Metropolis: Multiplied
Global analysis tracks dramatic growth of cities
Cities are creeping into some of the world’s most biologically diverse ecosystems, concludes a new analysis that takes a first crack at estimating how fast the world’s urban areas have grown over the last few decades – and how much they might grow in the future.
“The conversion of Earth’s land surface to urban uses is one of the most irreversible human impacts on the global biosphere,” a team led by Karen Seto of Yale University in New Haven, Connecticut writes in PLoS ONE. Although cities can play a positive role in boosting economies and making more efficient use of resources, urban growth also “drives the loss of farmland, affects local climate, fragments habitats, and threatens biodiversity.”
To get a clearer picture of just how the world’s cities are growing, the researchers scoured journals for studies that used satellite imagery to chart urban land conversion. In the end, their pile held 326 studies that covered nearly 300 urban areas.
After combining the data, they estimated that urban land area increased by 58,000 square kilometers from 1970 to 2000 – an area nearly the size of Mongolia. India, China, and Africa had the highest rates of urban land expansion, while North America had the largest overall change in total urban extent. By 2030, the researchers estimate that global urban land cover will increase by 430,000 to 12.6 million square kilometers, with growth most likely to be about 1.5 million square kilometers.
The team also looked at a variety of factors – from population growth to economic gains – that might be causing cities to get bigger. Overall, they found that cities are sprawling outwards faster than population, “suggesting that urban growth is becoming more expansive than compact.” In China, economic growth appears to account for about one-half of urban expansion, “but only moderately affects urban expansion in India and Africa, where urban land expansion is driven more by urban population growth.” Many cities, however, are growing due to a complex array of factors, which are “difficult to observe comprehensively at the global level, including international capital flows, the informal economy, land use policy, and generalized transport costs.”
Among some other trends spotted by the study:
- “Urban land expansion is growing faster in low elevation coastal zones than in other areas. This is likely to put millions of people at risk to climate change impacts such as storm surges and sea level rise.”
- City expansion near protected areas is as high as in other regions. “This will challenge conservation strategies,” the researchers note.
“It is likely that these cities are going to be developed in places that are the most biologically diverse,” says Seto. “They’re going to be growing and expanding into forests, biological hotspots, savannas, coastlines—sensitive and vulnerable places.”
The study probably understates global urban growth, the researchers note. “Expansion has been far greater than what our analysis shows because we only looked at published studies that used satellite data,” notes Seto. Just 48 “of the most populated urban areas have been studied using satellite data, with findings in peer-reviewed journals,” she explains. “This means that we’re not tracking the physical expansion of more than half of the world’s largest cities.” – David Malakoff | August 23, 2011
Source: Seto KC, Fragkias M, Güneralp B, Reilly MK, 2011 A Meta-Analysis of Global Urban Land Expansion. PLoS ONE 6(8): e23777. doi:10.1371/journal.pone.0023777
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