This would occur because the Southern Ocean near Antarctica is a key site of “upwelling,” in which deep ocean waters, which have picked up such nutrients as phosphorous and nitrogen from the depths (which end up there after marine organisms die and their bodies sink), rise and deliver that biological bounty to the surface. Then, the nutrients enter the global ocean circulation and are carried northward to more moderate climes.

But if warming gets severe enough, Southern Ocean upwelling can be suppressed by warm ocean surface waters. Meanwhile, many of the nutrients that do manage to rise will be consumed by the increasingly active biology of the mostly ice-free ocean around Antarctica — leaving far fewer nutrients for the rest of the world.

In this case, as organisms in the Southern Ocean die, more nutrients again sink to the bottom of that ocean, and stay there.

“So you have nutrients building up in the deep ocean, down where the biology can’t use them or get to them,” said Keith Moore, the lead author of the study in Science and a professor at the University of California at Irvine.

The researchers concede that the picture they paint is dire — and requires very high levels of warming that might never materialize. The temperature of the Antarctic Ocean in the scenario, for instance, would be 6 degrees Celsius (10.8 degrees Fahrenheit) warmer than it is now, and the sea ice ringing Antarctica would be almost entirely gone.

The computer modeling study also uses a worst-case scenario for the burning of fossil fuels to the year 2100 and even beyond it, ultimately triggering atmospheric concentrations of carbon dioxide just below 2,000 parts per million. That’s far in excess of the current level around 410 parts per million. It’s questionable that humanity would let things get that bad, and growing numbers of wind and solar installations and electric cars suggest that in future decades, we’ll be powering key aspects of life without fossil fuels.

Still, the authors said, it’s worth probing such extremes to understand how the climate system works, and they noted that for now, a high emissions scenario remains possible.

“These simulations paint a fairly dire picture of what I think will be catastrophic changes in the context of unmitigated climate warming,” said Matthew Long, one of the study’s authors and an oceanographer at the National Center for Atmospheric Research in Boulder, Colo. “Along the path to those very catastrophic events, we may cross thresholds that we don’t know about. And so, I think it’s important for people to reflect on the impact we’re having on the world’s ocean and consider that in the context of action to mitigate climate warming.”

A researcher who was not involved in the study but reviewed it for The Washington Post, oceanographer Lynne Talley of the Scripps Institution of Oceanography, said she found the scenario plausible if warming is strong enough.

“Maybe they’ve got an extreme answer here, but all the pieces are what you’d expect to happen in a more moderate forcing,” Talley said.

An accompanying essay in Science, by ocean experts Charlotte Laufkotter and Nicolas Gruber of the University of Bern and ETH Zurich, respectively, in Switzerland, added that “the mere possibility of a future Southern Ocean nutrient-trapping scenario is highly concerning, warranting dedicated efforts to further our understanding of the unique role of the Southern Ocean in the global climate system.”

Fortunately, the study assumes as its premise a level of global warming that we in the present have ample opportunity to prevent.

If you think that the Paris climate agreement will work, holding the warming of the planet to “well below” 2 degrees Celsius (3.6 degrees Fahrenheit) and bringing atmospheric carbon dioxide concentrations back down before they get much past 450 parts per million, then you can safely assume that such things will not occur.

On the other hand, if you don’t think the world can manage economic and population growth in the coming decades without a continual or even growing reliance on fossil fuels, this extreme scenario may be hard to get out of your mind.

Moore also said that he thinks the scenario presented in the study could at least begin to kick in at a lower temperature than the extreme ones in the paper. He said he’ll start to worry at a global temperature rise around 2.5 degrees Celsius (4.5 degrees Fahrenheit), when a lot of floating Antarctic sea ice could start to go.

“We don’t know exactly where that tipping point is,” Moore said. The study thus reinforces the importance of the Paris climate goals.

In the end, Long said, there is a value in describing what the worst-case scenario actually is — even if it is never actually realized.

“Human-driven climate warming is driving changes in the ocean that are epic in the context of Earth history,” he said. “They’re commensurate with some of the biggest, most fundamental reorganizations of the life support system of the planet. The scenario is unlikely, yet action remains stalled.”

March 9, 2018