Global drylands encompassing hyper-arid, arid, semiarid, and dry subhumid areas cover about 41 percent of the earth's terrestrial surface and are home to more than a third of the world's population. By analyzing observations for 1948–2008 and climate model simulations for 1948–2100, we show that global drylands have expanded in the last sixty years and will continue to expand in the 21st~century. By the end of this century, the world's drylands (under a high greenhouse gas emission scenario) are projected to be 5.8 × 10<sup>6</sup> km<sup>2</sup> (or 10%) larger than in the 1961–1990 climatology. The major expansion of arid regions will occur over southwest North America, the northern fringe of Africa, southern Africa, and Australia, while major expansions of semiarid regions will occur over the north side of the Mediterranean, southern Africa, and North and South America. The global dryland expansions will increase the population affected by water scarcity and land degradations.