| 英文摘要: | 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. Forests are the focus of efforts to mitigate harmful ecological and social impacts of land use, including agreements to reduce carbon dioxide emissions from deforestation and forest degradation (REDD+; refs 9, 10, 11). The goals are both scientific—to balance regional and global carbon budgets—as well as political, to reduce carbon emissions and stop species extinctions by defining national baselines and managing future anthropogenic change12. The Forest Resources Assessments (FRAs) of the United Nations Food and Agriculture Organization (FAO)—the authority for national and global accounting—recorded 40.8 × 106 km2 of forest in 2000, equalling 31% of Earth’s land area13. The FRAs rely on self-reporting by participating countries, raising concerns about subjectivity and consistency14, 15, 16. Although estimates from satellite images should provide a more objective base9, even these disagree significantly over the amount and distribution of forests worldwide. Figure 1 maps the consensus among eight global satellite data sets over the class ‘forest’ in or near the year 2000 (Methods). The densely canopied biomes of the tropical, temperate and boreal zones, and the treeless deserts, prairies and tundra show near-perfect agreement across all sources on the presence or absence of forests. Yet the data disagree over the planet’s semi-arid savannahs, shrublands and woodlands, and over the northern limits of the boreal forest. Although 102.2 × 106 km2 show perfect consensus among the eight data sets on either the presence or absence of forests, 9.4 × 106 km2 were identified as forest by four out of the eight sources. These sparsely forested regions are the areas of greatest remaining uncertainty.
| http://www.nature.com/nclimate/journal/v6/n2/full/nclimate2816.html
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