As politicians continue to debate climate change, researchers are hard at work trying to predict what humans can expect as the planet continues to warm. Even with plenty of research, the many variables involved in climate change make the future uncertain. What does seem certain, however, is that humans are ill-prepared.

A recent study published in the journal Paleoceanography has shown just how extreme the effects of rapid global warming can be. The study used chemical analyses and research on very small fossilized organisms (micropaleontology) to determine what happened to the Earth’s marine life during a global warming event in the past.

Researchers at Syracuse University found that a global warming event known as the Paleocene-Eocene Thermal Maximum (PETM) lowered the amount of dissolved oxygen in the world’s oceans. This caused oxygen minimum zones (OMZs, ocean layers of low oxygen saturation) to grow, leading to a mass extinction of life in the Earth’s oceans. Climate scientists are studying the PETM, which occurred 55 million years ago and is similar to the warming seen today, to determine what the effects of modern climate change might be.

“Global warming impacts marine life in complex ways, of which the loss of dissolved oxygen (a condition known as hypoxia) is a growing concern,” said Zunli Lu, a coauthor of the study and an assistant professor of Earth sciences at Syracuse University. “Moreover, it’s difficult to predict future deoxygenation that is induced by carbon emissions, without a good understanding of our geologic past.”

Lu and his colleagues looked at microfossils of organisms known as foraminiferas to determine what Earth’s oceans were like during the PETM. They focused on a specific type of iodine that exists only in oxygenated water to estimate the ambient oxygen levels of the planet’s ocean water during that time, then compared that data to climate models of the PETM. The researchers found that OMZs likely expanded during the PETM, possibly foreshadowing the effects of modern climate change.

“By comparing our fossil data with oxygen levels simulated in climate models, we think OMZs were much more prevalent 55 million years ago than they are today,” said Lu. “Deoxygenation, along with warming and acidification, had a dramatic effect on marine life during the PETM, prompting mass extinction on the seafloor.”