, but showed improved accuracy in agricultural areas and increased discrimination of small forest patches. Against lidar measurements, the Landsat-based estimates exhibited accuracy slightly less than that of the MODIS VCF (RMSE 016.8% for MODIS-based vs. 17.4% for Landsat-based estimates), but RMSE of Landsat estimates was 3.3 percentage points lower than that of the MODIS data in an agricultural region. The Landsat data retained the saturation artifact of the MODIS VCF at greater than or equal to 80% tree cover but showed greater potential for removal of errors through calibration to lidar, with post-calibration RMSE of 9.4% compared to 13.5% in MODIS estimates. Provided for free download at the Global Land Cover Facility (GLCF) website (www.landcover. org), the 30-m resolution GLCF tree cover dataset is the highest-resolution multitemporal depiction of Earth's tree cover available to the Earth science community.

Deforestation is a major driver of climate change 1 and the major driver of biodiversity loss 1,2 . 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 ecosystems 3 . Previous satellite-based estimates 4,5 of global forest area range from 32.1 × 10 6 km 2 to 41.4 × 10 6 km 2 . Here, we show that the major reason underlying this discrepancy is ambiguity in the term 'forest'. Each of the >800 o cial definitions 6 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 Change 7 . Applying the range to the first global, high-resolution map of percentage tree cover 8 reveals a discrepancy of 19.3 × 10 6 km 2 , some 13% of Earth's land area. The discrepancy within the tropics alone involves a di erence of 45.2 Gt C of biomass, valued at US$1 trillion. To more e ectively 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 9-11). 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 change 12 .The Forest Resources Assessments (FRAs) of the United Nations Food and Agriculture Organization (FAO)-the authority for national and global accounting-recorded 40.8 × 10 6 km 2 of forest in 2000, equalling 31% of Earth's land area 13 . The FRAs rely on self-reporting by participating countries, raising concerns about subjectivity and consistency 14-16 . Although estimates from satellite images should provide a more objective base 9 , 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 nearperfect 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 × 10 6 km 2 show perfect consensus among the eight data sets on either the presence or absence of forests, 9.4 × 10 6 km 2 were identified as forest by four out of the eight sources. These sparsely forested regions are the areas of greatest remaining uncertainty.There are two reasons for the uncertaint...

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