The UN Sustainable Development Goal 14 aims to "conserve and sustainably use the oceans, seas and marine resources for sustainable development". Achieving this goal will require rebuilding the marine life-support systems that deliver the many benefits society receives from a healthy ocean. In this Review we document the recovery of marine populations, habitats and ecosystems following past conservation interventions. Recovery rates across studies suggest that substantial recovery of the abundance, structure, and function of marine life could be achieved by 2050, should major pressures, including climate change, be mitigated. Rebuilding marine life represents a doable Grand Challenge for humanity, an ethical obligation, and a smart economic objective to achieve a sustainable future. The ability of the ocean to support human wellbeing is at a crossroads. The ocean currently contributes 2.5% of global GDP and provides employment to 1.5% of the global workforce 1 , with an estimated output of US$1.5 trillion in 2010, expected to double by 2030 1. And there is increased attention on the ocean as a source of food and water 2 , clean energy 1 , and as a means to mitigate climate change 3,4. At the same time, many marine species, habitats and ecosystems have suffered catastrophic declines 5-8 and climate change is further undermining ocean productivity and biodiversity 9-14 (Fig. 1). The conflict between growing human dependence on ocean resources and declining marine life under human pressures (Fig. 1) is focusing unprecedented attention on the connection between ocean conservation and human well-being 15. The UN Sustainable Development Goal 14 (SDG14 or "life below water") aims to "conserve and sustainably use the oceans, seas and marine resources for sustainable development" (https://sustainabledevelopment.un.org/sdg14). Achieving this goal will require rebuilding marine life, defined in the context of SDG14 as the life-support systems (populations, habitats, and ecosystems) that deliver the many benefits society receives from a healthy ocean 16,17. Here we show that, in addition to being a necessary goal, substantially rebuilding marine life within a human generation is largely achievable, if the required actions, prominently mitigating climate change, are deployed at scale. Slowing the decline of marine life and achieving net gains By the time the general public admired life below water through the "Undersea World of Jacques Cousteau" (1968-1976), the abundance of large marine animals was already greatly reduced 5-7,18. And the abundance of marine animals and habitats that support ecosystems services has shrunk to a fraction of what was in place when the first frameworks to conserve and sustain marine life were introduced in the 1980s (Fig. 1), to a fraction of pre-exploitation levels 5,6,19,20. Currently, at least one-third of fish stocks are overfished 21 , one-third to half of vulnerable marine habitats have been lost 8 , a substantial fraction of the coastal ocean suffers from pollution, eutrophication, oxygen d...

Climate change is shifting the ranges of species. Simple predictive metrics of range shifts such as climate velocity, that do not require extensive knowledge or data on individual species, could help to guide conservation. We review research on climate velocity, describing the theory underpinning the concept and its assumptions. We highlight how climate velocity has already been applied in conservation-related research, including climate residence time, climate refugia, endemism, historic and projected range shifts, exposure to climate change, and climate connectivity. Finally, we discuss ways to enhance the use of climate velocity in conservation through tailoring it to be more biologically meaningful, informing design of protected areas, conserving ocean biodiversity in 3D, and informing conservation actions.

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