As representatives from 195 nations gather in Paris to hammer out a global agreement to slash greenhouse gas emissions, a new study finds that the failure to do so could leave the world gasping for breath.

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Marine plants such as phytoplankton are estimated to produce more than half the Earth’s atmospheric oxygen, according to the National Oceanic and Atmospheric Administration. For the study, Sergei Petrovskii, an applied mathematics professor at the University of Leicester in the United Kingdom, calculated how unrestrained global warming could affect phytoplankton and thus the ocean’s ability to generate breathable air. He ran computer models that looked at what would happen to phytoplankton’s ability to photosynthesize at different temperatures.

If the world’s oceans warmed by 6 degrees Celsius—a realistic possibility if global emissions continue unabated—the tiny plants would halt oxygen production, according to the study, which was published Tuesday in the Bulletin of Mathematical Biology.

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By 2100, the earth at sea level could have atmospheric oxygen levels comparable to the top of Mount Everest today. “And as far as I know, people cannot normally stay on Everest without oxygen masks for more than a few minutes,” Petrovskii said.

The threat has been “mostly overlooked” by climate scientists, Petrovskii said, noting that such a global disaster would come with little notice.

Phytoplankton viewed through a microscope. (Photo: NOAA MESA Project)

“A distinct feature of this catastrophe is that there will be few warning signs and little change before it is too late,” he said. That’s because phytoplankton can continue to produce oxygen and photosynthesize at levels below 6 degrees of temperature rise.

“Under a 2-degree increase, we will probably see no change; the 4-degree increase would already be dangerously close,” Petrovskii said, adding that more research is needed to determine what increase in global temperatures would halt phytoplankton’s ability to photosynthesize.