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.
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, the 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.
The ocean contains just one-60th 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.
“If we start messing up ecosystems and the services they provide, it has knock-on effects,” said 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.”
The researchers highlighted the plight of the Virgin Gorda least gecko, a thumbnail-sized 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, raising the specter of extinction once again.