of the urban carbon pools
Ecology and Biodiversity of Plants and Ecosystems
Final Report Abstract
Vegetation, soils, buildings, and sometimes landfills store carbon in cities. Urbanization can increase or decrease organic carbon storage of the land use type, which it replaced, depending on the local climate, intensity of urban heat island effect, air pollution level and type. Moreover, even within the same city, organic carbon can be accumulating in the some land use types, while being depleted in the others. In arid and semi-arid climate, urbanization increases the accumulation of soil carbon in urban parks and gardens relative to rural areas, which soils are carbon poor. Buildings and landfills are the new pools of carbon distinguishing cities from natural ecosystems. Nowadays, buildings are mostly designed with concrete and steel, which contain a very small amount of carbon. Steel contains 0.25% of inorganic carbon. Although carbon storage of concrete is increasing over time as a result of carbonation, its storage is negligible in comparison to wooden buildings. A five-story residential building structured in laminated timber can store ~186 kgC/m2 in the primary structure; that is more than in the aboveground biomass of the natural forest with the highest carbon density (52 kgC/m2 typical for the Coast Range ecoregion of North America). Fraction of wooden buildings in cities is country specific and historically was higher in countries with plentiful forest resources. We propose to exploit the projected demand for urban buildings to mitigate climate change. By substituting an emerging class of bio-based structural materials storing carbon and having low embodied emissions, we can create a durable, human-made global carbon pool while simultaneously reducing CO2 emissions. Wood-based construction for 2.3 billion new urban dwellers can accumulate 0.25-20 GtC and reduce cumulative emissions of greenhouse gases from material manufacturing from 0.7-20 GtC to 0.3-10 GtC between 2020 and 2050. The transition to bio-based materials will become a successful climate mitigation strategy only if harvested forests are managed sustainably and wood from demolished timber buildings is preserved on land in various forms. In comparison to other engineered carbon sinks, the option of storing carbon in buildings has obvious benefits. It takes advantage of evolving construction processes that will occur in any case, and serves as a substitute for mineral-based structural materials that cause high processing CO2 emissions. Mass timber is a cheaper and safer way to store carbon than pumping CO2 underground37, and is a more useful option than burying logs in trenches under a thick layer of soil38 for long-term storage or direct conversion of roundwood into biochar. Most notably, the carbon pools in timber cities have to be maintained and gradually enhanced along with forest carbon pools to ensure long term lock-in of carbon on land, which can be achieved by encouraging long life span of timber buildings, stimulating market for used wood and technologies allowing to convert used wood into other long living products as well as sustainable management of forests.
Publications
- (2019) Urban trees, air quality, and asthma: An interdisciplinary review. Landscape and Urban Planning 187, 47-5
Eisenman, T.S., Churkina, G., Jariwala, S.P., Kumar, P., Lovasi, G.S., Pataki, D.E., Weinberger, K.R., Whitlow, T.H.
(See online at https://doi.org/10.1016/j.landurbplan.2019.02.010)
