Metadata

  • Author: Xiaoping Liu, Yinghuai Huang, Xiaocong Xu, Xuecao Li, Xia Li, Philippe Ciais, Peirong Lin, Kai Gong, Alan D. Ziegler, Anping Chen, Peng Gong, Jun Chen, Guohua Hu, Yimin Chen, Shaojian Wang, Qiusheng Wu, Kangning Huang, Lyndon Estes, Zhenzhong Zeng
  • Document Tags: data paper cities
  • URL: https://www.nature.com/articles/s41893-020-0521-x

There’s plenty of good stuff here. The thing that stuck with me was:

global urban extent has expanded by 9,687 km2 per year. This rate is four times greater than previous reputable estimates from worldwide individual cities, suggesting an unprecedented rate of global urbanization. The rate of urban expansion is notably faster than that of population growth

This one also struck a cord:

the global urban extent increased from 362,747 km2 to 653,354 km2 from 1985 to 2015, representing a net expansion of 80% […] 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, much of the newly developed urban lands were not used for housing but for other purposes (for example, commercial and industrial districts).

I’m not sure the last sentence follows from the previous ones. The economist in me probably thinks people consume more land per head (i.e., bigger houses, bigger plots, bigger cars that require bigger lanes). But, as they say, this is an empirical question…

Highlights

we map global annual urban dynamics from 1985 to 2015 at a 30 m resolution using numerous surface reflectance data from Landsat

global urban extent has expanded by 9,687 km2 per year. This rate is four times greater than previous reputable estimates from worldwide individual cities, suggesting an unprecedented rate of global urbanization. The rate of urban expansion is notably faster than that of population growth

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 resolution6,7. Development of this information lags behind that of state-of-the-art non-urban landcover change data

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.

we 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 (that is, the Global Human Settlement Layer6,19, the Global Urban Footprint20, the Global Urban Land 7 and the Global Artificial Impervious Area16; Supplementary Table 1). We extracted cells from the two fusion maps that changed from non-urban in 1985 to urban in 2015. We 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, we assessed the accuracy of all derived products over the past three decades Note: Approach the fused GAUD global urban extent maps are robust across different urban ecoregions and have relatively high mean accuracy.

Better accuracy is mainly attributed to (1) our GAUD data having urban regions that are consistent in different urban products and (2) inconsistent regions being reclassified using a locally adaptive random forest classifier.

the global urban extent increased from 362,747 km2 to 653,354 km2 from 1985 to 2015, representing a net expansion of 80%

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%

the rates of urbanization in Asia, Africa and South America accelerated during the 30 yr period but began to slow in North America, Europe and Australia

the bulk (~70%) of urban growth since 1992 occurred at the expense of agricultural land, followed by grasslands (~12%) and forests (~9%)

the dataset can be used to improve our understanding of how urban areas affect carbon cycles, a negative anthropogenic impact that has not been studied thoroughly30,31 because of the lack of accurate and continuous global urban extent maps. The GAUD data can also be used to study how urban expansion drives global changes in energy and water use and in turn triggers changes in water and energy fluxes between the land and atmosphere25,32. GAUD may also aid research into the impacts of rapid urban expansion on habitat and biodiversity loss or the extent that health risks arise from urban heat island effects.