Internationally coordinated expansion of the global protected area network to 17% could triple the average protection of species ranges and ecoregions; if projected land-use changes and consequent habitat loss until 2040 occur, currently feasible protection levels will not be achievable, and more than 1,000 threatened species face reductions in the range of over 50%. Protected areas are intended to mitigate pressures on biodiversity caused by anthropogenic factors such as habitat loss. To that end, an internationally agreed target aims to extend the protected area network to cover 17% of the world's land area by 2020. But biodiversity is unevenly distributed between countries and habitats, raising the question of which areas should be protected to maximize the effectiveness. Federico Montesino Pouzols et al. show that internationally coordinated expansion of the protected area network to the 17% target could triple the average protection of species ranges and ecoregions. However, within-country prioritization is considerably less efficient. Moreover, taking into account projected land-use changes and consequent habitat loss until 2040, current levels of protection will not be feasible to maintain, and over 1,000 threatened species face reductions in their range of over 50%. Thus, the authors suggest that for effective biodiversity conservation, land-use policy and protected area decisions must be coordinated at an international level. Protected areas are one of the main tools for halting the continuing global biodiversity crisis1,2,3,4 caused by habitat loss, fragmentation and other anthropogenic pressures5,6,7,8. According to the Aichi Biodiversity Target 11 adopted by the Convention on Biological Diversity, the protected area network should be expanded to at least 17% of the terrestrial world by 2020 ( http://www.cbd.int/sp/targets ). To maximize conservation outcomes, it is crucial to identify the best expansion areas. Here we show that there is a very high potential to increase protection of ecoregions and vertebrate species by expanding the protected area network, but also identify considerable risk of ineffective outcomes due to land-use change and uncoordinated actions between countries. We use distribution data for 24,757 terrestrial vertebrates assessed under the International Union for the Conservation of Nature (IUCN) ‘red list of threatened species’9, and terrestrial ecoregions10 (827), modified by land-use models for the present and 2040, and introduce techniques for global and balanced spatial conservation prioritization. First, we show that with a coordinated global protected area network expansion to 17% of terrestrial land, average protection of species ranges and ecoregions could triple. Second, if projected land-use change by 2040 (ref. 11) takes place, it becomes infeasible to reach the currently possible protection levels, and over 1,000 threatened species would lose more than 50% of their present effective ranges worldwide. Third, we demonstrate a major efficiency gap between national and global conservation priorities. Strong evidence is shown that further biodiversity loss is unavoidable unless international action is quickly taken to balance land-use and biodiversity conservation. The approach used here can serve as a framework for repeatable and quantitative assessment of efficiency, gaps and expansion of the global protected area network globally, regionally and nationally, considering current and projected land-use pressures.