Deforestation is a major driver of climate change1 and the major driver of biodiversity loss1,2. Yet the essential baseline for monitoring forest cover—the global area of forests—remains uncertain despite rapid technological advances and international consensus on conserving target extents of ecosystems3. Previous satellite-based estimates4,5 of global forest area range from 32.1 × 106 km2 to 41.4 × 106 km2. Here, we show that the major reason underlying this discrepancy is ambiguity in the term ‘forest’. Each of the >800 official definitions6 that are capable of satellite measurement relies on a criterion of percentage tree cover. This criterion may range from >10% to >30% cover under the United Nations Framework Convention on Climate Change7. Applying the range to the first global, high-resolution map of percentage tree cover8 reveals a discrepancy of 19.3 × 106 km2, some 13% of Earth’s land area. The discrepancy within the tropics alone involves a difference of 45.2 Gt C of biomass, valued at US$1 trillion. To more effectively link science and policy to ecosystems, we must now refine forest monitoring, reporting and verification to focus on ecological measurements that are more directly relevant to ecosystem function, to biomass and carbon, and to climate and biodiversity.
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Funding was provided by the following NASA programmes: Making Earth System Data Records for Use in Research Environments (NNX08AP33A-MEASURES), Land Cover and Land Use Change (NNX08AN72G-LCLUC), Carbon Cycle Science (NNH13ZDA001N-CARBON), and Earth System Science Research Using Data and Products from Terra, Aqua, and Acrimsat Satellites (NNH06ZDA001N-EOS). X.-P.S. was also supported by NASA’s Earth and Space Science Fellowship (NESSF) Program (NNX12AN92H). P.N. was also supported by the Norwegian Agency for Development Cooperation’s Department for Civil Society under the Norwegian Forest and Climate Initiative. The opinions expressed do not represent those of the Global Environmental Facility or the World Bank Group. Data processing and analysis were performed at the Global Land Cover Facility (www.landcover.org) in the Department of Geographical Sciences at the University of Maryland in service of the Global Forest Cover Change Project (www.forestcover.org), a partnership of the University of Maryland Global Land Cover Facility and NASA Goddard Space Flight Center. We thank A. Whitehurst, C. Jenkins and N. Aguilar-Amuchastegui for comments and T. B. Murphy for political insights.
The authors declare no competing financial interests.
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Sexton, J., Noojipady, P., Song, X. et al. Conservation policy and the measurement of forests. Nature Clim Change 6, 192–196 (2016) doi:10.1038/nclimate2816
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