The increase of impervious cover and decrease of tree cover within urban areas globally (2012-2017)

Trees in cities provide numerous benefits to society by altering the local physical, biological and social environment, providing billions of dollars in annual benefits. How tree and other cover types vary and are changing globally within urban areas is currently unknown. Photo-interpretation was used to determine current urban cover (tree, impervious, grass, other cover) percentages and recent changes in cover types throughout the world. Within existing urban areas, the average global urban tree cover had a slight, but statistically significant decline from 26.7 % to 26.5 % (c. 2012-2017), or a loss of about 40,000 ha per year. All continents exhibited a loss in urban tree cover except for Europe; the greatest decrease in percent tree cover was in Africa. Concurrent with tree loss was an increase in impervious cover among all continents, which globally had a statistically significant increase from 24.3 % to 25.9 % (326,000 ha/year). Urban tree cover was significantly different among forested (30.6 %), grassland (18.5 %) and desert regions (12.6 %). Understanding global urban cover type variation and changes can improve global assessments and help guide forest management to improve environmental quality in cities.