Forests, Trees and Water on a Changing Planet: A Contemporary Scientific Perspective

More than seven billion humans share the planet with approximately three trillion trees (Crowther et al., 2015), 46% less trees than at the start of human civilisation. Approximately 1.36 trillion of these trees exist in tropical and subtropical regions, 0.84 trillion in temperate regions and 0.84 trillion in the boreal region; overall nearly one-third are outside forests (Crowther et al., 2015). There is a wide variation in the ratio of trees to humans and whether or not this matters can be answered in many ways. Even so, we know that the majority of the four billion people facing severe water scarcity (Oki and Kanae, 2006; Rockström et al., 2014; Mekonnen and Hoekstra, 2016) live in areas where forests and trees outside forest are currently scarce.