New Research Suggests Ocean May Increase in Oxygen During Warming

The climate change Earth is experiencing today is similar to that during a period of rapid and intense global warming it experienced some 56 million years ago. Understanding the similarities can help scientists evaluate what is happening in today’s warming world, according to Ellen Thomas, Harold T. Stearns Professor of Integrative Sciences, Emerita.

Key to that understanding is figuring out how much oxygen was dissolved in large swaths of the oceans during that period of rapid warming, called the Paleocene-Eocene Thermal Maximum or PETM, when average temperatures increased by 5-8^o Celsius or 9-14^o Fahrenheit in a few thousand years, Thomas said.

“Most ocean life forms, including fish, need oxygen, so the loss of oxygen would mean loss of an important food source for many people, especially in tropics and subtropics,” she said. 

Thomas was among a group of climate researchers who recently published in Science about the results of their analysis of oxygen levels in several regions of the world’s oceans during the PETM. Their research suggests that oxygenation in the upper waters of the ocean, where most marine lifeforms dwell, increased in the eastern Pacific, the largest ocean. That’s potentially positive, as to how the world’s oceans may respond to current and future warming, because “effects of global warming may not be as bad as we thought,” Thomas said.

While oxygenation of the oceans is complex, climate researchers generally consider that three main factors determine oxygen levels at various regions throughout the oceans: temperature, how well the ocean waters are mixed at various depths, and the amount of organic matter available to consume the oxygen available, Thomas said.

This group of researchers used a novel approach to estimate oxygen levels as they were in the oceans millions of years in the past: they analyzed nitrogen isotopes in the microscopic shells of planktonic foraminifera, which are single-celled, shell-forming organisms that lived floating in the water column. The nitrogen isotope values provide information on oxygen levels in the waters where the organisms lived. This information from nitrogen isotopes was supported by data on the size of the shells of the foraminifera in the largest oxygen minimum zone on Earth — the depth zone where there’s the least amount of oxygen — in the eastern equatorial Pacific Ocean. The microscopic organisms had larger shells during the PETM, indicating that they could grow fast at the high temperatures, for which they need higher oxygen levels, Thomas said.

“The largest oxygen minimum zone in the largest ocean of the world did not become more severe, but in fact declined in size. That may have been one of the factors that prevented an extinction in the upper ocean waters during rapid global warming,” she said.

This data can be used to improve climate-ecosystem models to therefore better inform policy decisions. But Thomas said more research other regions of the global oceans is necessary to determine whether their findings can be confirmed.