| Summary: | Maintaining or restoring at least 50% of Earth’s land area as intact, natural ecosystems has been proposed as a solution to the world’s current biodiversity crisis. Several recent papers published in peer-reviewed journals claim that this proposal, widely known as Half Earth, is supported by science. I present a detailed review of the current state of the science “supporting” Half Earth to show that our current lack of knowledge regarding its potential ecological effects preclude rational, evidence-based judgments about Half Earth. Before it can be adopted through intergovernmental agreement, the science supporting Half Earth should be vetted through an IPCC-like organization. The Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) has an institutional framework that can provide IPCC-like science, and it should assess the potential effects of Half Earth and other global biodiversity conservation strategies. Specifically, the IPBES could assess and summarize for policymakers what is currently known about: (1) the amount of protected area (relative to ecoregion area) needed to achieve different biodiversity conservation objectives; (2) effectiveness and efficiency of protected areas relative to other conservation strategies; (3) likelihood of different land-area targets and other strategies successfully achieving conservation objectives; (4) feasibility and relative costs of different protected land-area targets versus other strategies, and (5) plausible economic and social impacts of different land-area targets and other global conservation strategies.
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