Global land cover data products are key sources of information in understanding the complex interactions between human activities and global change.They play a critical role in improving performances of ecosystem,hydrological and atmospheric models.Three freely available global land cover products developed in the United States are popularly used by the scientific community.These include two global maps developed separately by the United States Geological Survey (USGS) and the University of Maryland (UMD) with NOAA Advanced Very High Resolution Radiometer (AVHRR) data,and one developed by Boston University with the EOS Moderate Resolution Imaging Spectroradiometer (MODIS) data.They are compared with known land cover types at 250 available Fluxnet sites around the world.The overall accuracies are 37%,36% and 42%,respectively for the USGS,UMD and Boston global land cover maps.Some future global land cover mapping strategies are suggested.