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Abstract
Since 2000, eight groups from government and academia in both the EU and the US have created global maps that can be used to describe urban areas. Although most share common inputs, they differ by as much as an order of magnitude in their estimates of the total extent of the Earth's urban land (from 0.27–3.52 million sq. km). Based on a new Discrete Global Grid-based system for map comparison, we find inter-map correlations highest in North America ( r¯ = 0.83) and lowest in Asia (r¯ = 0.66). We conduct the first global accuracy assessment focused on urban areas using a two-tiered approach that draws on high-resolution Google Earth imagery and medium-resolution Landsat-based maps. Across a wide range of accuracy measures, spatial scales, and world regions, the new MODIS 500 meter-resolution global urban map has the highest accuracy (aggregate accuracy rank of 1.0), followed by a thresholded version of the Global Impervious Surface Area map (aggregate accuracy rank of 2.7).
Based on an analysis of the MODIS 500 meter-resolution land cover map and the 2007 World Database of Protected Areas, we observe 19,600 sq. km of urban incursions or inholdings in protected areas (PAs); 77 percent within developed countries. A simple spatial model of urban expansion that is driven by several demographic and urban density scenarios predicts a near doubling of urban areas through 2050 (from 0.66 to 1.22 million sq. km). Assuming that PAs have no ability to deter urban encroachment, this urban expansion will threaten an additional 12,000 sq. km of PA, a 61 percent increase over observed incursions. Despite the relative inaccessibility of Africa's PA, by 2050 that region faces the largest potential PA losses: 2,700 sq. km, an increase of 392 percent. Overall, developing countries account for 76 percent of all potential urban PA incursions through 2050. High-growth demographic scenarios combined with declining urban densities yield global PA incursions twice the size of those derived from lower-growth, more compact futures. Our findings highlight the need to account for demographic pressure and urban planning when designing sustainable conservation strategies in the context of a rapidly urbanizing world.
