Urban soil sealing in Europe

Soil sealing is the covering of the soil surface with materials like concrete and stone, as a result of new buildings, roads, parking places but also other public and private space. Depending on its degree, soil sealing reduces or most likely completely prevents natural soil functions and ecosystem services on the area concerned.

A diverse situation

EEA produced recently a high resolution soil sealing layer for the whole of Europe for the year 2006 based on the same satellite pictures as used for CORINE land cover data. An analysis of soil sealing in European cities shows that they perform very differently.  Furthermore, comparing their mean levels of soil sealing per built up area can differ much from comparing the values per inhabitant; it can even be reverse (figure 1).

Figure 1: Mean soil sealing in European capitals (UMZ – Urban Morphological Zone) and soil sealing per inhabitant

Such a contrasting pair of cities is Sofia and Helsinki with both having around 1 million inhabitants (map 1 and 2). Whereas Helsinki has a low overall soil sealing but higher values per inhabitant, Sofia shows the opposite pattern. Helsinki enables thus potentially more ecosystem functions within its city boundaries, although altered by human impacts. In contrary, Sofia with its low sealed area per inhabitant keeps the city still compact and can thus save space for other uses outside the city area.

Map 1 and 2: Degree of soil sealing (impermeability) in the UMZ of Sofia and Helsinki

See also the maps: Soil sealing and population density in the capitals of EEA countries

Why soil sealing matters – the example of the urban heat island effect

As mentioned above, soil sealing affect ecosystem services and the quality of life in a city in many ways.

One is the regulation of temperature. The example of Budapest (maps 3 and 4) shows well the effect of soil sealing on temperature. The green urban areas with no or very low soil sealing are much colder then the highly sealed built up areas.

Maps 3 and 4: Comparing the degree of soil sealing and the surface temperatures in Budapest, Hungary
sources of map on surface temperature: Richard Ongjerth, Péter Gábor, Sándor Jombach, 2007 and Péter Gábor, Sándor Jombach, Richard Ongjerth, 2008

Considering that the mean temperature in Europe rises and the number of heat waves is expected to increase, a high soil sealing will further exacerbate the already existent heat island effect of cities increasing their vulnerability against heat wave impacts. Map 5 shows the mean soil sealing of European cities underlaid by the projected increase in the number of tropical nights (T_min>20C). Cities with high levels of soil sealing in areas with a higher projected increase of the number of tropical nights are more vulnerable to climate change and need appropriate adaptation measures like boosting urban green also on walls and roof areas, white walls, building isolation, shadow blinds etc.

Map 5: degree of mean soil sealing in Europe's cities (UMZ) in 2006 and modelled change in number of tropical nights (T_min>20C) during summer between 1961-1990 and 2010-2040 indicating higher risks for heat waves

Another climate related effects is the reduction of the water infiltration potential of the soil, which increases the run-off of water and the risk of river flood. In the event of heavy rain falls, in cities with a high soil sealing, also the capacity of the sewage system might no longer be able to cope with the high run-off of water and cause surface flooding. Indeed, much of the flooding in England in the summer of 2007 was due to surface water (The Pitt Review, 2008).

Coping with soil sealing

From a European perspective, a low soil sealing per inhabitant is definitely of favour as it reduces our overall ecological footprint. Living in urban areas can provide such setting. Nevertheless, a high compactness and thus soil sealing of cities can also have negative impacts for the quality of life in them. Therefore, cities will need a smart urban design by reducing all soil sealing where not needed (parts of public places, parking lots, brownfields etc.) and using all potentials to maximise unsealed and green areas as well as further green elements like street trees, green walls and roofs while maintaining their compactness and urban density.

References

• The Pitt Review (2008). Learning Lessons from the 2007 floods. Cabinet Office, 22 Whitehall, London SW1A 2WH. 
• Gábor, P., Jombach, S. and Ongjerth, R. (2008). The relation between the biological activity and the land surface temperature in Budapest. URBIO 2008 Proceedings, 11 June 2008.