It’s Not Too Late to Stop Mass Extinction in the Ocean

A quarter of a billion years ago, things were not going well on planet Earth. That’s putting it mildly. Back then the planet was in the middle of the worst mass extinction event ever—much worse than the one that wiped out the non-avian dinosaurs 66 million years ago. During this earlier Permian-Triassic extinction event, some 70 percent of land species met their end. Greenhouse gases released by volcanic eruptions in Siberia poured into the skies, cooking the Earth and causing acid rain to fall back down onto the land. Things were so grim that paleontologists have termed this mass extinction the Great Dying.In the ocean, the situation was even more dire. Temperatures around the tropics shot up by 10 degrees Celsius and deep-sea currents slowed down, which starved the oceans of oxygen. Less than 5 percent of all marine species made it through the Great Dying. It would take tens of millions of years for life in the ocean to recover from rock bottom and return to its previous levels of diversity. Trilobites, a huge group of underwater creatures that had existed in the oceans for more than 250 million years, were wiped out altogether. On land, the dopey-looking Lystrosaurus rapidly spread over the newly barren planet.For oceanographers Curtis Deutsch and Justin Penn, the Great Dying can tell us a lot about where our present-day planet could be headed if we don’t get a handle on climate change.“These environmental changes are also happening in the modern ocean today,” says Penn, a research associate at Princeton University’s Department of Geosciences and coauthor of a new paper published in the journal Science. The overall oxygen content in the ocean has already fallen by around 2 percent since the mid-20th century. This led Deutsch and Penn to a natural question: If greenhouse gas emissions precipitated ocean extinctions in the distant past, what level of extinctions might climate change lead us to?To figure this out, the scientists looked at two future emissions scenarios. In one of them, fossil fuel emissions rapidly increase—way beyond current expected trends—and lead to warming of around 4.9 degrees Celsius by 2100. In the other scenario, lower emissions keep temperature increases to just under 2 degrees Celsius by the end of the century. It’s worth noting that this high-emissions projection is an unlikely worst-case scenario—it would require huge increases in coal use, even though such activity peaked in 2013. If countries stick to current policies, it’s more likely we’re headed for 2.7 degrees Celsius of warming, and if they uphold pledges made at COP26, humanity may be able to keep warming under 2 degrees Celsius .“There is still a huge range of possible futures,” says Deutsch. “We wanted to best bracket the plausible range of futures without being overly extreme in either direction.”The scientists used these two emissions scenarios to estimate what would happen to oxygen demand and supply in the ocean. Like us, marine animals need to breathe oxygen to survive, but higher temperatures reduce the amount of oxygen seawater can hold and slow down currents that would usually circulate oxygen between the surface and deep ocean. At the same time, warmer temperatures increase the amount of oxygen creatures need to go about their business. This increased demand and decreased availability of oxygen is thought to be one of the major reasons so many marine species died out during the Permian-Triassic extinction.