Even the cold, remote waters of Antarctica are no refuge from ocean acidification. Acidity in some places in the ocean around Antarctica could double compared to 1990 levels by the end of the century, according to new research. Even if emissions don’t continue their steep rise, ocean acidity is likely to be significantly higher on the Antarctic shelf than it is today, threatening many of the organisms that live there.
When ocean waters absorb carbon dioxide (CO2), their pH level goes down, meaning acidity increases. Although ocean acidification is a threat to waters worldwide, there hasn’t been a scientific consensus on how it might affect Antarctic continental shelf waters, which are largely covered by massive ice sheets. The new research, published in Nature Communications, paints a less-than-optimistic picture.
The new study reports that in three out of four emission scenarios in Antarctica, “[more acidic] waters are almost everywhere,” said study coauthor Cara Nissen, an ocean biochemical modeler at the University of Colorado Boulder.
Rising Risks to Marine Organisms
Before the Industrial Revolution, ocean water had a pH of about 8.2. Today, the world’s oceans sit at around 8.1. That may not sound significant, but it represents an increase in acidity of approximately 30%. (The pH scale is logarithmic, meaning that a drop of a single digit represents a tenfold increase in acidity levels.) Furthermore, even small changes can have big effects on creatures that are adapted to a specific pH level.
As acidity rises, levels of calcium carbonate in seawater decline, and marine organisms that build shells begin running out of material. The more acidic waters will also begin eating away at their shells, forcing the organisms to expend more energy to maintain them.
Acidity levels can affect nonshelled creatures as well, ratcheting up stress levels and disrupting their reproduction and development, especially when combined with other factors like warmer waters, pollution, and overfishing. For example, a study of Antarctic krill, a foundational component of the continent’s oceanic food web, showed that metabolism increases as acidity rises. Another study showed that the combination of temperature increases and acidification harms developing dragonfish.
“We know what ecosystem stressors matter. And we know that ocean acidification is a big one that the organisms care about,” Nissen said.
Oceanic Acidity Set to Steadily Increase
High-resolution modeling of ocean acidification around Antarctica was lacking, Nissen said, meaning there weren’t good estimates of what environmental changes delicate Antarctic ecosystems could soon face. In their study, Nissen and her coauthors used a computer model that simulates ocean biogeochemistry as CO2 levels rise both in the open ocean and in areas covered by sea ice. They fed it data from the Coupled Model Intercomparison Project Phase 6 (CMIP6) climate model that underpins the Intergovernmental Panel on Climate Change‘s global warming projections. Then, they ran their model on four different global warming scenarios, each assuming different levels of greenhouse gas emissions during this century.
The authors looked specifically at acidity levels in two existing and three proposed marine protected areas (MPAs) in Antarctica. The already-established areas, which currently cover more than 1.55 million square kilometers (600,000 square miles) of ocean around the continent, are protected from human activities such as fishing.
Under the most aggressive global warming scenario, pH in these regions could drop by 0.36—more than 3 times the pH decline since the Industrial Revolution. That amounts to acidity levels more than doubling. Even under intermediate scenarios, the ocean around Antarctica could be on track for an increase in acidity of between 40% and 80%, depending on the region and level of emissions. Only modeling of the lowest emission pathway held acidity level increases to moderate levels.
“It was surprising to us that even the intermediate emission scenarios showed that pronounced level of widespread ocean acidification,” Nissen said.
The study also points out that ocean acidification may put ecosystems on the Antarctic continental shelf at an inherently higher risk. That’s because cold, dense waters around Antarctica sink more readily to the seafloor, bringing lower pH levels down to where many marine organisms live. Fresh water from melting ice also has a lower buffering capacity than ocean water, meaning its acidity levels are more strongly affected by carbon absorption.
The new research seems to silence any hopes that the Antarctic shelf might be a place of refuge, said Clara Manno, a pelagic marine ecologist with the British Antarctic Survey who wasn’t affiliated with the research. “It tells you that [remote waters are] not a way to escape,” she said. Ocean acidification, she continued, is “very important. It’s everywhere.”
In addition, the rate at which acidity levels increase could be just as important as the size of the increase when it comes to organisms’ health, Manno explained. Faster increases caused by higher levels of CO2 in the atmosphere will make it more difficult for ocean life to adapt or move.
A Call for More Marine Protected Areas
The takeaways from the research are in line with long-standing recommendations for mitigating the impacts of climate change. First and foremost, keeping greenhouse gas emissions low is critical for ocean health, Nissen said. Protecting ocean waters from threats like pollution and fishing could also help the creatures there better cope with rising acidity levels.
“I think it’s all about reducing the stress on these ecosystems as much as possible,” Nissen said.
For that, she argues creating more MPAs will be key. These protected areas ensure ocean life is shielded from some anthropogenic threats and provide researchers areas for comparison when studying ecosystems. With enough protection, many organisms may be able to adapt to increased acidity. For now, only time will tell.
—Nathaniel Scharping (@nathanielscharp), Science Writer