Amount of vegetation indicates a city’s carbon dioxide emissions

The combined annual carbon dioxide emissions of urban areas within the EU are estimated to be four times the amount of carbon stored by vegetation in the area. This finding was presented by Finnish researchers, whose data indicates the direct impact of land use in cities on its carbon dioxide emissions.

Urban structure in Europe. The colours indicate the fraction of natural area. Fully built-up areas are marked with yellow, while the sparsely built areas are marked with red and natural areas with black.

Urban structure in Europe. The colours indicate the fraction of natural area. Fully built-up areas are marked with yellow, while the sparsely built areas are marked with red and natural areas with black.

The city centre of Helsinki emits nearly five kilograms of carbon per each square metre. In London the figure is twice as high. Moreover, the figures are manifold compared to the amount of carbon that Finnish coniferous forests are able to store, which is a few hundred grams a year.

“Built-up cities are a major source of carbon dioxide. The fraction of natural areas is small and there is very little greenery, so the vegetation that serves as a carbon sink does not have the capacity to compensate for the human carbon emissions,” says Annika Nordbo from the Department of Physics at the University of Helsinki.

Nordbo is a member of a research group which recently mapped out annual levels of net carbon exchange per square metre in a number of cities. For the study, the group applied direct meteorological measurements, taking into account all the emission sources within the range of an eddy-covariance system as well as the role of vegetation, which is often overlooked in inventory-based estimates.

“As a rule, emissions per unit area are the greater the denser the urban fabric,” says Nordbo.

The researchers identified a clear correlation between urban land use and emissions.

According to their findings, land use explains 84 per cent of the variation in annual carbon budgets between cities, although the cities included in the study were very different in terms of climate, structure and culture. In fact, the link between land use and urban carbon dioxide budgets allowed the researchers to determine the fraction of natural area with which a city would be carbon neutral.

“When the fraction of natural area is 80 per cent, the emissions of the city are offset by the carbon storage of vegetation,” says Nordbo.

“But such a city can hardly be called a city,” she says.

The researchers were able to show that the fraction of natural area can be used as a robust proxy for carbon dioxide budget using measurements from 14 stations around the world. The research material comprised land use data and direct carbon dioxide flux measurements. Based on that proxy and satellite mapping of the fraction of natural area, they were able to map annual carbon dioxide budgets. From these results, some 56 individual cities were extracted and their carbon dioxide budget based on the fraction of natural area was compared to inventory-based estimates.

The correlation between land use and carbon dioxide budget is so strong that the fraction of natural area could be used to successfully predict the results of inventory-based emission studies for the individual cities.

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Text: Minna Meriläinen-Tenhu
Photo: Nordbo et al., Geophysical Research Letters, Vol. 39, L20802, 5 PP., 2012, doi:10.1029/2012GL053087
31.10.2012
University of Helsinki, digital communications


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