An international team of researchers has used satellite images to map urban growth between 1985 and 2015. Each image captured 900 square meters of ground, a level of detail that allowed the scientists to study land-use change over time. In a paper in Nature Sustainability, they report that global urban extent has expanded by 9,687 km² per year—a rate four times greater than a previous estimate, suggesting an unprecedented rate of global urbanization.

Urban area dynamics during 1985–2015 at the global and continent scales. a, Global urban area dynamics over the past three decades. The data were produced using 30 m resolution Landsat data. The slope is derived from linear regression, with an uncertainty range at the 95^th confidence level. b, Slopes of urban area growth over continents. The bottom and upper bounds of the error bars indicate uncertainty ranges (95% confidence interval). Regressions over these continents are all significant with P values <0.01. Liu et al.

The rate of urban expansion is notably faster than that of population growth, the researchers noted, indicating that the urban land area already exceeds what is needed to sustain population growth.

Many global environmental problems are associated with cities, whose residents currently account for more than 55% of the world population. This percentage is projected to increase to 68% by 2050. Urban area composition is highly dynamic; however, our understanding of how cities change in space and over time is limited by the lack of spatially and temporally comprehensive urban land cover data at a high resolution.

Development of this information lags behind that of state-of-the-art non-urban landcover change data (for example, at 30m with an annual resolution showing global forest cover change). Detailed high-resolution maps of urban change are crucial for studying greenhouse gas emissions, urban heat island effects, population exposure to extreme weather events and air pollution, land-use change dynamics, ecosystem carbon losses, urban sustainability and development pathways. Such maps would also help urban planners make informed decisions regarding urban expansion in areas exposed to natural hazards, such as landslides, earthquakes, tsunamis, floods and disease.

The current understanding of urban growth is largely based on demographic (population) data rather than information describing the spatial and temporal patterns of urban land-cover change.

—Liu et al.

The researchers used Landsat imagery to map annual changes in urban extent at a 30 m resolution for the period 1985 to 2015 on the Google Earth Engine (GEE) platform. They first defined the extent of urbanized land between 1985 and 2015 by fusing four available global urban-extent maps with similar spatial resolutions for 1985 and 2015. They then extracted cells from the two fusion maps that changed from non-urban in 1985 to urban in 2015.

They then used an annual time series (1985– 2015) of the normalized urban areas composite index (NUACI) to detect the year of urbanization and green recovery (vegetative regrowth or new plantings in built environments) for each pixel within this urbanized extent. Finally, they assessed the accuracy of all derived products over the past three decades.

Among their findings:

• The global urban extent increased from 362,747 km² to 653,354 km² from 1985 to 2015, representing a net expansion of 80% and a significant average expansion rate of 9,687 km² per year (P value < 0.01).

• During the same period (1985–2015), data from the United Nations show that urban population, an essential driver of urban area expansion, increased by 52%.

• Thus, they concluded, much of the newly developed urban lands were not used for housing but for other purposes (e.g., commercial and industrial districts). This unbalanced growth of urban lands relative to population growth has been more pronounced in developing regions (for example, China and India).

• Approximately 69% of the newly developed urban areas are in Asia and North America, where urban areas increased by an average of 4,970 ± 319 km² yr^–1 and 2,358 ± 150 km² yr^–1, respectively. The expansion rates in Asia and North America substantially exceeded those of Europe (1,883 km² yr^–1).

• Urban growth rates were smaller in Africa, Australia and South America, averaging less than 1,000 km² yr^–1.

• The United States, China and India, the three countries with the most cities, experienced different urbanization trajectories. Urban expansion in China and India occurred mainly in some large cities (that is, those with more than 300,000 inhabitants) and their surrounding rural areas by encroaching upon agricultural lands. A much greater number of cities accounted for the bulk of US urban growth.

• Regionally, the dominant types of land converted to urban areas are as follows: (1) agricultural land in China, India, South Korea, Japan, Europe, Southeast Asia countries and the central United States; (2) forest land in northern Europe and the eastern United States; (3) other cover types in Saudi Arabia and the western United States.

Resources

• Liu, X., Huang, Y., Xu, X. et al. (2020) “High-spatiotemporal-resolution mapping of global urban change from 1985 to 2015.” Nat Sustain doi: 10.1038/s41893-020-0521-x