Not since an asteroid wiped out the dinosaurs — along with at least half of all other beings on Earth — has life in the ocean been so at risk.

Warming waters are cooking creatures in their own habitats. Many species are slowly suffocating as oxygen leaches out of the seas. Even populations that have managed to withstand the ravages of overfishing, pollution and habitat loss are struggling to survive amid accelerating climate change.

If humanity’s greenhouse gas emissions continue to increase, according to a study released Thursday, roughly a third of all marine animals could vanish within 300 years.

The findings, published in the journal Science, reveal a potential mass extinction looming beneath the waves. The oceans have absorbed a third of the carbon and 90 percent of the excess heat created by humans, but their vast expanse and forbidding depths mean scientists are just beginning to understand what creatures face there.

Yet the study by Princeton University earth scientists Justin Penn and Curtis Deutsch also underscores how much marine life could still be saved. If the world takes swift action to curb fossil fuel use and restore degraded ecosystems, the researchers say, it could cut potential extinctions by 70 percent.

“This is a landmark paper,” Malin Pinsky, a Rutgers University biologist who did not contribute to the paper, said in an interview. “If we’re not careful, we’re headed for a future that I think to all of us right now would look quite hellish. … It’s a very important wake-up call.”

The world has already warmed more than 1 degree Celsius (1.8 degrees Fahrenheit) since the preindustrial era, and last year the oceans contained more heat energy than at any point since record-keeping began six decades ago.

These rising ocean temperatures are shifting the boundaries of marine creatures’ comfort zones. Many are fleeing northward in search of cooler waters, causing “extirpation” — or local disappearance — of once-common species.

Polar creatures that can survive only in the most frigid conditions may soon find themselves with nowhere to go. Species that can’t easily move in search of new habitats, such as fish that depend on specific coastal wetlands or geologic formations on the sea floor, will be more likely die out.

Using climate models that predict the behavior of species based on simulated organism types, Deutsch and Penn found that the number of extirpations, or local disappearances of particular species, increases about 10 percent with every 1 degree Celsius of warming.

The researchers tested their models by using them to simulate a mass extinction at the end of the Permian period, when catastrophic warming triggered by volcanic eruptions wiped out roughly 90 percent of all life on Earth. Because the models successfully replicated the events of 250 million years ago, the scientists were confident in their predictions for what might happen 300 years in the future.

Penn and Deutsch’s research revealed that most animals can’t afford to lose much more than 50 percent of their habitat — beyond that number, a species tips into irreversible decline. In the worst-case emissions scenarios, the losses would be on par with the five worst mass extinctions in Earth’s history.

These changes are already starting to unfold. In the 1980s, a heat wave in the Pacific eliminated a small, silvery fish called the Galapagos damsel from the waters off Central and South America. A hot spot along the coast of Uruguay has driven mass die-offs of shellfish and widespread shifts in fishermen’s catch. Japanese salmon fisheries have plummeted as sea ice retreats and warmer, nutrient-depleted waters invade the region.

The danger of warming is compounded by the fact that hotter waters start to lose dissolved oxygen — even though higher temperatures speed up the metabolisms of many marine organisms, so that they need more oxygen to live.

The ocean contains just one-sixtieth as much oxygen as the atmosphere, even less in warmer areas where water molecules are less able to keep the precious oxygen from bubbling back into the air. As global temperatures increase, that reservoir declines even further.

The heating of the sea surface also causes the ocean to stratify into distinct layers, making it harder for warmer, oxygenated waters above to mix with the cooler depths. Scientists have documented expanding “shadow zones” where oxygen levels are so low that most life can’t survive.

Deoxygenation poses one of the greatest climate threats to marine life, said Deutsch, one of the study’s co-authors. Most species can expend a bit of extra energy to cope with higher temperatures or adjust to rising acidity. Even some corals have found ways to keep their calcium carbonate skeletons from eroding in more acidic waters.

“But there’s no price organisms can pay to get more oxygen,” Deutsch said. “They’re just sort of stuck.”

This climate-driven marine die-off is just one piece of a broader biodiversity crisis gripping the entire globe. A recent report from the Intergovernmental Panel on Climate Change found that warming has already contributed to the disappearance of at least 400 species. A separate U.N. panel has found that about 1 million additional species are at risk of extinction as a result of overexploitation, habitat destruction, pollution and other human disruption of the natural world.

A comprehensive new assessment published Wednesday in the journal Nature showed that more than 20 percent of reptile species could vanish. Turtles and crocodiles are most at risk, with more than half of each group at least vulnerable to extinction in the near future.

The consequences for communities that rely on reptiles for food, pest management, culture and other services could be profound.

“If we start messing up ecosystems and the services they provide, it has knock-on effects,” said assessment co-author Neil Cox, manager of the biodiversity assessment unit at the International Union for Conservation of Nature. “I think threats to biodiversity are as severe as climate change, we’re just underestimating them.”

Yet the two crises are closely intertwined, added Blair Hedges, an evolutionary biologist at Temple University and contributor to the reptile assessment. Climate change can accelerate the demise of populations already destabilized by habitat degradation or hunting. Ecosystems that lose key species may be less able to pull carbon out of the atmosphere or buffer against climate impacts.

The researchers highlighted the plight of the Virgin Gorda least gecko, a thumbnail-size reptile that dwells in moist pockets of soil on Caribbean hillsides. The creation of national parks on islands where the gecko is found helped avert habitat loss that could have doomed the species. But now its home is drying out from climate change, once again raising the specter of extinction.

“If you have multiple threats … working together, often even when you think one of them is under control, then the other one turns out to be even more of a threat,” Hedges said.

Though the danger to animals — and the humans who depend on them — is undeniably dire, Pinsky, the Rutgers biologist, urged against giving in to despair.

In an analysis for Science that accompanied Penn and Deutsch’s report, he and Rutgers ecologist Alexa Fredston compared marine animals to canaries in a coal mine, alerting humanity to invisible forces — such as dangerous carbon dioxide accumulation and ocean oxygen loss — that also threaten our ability to survive. If people can take action to preserve ocean wildlife, we will wind up saving ourselves.

“It’s scary, but it’s also empowering,” Pinsky told The Post.

“What we do today and tomorrow and the rest of this year and next year can have really important consequences,” he added. “This is not ‘once in a lifetime’ but maybe ‘once in a humanity’ moment.”