The first Global Assessment of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) found widespread, accelerating declines in Earth's biodiversity and associated benefits to people from nature. Addressing these trends will require science-based policy responses to reduce impacts, especially at national to local scales. Effective scaling of science-policy efforts, driven by global and national assessments, is a major challenge for turning assessment into action and will require unprecedented commitment by scientists to engage with communities of policy and practice. Fulfillment of science's social contract with society, and with nature, will require strong institutional support for scientists' participation in activities that transcend conventional research and publication.