Here, we ask how much land is needed to protect biodiversity that is not already covered in the existing global protected area network. Unfortunately, reliable estimates of unprotected ecosystem services are difficult to derive given the scarcity of necessary global spatial data. However, global data are available for carbon storage and we therefore use this as an illustrative example to estimate a bare minimum extent of additional protected areas required to cover 75 and 90% (arbitrarily chosen) of global biomass carbon in natural habitats. The carbon stored in natural habitats is of global importance for slowing climate change, and global deforestation and forest degradation account for c. 12% of global CO2 emissions annually (van der Werf et al., 2009). Curbing deforestation is seen as a cost-effective way of reducing CO2 emissions (McKinsey & Company, 2009) and consequently an international financial mechanism to reduce emissions from deforestation and forest degradation (REDD+) has been adopted by the United Nations Framework Convention on Climate Change (UNFCCC, 2010). The protection of natural habitats is an effective way to ensure continued carbon storage, assuming effective management to prevent degradation (Scharlemann et al., 2010)