The spatial extent of marine and terrestrial protected areas (PAs) was amongst the most intensely debated issues prior to the decision about the post-2020 Global Biodiversity Framework (GBF) of the Convention on Biological Diversity. Positive impacts of PAs on habitats, species diversity and abundance are well documented. Yet, biodiversity loss continues unabated despite efforts to protect 17% of land and 10% of the oceans by 2020. This casts doubt on whether extending PAs to 30%, the agreed target in the Kunming-Montreal GBF, will indeed achieve meaningful biodiversity benefits. Critically, the focus on area coverage obscures the importance of PA effectiveness and overlooks concerns about the impact of PAs on other sustainability objectives. We propose a simple means of assessing and visualising the complex relationships between PA area coverage and effectiveness and their effects on biodiversity conservation, nature-based climate mitigation and food production. Our analysis illustrates how achieving a 30% PA global target could be beneficial for biodiversity and climate. It also highlights important caveats: i) achieving lofty area coverage objectives alone will be of little benefit without concomitant improvements in effectiveness, ii) trade-offs with food production particularly for high levels of coverage and effectiveness are likely and iii) important differences in terrestrial and marine systems need to be recognized when setting and implementing PA targets. The CBD's call for a significant increase in protected area will need to be accompanied by clear PA effectiveness goals to reduce and revert dangerous anthropogenic impacts on socio-ecological systems and biodiversity.