In Warming World Oceans Risk Mass Extinctions, Model Shows

At first, the scientists chose a straightforward title for their research: “Marine Extinction Risk From Climate Warming.”

But as publication approached, something nagged at them. Their findings illustrated two drastically different outcomes for ocean life over the next three centuries depending on whether greenhouse gas emissions were sharply curbed or continued apace. Somehow it seemed the study’s name conjured only doom.

“We were about to send it in and I thought, ‘Gee, it sounds like a title that only has the dark side of the result,'” said Curtis Deutsch, a professor of geosciences at Princeton University who studies how climate change affects the ocean. “Not the bright side.”

So he and his co-author, Justin L. Penn, added an important word they hoped would highlight their finding that the grim scenario outlined by their results could still be, well, avoided.

On Thursday they published “Avoiding Ocean Mass Extinction From Climate Warming” in Science. It is the latest research that crystallizes the powerful yet paralyzed moment in which humanity finds itself. The choices made today regarding greenhouse gas emissions stand to affect the very future of life on Earth, even though the worst impacts may still feel far away.

Under the high emissions scenario that the scientists modeled, in which pollution from the burning of fossil fuels continues to climb, warming would trigger ocean species loss by 2300 that was on par with the five mass extinctions in Earth’s past. The last of those wiped out the dinosaurs.

“It wasn’t an ‘Aha’ moment per se,” said Dr. Penn, a postdoctoral researcher at Princeton, recalling the first time he looked at a graph comparing those past extinctions with their grim forecast. “It was more of an ‘Oh my God’ moment.”

On the other hand, reining in emissions to keep within the upper limit of the Paris climate agreement would reduce ocean extinction risks by more than 70 percent, the scientists found. In that scenario, climate change would claim about 4 percent of species by the end of this century, at which point warming would stop.

“Our choices have huge impacts,” Dr. Deutsch said.

While there is broad consensus that a shift away from coal and expanded wind and solar energy make the worst-case scenario, unlikely oil and gas use continues to increase and the world is not on track to meet the lower-emissions scenario modeled by the scientists .

The new study builds on Dr. Deutsch and Dr. Penn’s earlier work: creating a computer simulation that detailed the worst extinction in Earth’s history some 252 million years ago. Often called “the Great Dying,” it claimed more than 90 percent of species in the oceans. The cause was global warming, triggered by volcanic eruptions. The oceans lost oxygen, and fish succumbed to heat stress, asphyxiation or both. The computer model found more extinctions at the poles as compared with the tropics, and the fossil record confirmed it.

To forecast the effects from global warming that is now driven by human activity, the scientists used the same model, with its intricate interplay between sunlight, clouds, ocean and air currents, and other forces like the chemical dances between heat and oxygen, water and air. They also took into account how much fish habitats could shift, estimating thresholds for survivability.

“It’s a lot of time spent on the computer,” Dr. Penn said.

While the study focused on the effects of warming and oxygen loss, ocean acidification and other snowball effects could worsen the species loss it predicted.

The ocean has long acted as a quiet safeguard against climate change, absorbing vast amounts of the carbon dioxide and trapped heat as people burned fossil fuels and razed forests. But that service has come at a cost. Last year, the ocean reached its highest temperature and lowest oxygen content since humans started keeping track. Changes to the ocean’s chemistry are already threatening fish. Coral reefs are in steep decline.

‘How screwed are we?’ I get that all the time,” Dr. Deutsch said. “If we don’t do anything, we’re screwed.”

Nations are still far from taking the necessary steps to prevent catastrophic climate change. Last month the secretary of the United Nations, António Guterres, general that a critical goal — restricting average global warming to 1.5 degrees Celsius (2.7 degrees Fahrenheit) since preindustrial times — was “on life support.”

The International Energy Agency, a group created to ensure a stable worldwide energy market, said last year that countries must immediately stop approving new fossil fuel projects. They have not stopped, and Russian’s invasion of Ukraine has added to calls for more drilling in the name of energy security.

In an interview, Drs. Deutsch and Penn said they feel like the ignored scientists in “Don’t Look Up,” the recent movie in which a comet hurtling toward Earth is a metaphor for climate change. As in the film, the planet is at a pivotal moment, giving people living today outsized power in determining the future.

“Great power brings great responsibility,” Dr. Deutsch said. “And we’re learning about our power, but not about our responsibility — to future generations of people, but also to all the other life that we’ve shared the planet with for millions of years.”

Pippa Moore, a professor of marine science at Newcastle University in England who studies the impacts of climate change on the ocean and was not involved with the study, called it comprehensive.

“This paper adds to the huge body of evidence that unless more is done to curb our greenhouse gas emissions, our marine systems are of course to see a massive shift in where marine species live and, as shown in this paper, significant extinction events that could rival previous mass extinction events,” she said.

Brad Plumer contributed reporting.

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