Even though researchers have paid a great deal of attention to wetland loss and status, the actual extent of wetland loss on a global scale, especially the loss caused directly by human activities, and the actual extent of currently surviving wetlands remains uncertain. This paper simulated the potential distribution of global wetlands by employing a new Precipitation Topographic Wetness Index (PTWI) and global remote sensing training samples. The results show earth would have approximately 29.83 million km(2) of wetlands, if humans did not interfere with wetland ecosystems. By combining datasets related to global wetlands, we found that at least 33% of global wetlands had beett lost as of 2009, including 4.58 million km(2) of non-water wetlands and 2.64 million km(2) of open water. The areal extent of wetland loss has been greatest in Asia, but Europe has experienced the most serious losses. Wetland-related datasets suffer from major inconsistencies, and estimates of the areal extent of the remaining global wetlands ranged from 1.53 to 14.86 million km(2). Therefore, although it is challenging, thematic mapping of global wetlands is necessary and urgently needed. (C) 2017 Elsevier B.V. All rights reserved.