Environmental science: Judging the Earth by its cover

The Earth is gaining tree cover and losing bare ground cover - mainly through human activity - reports a paper published online this week in Nature.

Land changes are both a consequence and a cause of global environmental change. However, the full extent of these changes - considered over long time periods at both local and global scales and specifically incorporating changes in crop, grass and other non-forested lands - remains unknown.

Xiao-Peng Song and colleagues use satellite imaging data to map the changes in land cover across the globe between 1982 and 2016. They divided land cover into three categories: bare ground, short vegetation cover and tree canopy cover - the latter being tall vegetation over 5 metres in height.

Despite the widespread and prominent agriculture-driven deforestation in the tropics, the authors note that the world’s tree canopy cover increased by 7% during the study period - gaining an extra 2.24 million square kilometres as a result of net gains in subarctic, subtropical and temperate climates. Global bare ground cover decreased by 1.16 million square kilometres, with Asian agricultural regions showing the most notable transformations. On a regional scale, they found change to be variable, with mountainous areas gaining tree canopy cover, whereas many arid and semi-arid systems (including Australia, China and the southwestern United States) lost vegetation cover.

Using a probability-sampling approach, the authors determine that 60% of the land changes that they observed are associated with direct human activities and 40% are a result of indirect drivers such as climate change.

doi: 10.1038/s41586-018-0411-9

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