Humanity's ongoing disruption of atmospheric carbon dioxide (CO₂) concentrations, temperatures, and global nutrient cycles is starving the oceans of oxygen. Although we can measure ongoing changes to ocean circulation and marine life, we still don't know how far the oxygen loss and the damage to ocean ecosystems could go. One way to foresee the oceans' future is to look back at how our seas and their inhabitants responded to higher CO₂ levels and warmer climates in the past.