Most objects in a city contain carbon; the material is stored in vegetation, soils, buildings, furniture, books, landfills, people and pets. Carbon is stored in organic form in living biomass such as trees, grasses or in artifacts derived from biomass, such as wooden furniture, building structures, paper, clothes and shoes made from natural materials. Inorganic carbon or fossil carbon, meanwhile, is primarily stored in objects fabricated by people, such as concrete, plastic, asphalt and bricks.
The key difference between organic and inorganic forms of carbon is in how they return to the gaseous state. Organic carbon can be returned to the atmosphere without applying additional artificial energy through decomposition of organic matter, whereas energy input, such as burning, is needed to release inorganic carbon.
Densely populated and built-up cities store most carbon in buildings. A sparsely populated city, in contrast, stores most carbon in natural pools, such as vegetation and soils. Depending on what the land was used for before urbanization, the soils underneath the city's buildings or roads may be carbon-rich (if the city replaced forest or productive grassland) or carbon-poor (if the city was built in a desert).
Under impervious surfaces organic carbon does not decompose because of the lack of oxygen. So the centres of old towns often contain a deep layer of organic soil, which may be up to several metres deep. In contrast to these sealed soils, soils in vegetated urban areas can either accumulate or release carbon depending on the climate and their management regime. In a desert city, soils in urban parks and gardens tend to accumulate carbon because parks are usually watered and fertilized.
In a city that replaced forest, soils in green spaces are likely to lose carbon because the natural carbon cycle has been interrupted. In the autumn, leaves are probably collected and removed from the park. As a result the soil would receive less carbon from decomposition of leaf litter, soil temperature would increase because of thin or missing litter buffer and, in the absence of irrigation, soil moisture would decrease.
Buildings in a densely populated city are mostly made of concrete and so store most carbon in inorganic form. Use of wood in residential houses depends on a country's cultural traditions, wood prices, fire regulations and climate. Residential houses in North America have more carbon per unit of house area than houses in Germany or France, probably because of differences in wood prices, fire regulations and associated insurance costs. Within the US, residential houses in the north have more wood per house unit area than houses in the south.
Nature versus urban
So how does a city compare with a more natural habitat? Cities have more diverse carbon pools than natural ecosystems and these pools are controlled by humans (Churkina, 2008). Natural ecosystems store carbon in two major pools, such as soil and vegetation, and they are self-maintaining. This natural carbon storage depends on the rates of carbon uptake by plants and carbon removal/release through plant respiration, soil decomposition, fire and harvesting. Carbon uptake and release in natural ecosystems depend on sunlight for energy as well as on other drivers, such as rainfall and air temperature. An ecosystem accumulates carbon if the rate of uptake is larger than the rate of removal/release. As well as storing carbon in soil and vegetation, cities also incorporate the material in human-made objects, such as buildings, books and furniture. Because of the larger number of carbon pools, the carbon density – that is, the amount of organic carbon stored per unit area – of urban areas is higher (23–42 kgC/m2) than the carbon density of tropical forests (4–25 kgC/m2) (Churkina et al., 2009).
Storing more
Cities can be designed to store more carbon in both organic and inorganic forms. On the organic side, planting more trees will increase carbon uptake as well as carbon storage in the short term. Domestic gardening, using water and fertilizers, promotes plant productivity and carbon cycling. A mowed lawn, for example, often provides greater storage of organic carbon than the natural cover it replaces. But cutting and pruning plants and removing plant litter extracts nutrients from the system. Recycling of plant litter and especially grass clippings in cities should be encouraged. This recycling would return nutrients from litter to ecosystem and reduce fertilizer use and associated emissions of greenhouse gases. For instance less fertilizer is needed if grass clippings are left to decompose on the turf surface rather than composted or bagged and sent to landfill.
"Anybody can increase storage of organic carbon in their house by buying wooden furniture instead of plastic and using natural materials like wool, cotton and silk for interior design." Galina Churkina
In densely built-up cities, additional carbon storage in buildings would be an option. Using more wood in house construction and in furniture could be an important means of increasing organic carbon storage. Anybody can increase storage of organic carbon in their house by buying wooden furniture instead of plastic and using natural materials like wool, cotton and silk for interior design. Using carbon-rich construction materials and furniture with a long lifetime would increase inorganic carbon storage in cities.
Any option for increasing carbon storage in cities should be assessed together with its associated carbon dioxide emissions and the additional benefits or issues it may bring to urban dwellers. Use of wood in buildings, instead of brick, aluminum, steel and concrete, can increase carbon storage in human settlements and reduce emissions of greenhouse gases related to the construction and lifecycle of buildings. Production of bricks and concrete is much more energy intensive than fabrication of wooden construction materials, and it is accompanied by high carbon dioxide emissions from fossil-fuel burning. In addition, wooden houses stay cooler inside than concrete buildings in summer and so require less energy for cooling. That said, any increase in wood use in building construction would have implications for its production. Rising demand for wood must be accompanied by an increase in the area of forest under management for long-term sustainable timber production.
Wooden houses are more prone to fire than those made of brick or concrete, which means that potential carbon dioxide emissions from burning are an issue in settlements where wooden houses dominate.
Urban cool
In addition to offering carbon storage and uptake, city trees cool surfaces by up to 10 °C through shading and transpiration of water. This reduces energy use in summer and carbon dioxide emissions from energy production for air conditioning and fans.
Although cities with large parks and open green space have the potential to uptake and store more carbon dioxide as well as to reduce the urban "heat island" effect in summer, they probably create larger distances for people to travel to work and to shop. Unless there is a well developed and efficient public transport system, longer daily travel distances lead to higher carbon emissions from road transportation.
Maintenance of urban vegetation is associated with higher carbon emissions than natural vegetation due to the use of fuel-driven machinery, such as lawnmowers and petroleum-based fertilizers. Carbon dioxide is also emitted during fertilizer production and by the generation of energy necessary for sprinklers. Carbon will accumulate in urban gardens and parks if they are fertilized or irrigated. Use of nitrogen fertilizer is associated with emissions of N2O, another powerful greenhouse gas. As a result, carbon uptake and accumulation in a park may be accompanied by high N2O emissions and an overall increase in greenhouse gas emissions. Although vegetated surfaces filter water better than bare or impervious ones, if large green areas are irrigated they may increase water use in summer, especially if grown in arid or semi-arid areas.
Cities can be designed to store carbon for long periods of time. The question is whether storage of the organic or inorganic form is more beneficial. We need further research to quantify the benefits and disadvantages of each form of carbon storage.
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