Expert Commentary

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

2011 study by Yale University, Texas A&M, and Boston University on the growing environmental impact of low-density urban and suburban sprawl.

According to the U.S. Census Bureau, the world’s population reached 7 billion in 2012, and is on track to add another billion by 2030. As the global population swells, so do cities: According to a United Nations report, more than half of the world’s residents now live in urban areas and by 2050 more than 70% will. And as both the number of people and the area of developed land on Earth increase, so does the potential for negative effects on the environment.

A 2012 study in the Proceedings of the National Academy of Sciences, “Global Forecasts of Urban Expansion to 2030 and Direct Impacts on Biodiversity and Carbon Pools,” looked at how the projected increase in urban area will affect the planet. The researchers, based at Yale University, Texas A&M and Boston University, begin the study by noting that traditionally cities were extremely dense and expanded slowly as their population increased. However, “Today, urban areas around the world are expanding on average twice as fast than their populations,” in part because of the spread of low-density urban sprawl, be it suburbs in developed nations or shantytowns in the developing world.

To better understand what the world of 2030 might look like if current population trends continue, the scholars used year 2000 data on global land cover and biodiversity, urban growth projections, and GDP forecasts to develop more than a thousand probabilistic forecasts. They focus on the negative environmental impact of urban sprawl on 34 biodiversity hotspots as well as the loss of natural carbon sequestration through deforestation.

The study’s findings include:

  • By 2030, approximately 5 billion of the world’s 8 billion residents — more than 60% — will live in urbanized areas.
  • Nearly 6 million square kilometers of land could be converted to urban areas by the same year. Of this, 1.2 million square kilometers (20%) have a greater than 75% chance of being converted to urban uses. “If all areas with high probability undergo urban land conversion, there will be a 185% increase in the global urban extent from circa 2000” — a near tripling.
  • The highest projected rate of urbanization is in Africa, up 590% from 2000 levels by 2030. “Expansion will be concentrated in five regions: The Nile River in Egypt, the coast of West Africa on the Gulf of Guinea, the northern shores of Lake Victoria in Kenya and Uganda and extending into Rwanda and Burundi, the Kano region in northern Nigeria, and greater Addis Ababa, Ethiopia.”
  • Almost half of the projected increase in global urban areas will occur in Asia. “In China, urban expansion is forecasted to create a 1,800-km coastal urban corridor from Hangzhou to Shenyang. In India, urban expansion is forecasted to be clustered around seven state capital cities.”
  • While the proportion of North Americans living in urban areas was already high in the year 2000 (78%), the amount of urban land cover is expected to nearly double by 2030.
  • “Less than 1% of all [biodiversity] hotspot areas were urbanized circa 2000. By 2030, new urban expansion will take up an additional 1.8% of all hotspot areas. Five biodiversity hotspots are forecasted to have the largest percentages of their areas to become urban: The Guinean forests of West Africa (7%), Japan (6%), the Caribbean Islands (4%), the Philippines (4%), and the Western Ghats and Sri Lanka (4%).”
  • “The highest rates of growth in urban area are forecasted to take place in regions that were relatively undisturbed by urban development circa 2000: The Eastern Afromontane, the Guinean Forests of West Africa, and the Western Ghats and Sri Lanka hotspots. Urban areas in these three hotspots are forecasted to increase by [approximately] 1,900%, 920%, and 900% over their 2000 levels, respectively.”
  • “If all areas with high probability become urban, the urban land cover in the biodiversity hotspots around the world will increase by more than 200%  between 2000 and 2030, with substantial variations in the rate and amount of increase across individual hotspots.”
  • In tropical regions with high probability of urban expansion, resulting biomass loss is estimated to be the equivalent of 1.38 billion tons of carbon by 2030. This is “equal to approximately 5% of emissions from tropical deforestation and land-use change.” In addition, “the soil carbon losses associated with expanding impervious surface cover have the potential to reduce soil carbon pools by 66%.”

The researchers state: “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.” In particular, policies could focus on increasing growth and density within current cities rather than urbanizing new areas.

Keywords: biodiversity, carbon, greenhouse gases, global warming, urbanization, density

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