Highlights

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We review prominent databases and provide the first spatially explicit comparison of estimates.
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Current estimates of degraded lands are hindered by missing and sometimes unreliable information.
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Global estimates of total degraded land range widely from less than 1 Gha to over 6 Gha.
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Additional work is needed to ensure we do not overestimate this potential of degraded lands.
Degraded lands have often been suggested as a solution to issues of land scarcity and as an ideal way to meet mounting global demands for agricultural goods, but their locations and conditions are not well known. Four approaches have been used to assess degraded lands at the global scale: expert opinion, satellite observation, biophysical models, and taking inventory of abandoned agricultural lands. We review prominent databases and methodologies used to estimate the area of degraded land, translate these data into a common framework for comparison, and highlight reasons for discrepancies between the numbers. Global estimates of total degraded area vary from less than 1 billion ha to over 6 billion ha, with equally wide disagreement in their spatial distribution. The risk of overestimating the availability and productive potential of these areas is severe, as it may divert attention from efforts to reduce food and agricultural waste or the demand for land-intensive commodities.