Helen Pillar ^1* Elizabeth Hetherington ² Lisa A. Levin ² Laura Cimoli ³ Jonathan Lauderdale ⁴ Jesse Van Der Grient ⁵ Kristen Johannes ² Patrick Heimbach ^1,6 Leslie Smith ⁷ Charles I. Addey ⁸ Pavanee Annasawmy ⁹ Sandra Antonio ¹⁰ Narissa Bax ^11,12 Henri F. Drake ¹³ Elva G. Escobar-Briones ¹⁰ Laura G. Elsler ¹⁴ Mara Freilich ¹⁵ Natalya D. Gallo ^16,17 Fanny Girard ⁸ Matthew J. Harke ¹⁸ Daniel O. Jones ¹⁹ Siddhi Joshi ²⁰ Xinfeng Liang ²¹ Paige J. Maroni ²² Otmane Sarti ²³ Paris V. Stefanoudis ^24,25 Olivier Sulpis ²⁶ David Trossman ²⁷

• ¹ Oden Institute for Computational and Engineering Sciences, Cockrell School of Engineering, The University of Texas at Austin, Austin, Texas, United States
• ² Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California, United States
• ³ Department of Applied Mathematics and Theoretical Physics, Faculty of Mathematics, School of Physical Sciences, University of Cambridge, Cambridge, England, United Kingdom
• ⁴ Department of Earth, Atmospheric and Planetary Sciences, School of Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
• ⁵ South Atlantic Environmental Research Institute, Stanley, Falkland Islands
• ⁶ Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas, United States
• ⁷ Your Ocean Consulting, LLC, Knoxville, Tennessee, United States
• ⁸ Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii, United States
• ⁹ FRB-CESAB, Fondation pour la Recherche sur la Biodiversité – Centre de Synthèse et d'Analyse sur la Biodiversité, Montpellier, France
• ¹⁰ National Autonomous University of Mexico, México City, México, Mexico
• ¹¹ Pinngortitaleriffik Greenland Institute of Natural Resources, Greenland Climate Research Centre, Nuuk, Greenland
• ¹² Centre for Marine Socioecology, Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Australia
• ¹³ Department of Earth System Science, School of Physical Sciences, University of California, Irvine, Irvine, California, United States
• ¹⁴ Department of Nutrition, T.H. Chan School of Public Health, Harvard University, Boston, United States
• ¹⁵ Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, Rhode Island, United States
• ¹⁶ Department of Biological Sciences , University of Bergen, Bergen, Hordaland, Norway
• ¹⁷ Bjerknes Center for Climate Research, Faculty of Mathematics and Natural Sciences, University of Bergen, Bergen, Hordaland, Norway
• ¹⁸ Gloucester Marine Genomics Institute (GMGI), Gloucester, Massachusetts, United States
• ¹⁹ National Oceanography Centre, University of Southampton, Southampton, England, United Kingdom
• ²⁰ Independent Researcher, London, United Kingdom
• ²¹ School of Marine Science and Policy, College of Earth, Ocean, and Environment, University of Delaware, Newark, Delaware, United States
• ²² Minderoo-UWA Deep-Sea Research Centre, School of Biological Sciences, Perth, Australia
• ²³ Abdelmalek Essaadi University, Tétouan, Tanger-Tetouan-Al Hoceima, Morocco
• ²⁴ Museum of Natural History, University of Oxford, Oxford, England, United Kingdom
• ²⁵ Nekton Foundation, Oxford, England, United Kingdom
• ²⁶ UMR7330 Centre Européen de Recherche et d'enseignement de Géosciences de l'environnement (CEREGE), Aix En Provence, Provence-Alpes-Côte d'Azur, France
• ²⁷ Earth System Science Interdisciplinary Center, College of Computer, Mathematical and Natural Sciences, University of Maryland, College Park, College Park, Maryland, United States

A defining aspect of the Intergovernmental Panel on Climate Change (IPCC) assessment reports (AR) is a formal uncertainty language framework that emphasizes higher certainty issues across the reports, especially in the executive summaries and short summaries for policymakers. As a result, potentially significant risks involving understudied components of the climate system are shielded from view. Here we seek to address this in the latest, sixth assessment report (AR6) for one such component - the deep ocean - by summarizing major uncertainties (based on discussions of low confidence issues or gaps) regarding its role in our changing climate system. The goal is to identify key research priorities to improve IPCC confidence levels in deep ocean systems and facilitate the dissemination of IPCC results regarding potentially high impact deep ocean processes to decision-makers. This will accelerate improvement of global climate projections and aid in informing efforts to mitigate climate change impacts. An analysis of 3000 pages across the six selected AR6 reports revealed 219 major science gaps related to the deep ocean. These were categorized by climate stressor and nature of impacts. Half of these are biological science gaps, primarily surrounding our understanding of changes in ocean ecosystems, fisheries, and primary productivity. The remaining science gaps are related to uncertainties in the physical (32%) and biogeochemical (15%) ocean states and processes. Model deficiencies are the leading cited cause of low certainty in the physical ocean and ice states, whereas causes of biological uncertainties are most often attributed to limited studies and observations or conflicting results. Key areas for coordinated effort within the deep ocean observing and modeling community have emerged, which will improve confidence in the deep ocean state and its ongoing changes for the next assessment report. This list of key “known unknowns’’ includes meridional overturning circulation, ocean deoxygenation and acidification, primary production, food supply and the ocean carbon cycle, climate change impacts on ocean ecosystems and fisheries, and ocean-based climate interventions. From these findings, we offer recommendations for AR7 to avoid omitting low confidence-high risk changes in the climate system.