14-06-2025
Yikes: We Just Crossed a Planetary Boundary 66 Million Years in the Making
Here's what you'll learn when you read this story:
Planetary boundaries outline how far past pre-industrial conditions Earth can get before anthropogenic activity has detrimental effects on the environment.
While we have been dangerously close to the ocean acidity boundary for some time, it is now being breached, and the shells of some organisms are dissolving because of chemical reactions that deprive them of calcium carbonate.
The boundary for ocean acidity was originally at 20% post-industrial, but researchers have now moved it back to 10% as a call to action before entire ecosystems start to disappear.
When the asteroid that triggered the end-Cretaceous Mass Extinction crashed to Earth (and decimated the dinosaurs), it hit rocks that released sulfur and caused pH levels in the oceans to plummet. Ocean acidity levels became intolerable and led to more than half of all marine life dying out. 66 million years later, it's happening again.
This time, there is no asteroid—just us humans. Anthropogenic activity such as deforestation and the burning of fossil fuels has released enormous amounts of emissions, primarily in the form of carbon dioxide. That CO2 has caused ocean pH levels to fall off a cliff and has lead to a 30% increase in acidity, which underwater habitats ranging from coral reefs to the deep ocean.
The planetary boundary for ocean acidification—the limit of what Earth can tolerate before the onset of destructive consequences—is a 20% drop in the concentration of calcium carbonate (a common base often found in things like limestone and seashells) from pre-industrialization levels. It was already looming by 2020, but until recently, we had not breached.
Now, however, researchers at the UK's Plymouth Marine Laboratory (PML) are nervous. In an effort led by biological oceanographer Helen Findlay (who is also Chair off the North East Atlantic Acidification Hub and an Executive Council Member for the Global Ocean Acidification Observing Network), experts found that the ocean acidification boundary had already been crossed by as much as 60% of subsurface ocean waters located 200 meters (about 656 feet) below the surface.
'The planetary boundaries assessment defines nine large scale Earth-system processes and associated boundaries that, if crossed, could generate unacceptable environmental change,' Findlay and her team said in a study recently published in the journal Global Change Biology.
Creatures that make their own shells through calcification rely on calcium and carbonate molecules already floating around in the ocean. Too much carbon dioxide can throw off this process. When CO2 is absorbed by seawater, it reacts with water molecules to form carbonic acid (H2CO3), which easily breaks apart into hydrogen (H+) ions and bicarbonate (HCO3-) ions. The lonely hydrogen ions lower the pH of water, and the bicarbonate ions bring their own problems. Species that make their own shells need carbonate ions (CO32-) to bond to calcium, but they can't access them if they're already locked up in bicarbonates. So, as the amount of CO2 in the ocean goes up, the number of available carbonate ions gos down, and the organisms end up in a sticky spot.
And the problems don't stop there. As rising temperatures heat up the ocean, warmer oceans hold less oxygen. This warmer water is more buoyant and does not mix as well with deeper, colder and more oxygenated water, so the shorter supply is used up faster. More oxygen breathed in than replenished leads to a deficit of oxygen known as hypoxia. Less oxygen means calcifying organisms have to use more energy to build and maintain their shells, the additional exposure to conditions with low oxygen can be even more dangerous for them. Low enough pH levels can cause shells and exoskeletons to actually dissolve, which is what led Findlay to suggest that the boundary of 20% less calcium carbonate (than existed in pre-industrial times) should be reset to 10%. This adjustment should give the ocean life affected a chance to recover and flourish again.
And if carbon emissions continue to rise, thing will continue to look bleaker and bleaker for marine life. A separate but alarming NOAA experiment showed that some species of pteropods—tiny mollusks (also known as sea butterflies for the wing-like appendages they use to swim) that produce their own shells—could soon find themselves unable to maintain the shells they need to survive. In the study, researchers placed pteropod shells in water with carbonate levels adjusted to reflect projected carbonate levels for the year 2100. The shells dissolved after only 45 days. And even in today's oceans, pteropod shells off the coast of Antarctica have already been found to be dissolving.
It might seem inconsequential for such small creatures to vanish, but the reality is that pteropods—along with other organisms considered zooplankton—form the base of an extensive food web that could suffer immensely if disrupted. Organisms that do not calcify will feel the effects of ocean acidification in other ways. If there is a significant enough change in ocean chemistry, for example, it will become difficult for some species of fish to detect predators.
Findlay also found that the polar oceans have experienced the most significant change at the surface level, but the most unnerving shifts in the subsurface have happened in low-latitude and subpolar regions. If deep-water corals are not able to build exoskeletons, entire ecosystems that depend on them for food and shelter could be wiped out. And ecosystem loss could lead to entire populations ending up isolated in smaller areas where they are more vulnerable to dying out.
'The main advancement lies in shifting from an assessment based primarily on the changing chemistry to a more holistic approach that considers uncertainties, regional variations, subsurface impacts and the biological consequences of exceeding the boundary,' she said.
There may be no extinction-level asteroid headed for Earth anytime soon, but if carbon emissions continue at the current rate, we could be creating a lethal asteroid effect of our own.
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