Climate change could cause a mass marine extinction event, study says

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.”

How protecting the ocean can save species and fight climate change

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….