The Global Deserts Outlook is the first thematic report in the Global Environment Outlook (GEO) series of environmental assessments by the United Nations Environment Programme (UNEP). This GEO report, prepared by experts from across the globe, traces the history and astonishing biology of the deserts and assesses likely future changes.Almost one-quarter of the earth's land surface - some 33.7 million km² - has been defined as 'desert' in some sense. These deserts are inhabited by over 500 million people, significantly more than previously thought. The world's deserts are facing dramatic changes as a result of global climate change, high water demands, tourism and salt contamination of irrigated soils. Better management of water supplies will be the key challenge for the future of deserts but could, if successful, be a beacon of hope and good practice for other water-short parts of the globe. These are among the findings of UNEP's Global Deserts Outlook launched to mark World Environment Day on 5 June.