Towards a Less Habitable Ocean.

Ocean warming and associated deoxygenation caused by anthropogenic global warming are impacting marine ecosystems. This article contextualizes and provides perspectives on key insights from a recently published study by Fröb et al. in Earth's Future (2024). The authors employ historical and high‐emission scenario simulations through a state‐of‐the‐art Earth system model to detect abrupt and persistent changes in the viability of marine habitats by leveraging an ecophysiological framework that quantifies how temperature and oxygen jointly limit the distribution of life in the ocean for a number of ecophysiotypes. A changepoint analysis is used to objectively detect shifts in decadal to multi‐decadal mean states in potential marine habitats. They observe a decrease in the ocean volume capable of providing viable habitats for those ecophysiotypes with positive sensitivity to hypoxia. About half of these decreases occur abruptly, thus highlighting potential risks on the capacity of marine organisms to cope with a changing environment. Plain Language Summary: The unabated emission of atmospheric carbon dioxide from human activities threatens all ecosystems on Earth. In the ocean, marine organisms are encountering a warmer, less‐oxygenated and acidified environment. While many of these changes occur gradually, allowing (in some cases) marine organisms time for adaptation, some alterations may happen abruptly, jeopardizing their ability to cope. This paper comments on the recently published study of Fröb et al., which explores the occurrence of such abrupt changes in the ability of marine ecosystems to provide viable habitats, drawing upon the well‐established concept of the "metabolic index." This index assesses whether the resting metabolic requirements of organisms are met by the availability of oxygen at a given temperature. By employing historical (1850–2014) and a high‐emission scenario (2015–2100) as simulated by an Earth system model, the authors uncover that global warming has already begun to drive abrupt alterations in ocean habitability. These findings provide further evidence that ocean biodiversity is poised to undergo major changes during this century. Key Points: The ocean's capacity to provide viable habitats is being compromised by the combined threat of ocean warming and associated deoxygenationFor marine organisms to survive, changes in environmental conditions should be slower than their ability to adaptGlobal warming is leading to more frequent and severe abrupt changes in ocean habitability [ABSTRACT FROM AUTHOR]