Latest Research on Urban Expansion : Mar 2022

Global forecasts of urban expansion to 2030 and direct impacts on biodiversity and carbon pools

Urban land-cover change threatens biodiversity and affects ecosystem productivity through loss of habitat, biomass, and carbon storage. However, despite projections that world urban populations will increase to nearly 5 billion by 2030, little is known about future locations, magnitudes, and rates of urban expansion. Here we develop spatially explicit probabilistic forecasts of global urban land-cover change and explore the direct impacts on biodiversity hotspots and tropical carbon biomass. If current trends in population density continue and all areas with high probabilities of urban expansion undergo change, then by 2030, urban land cover will increase by 1.2 million km2, nearly tripling the global urban land area circa 2000. This increase would result in considerable loss of habitats in key biodiversity hotspots, with the highest rates of forecasted urban growth to take place in regions that were relatively undisturbed by urban development in 2000: the Eastern Afromontane, the Guinean Forests of West Africa, and the Western Ghats and Sri Lanka hotspots. Within the pan-tropics, loss in vegetation biomass from areas with high probability of urban expansion is estimated to be 1.38 PgC (0.05 PgC yr−1), equal to ∼5% of emissions from tropical deforestation and land-use change. Although urbanization is often considered a local issue, the aggregate global impacts of projected urban expansion will require significant policy changes to affect future growth trajectories to minimize global biodiversity and vegetation carbon losses.[1]


Urban mobility and urban form: the social and environmental costs of different patterns of urban expansion

The question of the environmental or social costs of urban form is increasingly attracting attention in spatial policy, but scientific debate in this field is often marred by prejudices and abstract visions; empirical analyses are very rare. The present study aims at establishing, in the metropolitan area of Milan, whether different patterns of urban expansion could be associated with specific environmental costs—in particular, for land consumption and mobility generation. Different typologies of urban expansion were defined, and an impact index weighting differently journey-to-work trips with reference to mode and time length was built at the municipality level. The statistical analysis confirmed the expected “wasteful” character of sprawling development patterns in terms of land consumption, though suggesting that recent urban development is becoming relatively ‘virtuous’ with respect to the past. With reference to the mobility generated, higher environmental impacts were proved to be associated with low densities, sprawling development, more recent urbanisation processes and residential specialisation of the single municipalities. Public transport seems to be strongly influenced, both in terms of efficiency and competitiveness, by the structural organisation of an urban area: the more dispersed and less structured the development, the lower its level of efficiency and competitiveness and consequently its share of the mobility market. On the contrary, trip times for private transport appear to be correlated not so much to urban dimension or density as to the presence of recent housing development, indicating the emergence of new models of lifestyle and mobility which are very different from those of the past.[2]


The dimensions of global urban expansion: Estimates and projections for all countries, 2000–2050

Our study of the expansion of a representative sample of 30 cities showed that 28 of them expanded more than 16-fold during the twentieth century. More generally, cities are now expanding at twice their population growth rates, on average, and now cover almost 0.5% of the planet’s land area. We created a new dataset comprising the universe of all 3646 named metropolitan agglomerations and cities that had populations in excess of 100,000 in the year 2000, their populations in that year, and their built-up area identified in the Mod500 map, currently the best of eight satellite-based global maps of urban land cover. Using this dataset, we estimated urban land cover in smaller cities and towns in all countries and calculated total urban land cover in every country in the year 2000. We then employed multiple regression models that could explain more than 90% of the variations in our urban land cover estimates amongst countries. Then, using U.N. urban population projections in combination with three realistic density change scenarios based on our previous global and historical study of densities, we projected urban land cover in every country and world region from 2000 to 2050. According to our medium projection, urban land cover in developing countries will increase from 300,000 km2 in 2000 to 770,000 km2 in 2030 and to 1,200,000 km2 in 2050. Containing this expansion is likely to fail. Minimal preparations for accommodating it – realistic projection of urban land needs, the extension of metropolitan boundaries, acquiring the rights-of-way for an arterial road grid that can carry infrastructure and public transport, and the selective protection of open space from incursion by formal and informal land development – are now in order.[3]


Direct and indirect loss of natural area from urban expansion

Global losses of natural area are primarily attributed to cropland expansion, whereas the role of urban expansion is considered minor. However, urban expansion can induce cropland displacement, potentially leading to a loss of forest elsewhere. The extent of this effect is unknown. This study shows that indirect forest losses, through cropland displacement, far exceed direct losses from urban expansion. On a global scale, urban land increased from 33.2 to 71.3 million hectares (Mha) between 1992 and 2015, leading to a direct loss of 3.3 Mha of forest and an indirect loss of 17.8 to 32.4 Mha. In addition, this urban expansion led to a direct loss of 4.6 Mha of shrubland and an indirect loss of 7.0 to 17.4 Mha. Guiding urban development towards more sustainable trajectories can thus help preserve forest and other natural area at a global scale.[4]


Complexity and performance of urban expansion models

Urban expansion and spatial patterns of urban land have a large effect on many socioeconomic and environmental processes. A wide variety of modelling approaches has been introduced to predict and simulate future urban development. These models are often based on the interpretation of various determining factors that are used to create a probability map. The main objective of this paper is to evaluate the performance of different modelling approaches for simulating spatial patterns of urban expansion in Flanders and Brussels in the period 1988–2000. Hereto, a set of urban expansion models with increasing complexity was developed based on: (i) logistic regression equations taking various numbers of determining variables into account, (ii) CA transition rules and (iii) hybrid procedures, combining both approaches. The outcome of each model was validated in order to assess the predictive value of the three modelling approaches and of the different determining variables that were used in the logistic regression models. The results show that a hybrid model structure, integrating (static) determining factors (distance to the main roads, distance to the largest cities, employment potential, slope and zoning status of the land) and (dynamic) neighbourhood interactions produces the most accurate probability map. The study, however, points out that it is not useful to make a statement on the validity of a model based on only one goodness-of-fit measure. When the model results are validated at multiple resolutions, the logistic regression model, which incorporates only two explanatory variables, outperforms both the CA-based model and the hybrid model.[5]


Reference

[1] Seto, K.C., Güneralp, B. and Hutyra, L.R., 2012. Global forecasts of urban expansion to 2030 and direct impacts on biodiversity and carbon pools. Proceedings of the National Academy of Sciences, 109(40), pp.16083-16088.

[2] Camagni, R., Gibelli, M.C. and Rigamonti, P., 2002. Urban mobility and urban form: the social and environmental costs of different patterns of urban expansion. Ecological economics, 40(2), pp.199-216.

[3] Angel, S., Parent, J., Civco, D.L., Blei, A. and Potere, D., 2011. The dimensions of global urban expansion: Estimates and projections for all countries, 2000–2050. Progress in Planning, 75(2), pp.53-107.

[4] van Vliet, J., 2019. Direct and indirect loss of natural area from urban expansion. Nature Sustainability, 2(8), pp.755-763.

[5] Poelmans, L. and Van Rompaey, A., 2010. Complexity and performance of urban expansion models. Computers, Environment and Urban Systems, 34(1), pp.17-27.

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