The year 2021 marked the highest temperature and likely the lowest oxygen content for the oceans since human records began(1,2). These changes have put marine species on the front lines of climate change. For example, marine species' geographical ranges are shifting faster and experiencing more contractions than those of terrestrial species (3, 4). However, whether climate change poses an existential threat to ocean life has been less clear. Marine species are often considered to be more resilient to extinction than terrestrial ones, and human-caused global extinctions of marine species have been relatively rare (5). On page 524 of this issue, Perm and Deutsch (6) present extensive modeling to reveal that runaway climate change would put ocean life on track for a mass extinction rivaling the worst in Earth's history. Furthermore, they reveal how keeping global warming below an increase of 2? compared with preindustrial levels could largely prevent these outcomes. The topic of climate change and species extinction on land has been fraught with controversy. This is in part because of debates over suitable methods of predicting extinctions and in part because of the relatively few documented extinctions to date (7). The marine research community has largely avoided making projections of extinction risk (8), even though experts widely see climate change as a major threat to the global oceans (9). This has left the watery 70 of Earth's surface as a giant blank spot in the future projection of life on Earth.
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