The oceans just hit an ominous milestone

You’re the product of stability on a planetary scale. Around 12,000 years ago, Earth warmed from an ice age into the relatively consistent climate that allowed humans to adopt agriculture, literally putting down roots. That stability, though, is now shattered, as more than 8 billion people rapidly heat the planet, ravage its ecosystems, and plunder its resources. 

In a new report, scientists warn that we’ve crossed yet another “planetary boundary,” a threshold that keeps Earth’s systems hospitable to life — a sort of global resilience that allows the planet to absorb shocks. This time, it’s the relentless acidification of the seas that’s crossed into dangerous territory, threatening all manner of marine life, including the organisms at the base of the food web. Of the nine total planetary boundaries, this is the seventh that’s been breached. 

“What this health check again and again shows is that we have one interlinked, interconnected Earth system,” said Levke Caesar, co-lead of the Planetary Boundaries Science Lab at the Potsdam Institute for Climate Impact Research and a co-author of the report. “It actually would be fatal if we just concentrate on climate change, because, as we see, there are six other boundaries that have been transgressed. And we’re actually also increasing the pressure on all of these seven boundaries.”

Think of a planetary boundary as a warning sign on a road. At the end of the road is a cliff, representing a tipping point, in which an Earth system dramatically changes, often irreversibly. Researchers are worried, for instance, that parts of the Amazon may be nearing a transformation from rainforests to savannas, due to the compounding crises of climate change and deforestation. If that’s the cliff, the concept of a planetary boundary is a big yellow “CLIFF AHEAD” sign, a warning from scientists that we could be approaching a catastrophic shift. 

Or to use another metaphor: A planetary boundary is your doctor warning you that you have high blood pressure, and the tipping point is the ensuing heart attack. “We are setting the planetary boundaries in such a way that as long as we are within the safe operating space, we should prevent the crossing of tipping points,” Caesar said.

Before ocean acidification was added to the list this year, the researchers warned that six other boundaries have already been crossed: The climate is changing rapidly; humans are using too much freshwater; we’re driving species to extinction and transforming the biosphere; forests are shrinking; fertilizers are polluting water bodies; and “novel entities” like chemicals and plastics are streaming into the environment. 

Ocean acidification is intimately intertwined with the planetary boundary of climate change because seawater absorbs carbon dioxide from the atmosphere, and indeed has soaked up a quarter of humanity’s CO2 emissions. That’s helped keep the planet from warming even faster, but also creates carbonic acid. Accordingly, the report notes that ocean acidity has grown 30 to 40 percent since widespread burning of fossil fuels started in the industrial era.

This is perilous news for oceanic lifeforms. Many organisms, like corals, mollusks, and crustaceans, build shells for protection, but carbonic acid reduces the amount of calcium carbonate available for them to do so. And what they do manage to build will be continuously eroded by the increasing acidity of seawater, requiring them to add new layers to their homes to keep up. “Acidification affects a lot of marine organisms physiologically,” said Peter Roopnarine, curator of geology at the California Academy of Sciences, who wasn’t involved in the report. “Whether that’s for a properly functioning respiratory system, or for building an external or internal skeleton. It affects everything from the larvae, or the development of these organisms, all the way up to the adults.”

Acidification and rising water temperatures also cause coral bleaching, when the organisms get stressed and release the symbiotic algae that provide them energy. Because scientists have evidence that organisms are already being harmed in this way, the new report confirms that we’ve crossed the planetary boundary for ocean acidification. “There’s some real concern that as our water acidifies, that there will be fairly significant ecosystem impacts to the marine calcifying species,” said Jennie Rheuban, a research specialist at the Woods Hole Oceanographic Institution, who wasn’t involved in the report.

This acidification could also shake the very foundation of the oceanic food web. Phytoplankton are tiny photosynthetic organisms that soak up CO2 and expel oxygen, just like plants on land. Indeed, these plankton gobble up half of the CO2 sequestered worldwide by photosynthesis, and produce half of Earth’s atmospheric oxygen. 

Unfortunately, many species of phytoplankton build shells, and may struggle as the oceans relentlessly acidify. These organisms sequester loads of carbon and serve as a critical food source for small creatures known as zooplankton, which in turn are consumed by larger animals like fish. 

But not all oceans are acidifying the same way. Colder locales, like the Arctic or Southern Ocean that encircles Antarctica, more readily acidify than tropical waters. That means the polar regions — which may be remote but host a bevy of species like whales — could be approaching a tipping point where organisms aren’t able to build shells. “The Southern Ocean is about to tip,” said Ken Johnson, a senior scientist who studies the region at the Monterey Bay Aquarium Research Institute but wasn’t involved in the report. “The water is changing faster. We’re creeping right up to the edge of that tipping point.”

Further complicating matters, planetary boundaries often reinforce one another. In addition to acidification, ocean life is dealing with the breaching of the climate boundary, as the seas have absorbed around 90 percent of the excess heat that humans have added to the atmosphere. That’s steadily raised the average ocean temperature, but is also driving extreme marine heat waves, further stressing the organisms there. (Oxygen concentrations also drop as temperatures rise, compounding their pain.) They’re also dealing with the “introduction of novel entities” boundary being breached, as chemicals and plastic pollution injure and kill oceanic species. 

Those overlapping crises are not only making it more difficult for organisms to survive, but may also be making it more difficult for the ocean to keep sequestering carbon. That’s what Caesar and her colleagues will be exploring next. “What is happening under these different stressors?” Caesar said. “Because it’s not just ocean acidification. The ocean is also losing oxygen. It’s warming. How is this impacting this ocean buffering capacity?”

The good news is that by rapidly reducing greenhouse gas emissions, we can pull back from the cliff of ocean acidification. In fact, human intervention is why two of the nine planetary boundaries haven’t been crossed. Levels of ozone in the stratosphere are now within safe levels, the report notes, thanks to international agreements banning ozone-depleting substances. And atmospheric aerosols are also within the safe operating space, thanks to countries better regulating the burning of dirty fuels like coal and switching to renewable power like solar and wind. 

The number of overlapping and reinforcing problems makes the ocean a particular challenge. But by doing things like restoring coastal ecosystems and reducing plastic pollution, humans can reduce the pressures we’re putting on them. “That can be really hard to tackle, addressing these problems when there isn’t sort of a silver bullet where you change one thing and you fix all the problems,” Rheuban said. “There’s quite a number of different issues that need to be solved.”