16 million years ago, oceans were warmer than today yet had more oxygen
This discovery flipped the expectation that warming alone drives oceans toward severe oxygen loss. So, what did?
A 2021 study predicting that Earth's oxygen-rich atmosphere could largely disappear in about a billion years is making the rounds again. Researcher Kazumi Ozaki from Toho University and Christopher Reinhard of Georgia Tech ran nearly 400,000 computer simulations, estimating that Earth's atmosphere would remain above 1% of today's oxygen level for another 1.08 billion years on average. The study, published in Nature Geoscience, resurfaced after BBC Sky at Night revisited it on September 17, 2026.
Now, a separate study published January 16, 2026, in Communications Earth & Environment has revealed another surprising chapter in oxygen's history — this time in the oceans. Researchers found that two of today's major low-oxygen regions were actually better oxygenated during an exceptionally warm period millions of years ago. The findings support long-term model projections suggesting that some oxygen-poor zones could eventually become better oxygenated again, although regional ocean conditions will play a major role.
The key biosignature of life
Oxygen is one of the key biosignatures scientists look for when searching for signs of life beyond Earth. However, like every other living thing, it is not a permanent feature of the Earth’s atmosphere, which means it can either thrive or perish in the future. The 2021 study suggested that Earth's oxygen-rich atmosphere may ultimately make up only about 20% to 30% of the planet's total history, per SkyAtNight magazine. Oxygen-producing microorganisms helped drive the Great Oxidation Event roughly 2.4 billion years ago, when atmospheric oxygen levels rose dramatically. Earth, however, is not the same as it was then.
Where does an ocean's vulnerability stem from?
According to a 2017 study published in the journal Nature, the global ocean has lost more than 2% of its total dissolved oxygen inventory between 1960 and the study period. Vulnerability especially looms in oxygen-minimum (OMZ) and oxygen-deficient zones (ODZ). The Arabian Sea has long been identified as an OMZ as well as one of the most intensely oxygen-poor basins in the northern Indian Ocean. But today, scientists proclaim that heat alone didn’t push the sea to this vulnerable state.
The cue comes from reconstructed models of the Miocene Climatic Optimum, about 17 to 14.8 million years ago. This period had similar temperatures and atmospheric conditions that are predicted to occur after 2100. The Arabian Sea was still oxygen-poor during this period, but it was better oxygenated than it is today despite the much warmer global climate. This discovery flipped the expectation that warming alone drives oceans toward severe oxygen loss.
Shells keep the score
Researchers, led by Alexandra Auderset at the University of Southampton, investigated fossilized foraminifera, or microscopic shell-producing organisms buried on the seafloor, to trace the historical chronicles of the Arabian Sea. The Arabian Sea remained better oxygenated through the warm Miocene Climatic Optimum before gradually losing oxygen as the climate cooled. Researchers found that around 12.1 million years ago, the Arabian Sea became severely oxygen-poor for long periods.
Once these insights were derived from the sediment cores, Auderset’s team set their focus on determining the factors that might have played a role in oxygen depletion, if not heat alone. Several factors came to light. Oxygen circulation and the ocean's interior ventilation, for instance. They also found that different patches of the ocean responded differently to low-oxygen environments, for example, the Arabian Sea off the west coast of India. The waters across the eastern tropical Pacific, too.
According to a 2023 report published in Nature, Pacific waters also became better oxygenated during the Miocene period than today. The only difference: the Pacific developed persistent denitrification about 2.6 million years earlier than the Arabian Sea, showing that the two regions did not respond identically to the same global climate changes. Inflows and ocean circulation also mattered. Because the Arabian Sea is partly enclosed by the Arabian Peninsula and India, its deep-water exchange and oxygen replenishment differ from those of the open Pacific.
What is stealing oxygen from the sea?
Today, parts of the Arabian Sea have so little oxygen that only limited marine life can survive there. When oxygen drops that low, some microbes switch to using other chemicals to survive, disrupting the ocean’s nitrogen balance and potentially affecting the food web. Monsoon winds can also change how much oxygen stays in the water. They can bring more nutrients to the surface, which feeds more marine life. But when that extra material later breaks down, it uses up oxygen deeper in the sea. “Our results suggest that ocean oxygen loss, already underway today, is strongly shaped by local oceanography,” said Auderset.
So warmer oceans do not always lose oxygen in the same way. What happens depends a lot on local ocean conditions. The researchers say regional factors including circulation, monsoon-driven upwelling and inflows from surrounding seas could strongly influence what happens next. “Our research shows ocean response to climate warming is complex, and this means that we will need to be ready to adapt to changing ocean conditions,” Auderset concluded.
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