For a long time, it was assumed that rising ocean temperatures would inevitably reduce dissolved oxygen levels. According to Henry's law, warm water holds fewer gases; this raised fears that as the oceans warm, hypoxic zones — areas with low oxygen dangerous to marine life — would expand across the planet.
Scientists studied microscopic remains of ancient marine organisms — foraminifera — extracted from seafloor sediments in the Arabian Sea. Their calcareous shells preserve isotopic “signatures” of the surrounding environment. Special attention was given to the Miocene period — an era about 17 to 14 million years ago, when Earth's atmosphere and average temperatures were comparable to the conditions projected for the planet after 2100.
The results were unexpected. The waters of the ancient Arabian Sea during the Miocene contained more oxygen than they do today. And this pattern was not limited to that region: large areas of the ancient Pacific Ocean also remained well supplied with oxygen. Moreover, during this period the ocean was highly biologically productive: the biomass of marine organisms was significantly higher than it is today.
One key factor may have been changes in oceanic and atmospheric circulation. Stronger winds and sea currents can actively mix the water and increase its contact with the atmosphere. As a result, oxygen enters the ocean more quickly.
Scientists now face a new task: to understand which mechanisms allow warm oceans to maintain high oxygen levels. The answer could help predict the future of marine ecosystems more accurately — and perhaps even find ways to support their productivity. That means the picture becomes less uniformly bleak and perhaps a more optimistic one.
