Ocean acidification doesn’t get the attention it deserves. It’s overshadowed by its more visible cousin, ocean warming, and it lacks the dramatic imagery of bleached coral or plastic-choked turtles. But among marine scientists, acidification is increasingly viewed as one of the most serious long-term threats to ocean ecosystems - and one of the hardest to reverse.

The basic chemistry is well understood. The ocean absorbs roughly 25-30% of the carbon dioxide humans emit into the atmosphere. When CO2 dissolves in seawater, it reacts with water to form carbonic acid. This lowers the pH of the water - making it more acidic - and reduces the concentration of carbonate ions that marine organisms need to build shells and skeletons.

Since the start of the industrial era, ocean surface pH has dropped from approximately 8.2 to 8.1. That 0.1 unit decrease represents a roughly 30% increase in acidity (pH is logarithmic). It’s the fastest rate of ocean chemistry change in at least 50 million years.

What’s Happened Since 2024

Several significant research findings have been published in the past two years.

A 2025 study published in Nature Climate Change found that acidification is proceeding faster in polar regions than previously modelled. Cold water absorbs more CO2 than warm water, so the Arctic and Southern Oceans are acidifying roughly 50% faster than tropical waters. This has direct implications for polar marine food webs, which depend on pteropods (tiny swimming snails) and other organisms with calcium carbonate shells.

The pteropod findings are particularly concerning. Research from the British Antarctic Survey showed that Antarctic pteropod shells are now measurably thinner than specimens collected in the 1980s. Pteropods are a critical food source for fish, seabirds, and whales in polar ecosystems. Their decline would cascade through the entire food web.

Closer to home, monitoring by CSIRO and the Integrated Marine Observing System (IMOS) shows that Australian waters are tracking global acidification trends. The waters around Tasmania and the Great Australian Bight are acidifying fastest among Australian marine regions, consistent with the global pattern of higher acidification in cooler waters.

The Coral Question

Acidification’s relationship with coral reefs is complex and not as straightforward as it’s sometimes presented.

Corals build their skeletons from aragonite, a form of calcium carbonate. As ocean acidity increases and carbonate ion concentrations decrease, it becomes energetically more expensive for corals to build and maintain their skeletons. Think of it like trying to build a sandcastle while someone is slowly dissolving the sand.

However, the impact on any given coral species depends on several factors: the species’ physiology, the local water chemistry, temperature, light levels, and the coral’s evolutionary history. Some coral species have shown surprising resilience to moderate acidification in laboratory experiments, while others are acutely sensitive.

A 2025 meta-analysis in Science reviewed 94 experimental studies on coral responses to acidification. The findings were sobering but nuanced. At pH levels projected for 2100 under high-emission scenarios (around 7.8), most coral species showed significant reductions in calcification rates - typically 20-40% decreases. But at moderate acidification levels projected for 2050 (pH around 7.95), the effects were more variable, with some species maintaining near-normal calcification rates.

The catch is that acidification doesn’t happen in isolation. Corals on the Great Barrier Reef are simultaneously dealing with warming (which causes bleaching), pollution, sedimentation, and storm damage. The interaction between acidification and warming appears to be worse than either stressor alone - a finding that AIMS researchers have documented in field studies across the reef.

Shell-Building Organisms

Beyond coral, acidification threatens a wide range of organisms that build shells or skeletons from calcium carbonate.

Oysters are among the most studied. The Pacific oyster industry in the US Pacific Northwest experienced severe production losses in the late 2000s when acidified upwelling waters made it difficult for oyster larvae to form their initial shells. The industry adapted by monitoring water chemistry and timing larval production to avoid the most acidic conditions, but it was a $110 million wake-up call about the economic costs of acidification.

Australian oyster growers haven’t experienced the same acute impacts yet, but monitoring programs are in place. The Australian Shellfish Quality Assurance Program now includes pH monitoring in several growing regions.

Sea urchins show reduced shell growth and weakened spines under acidified conditions. Research published in early 2026 from the University of Sydney demonstrated that purple sea urchins (Heliocidaris erythrogramma) raised in acidified water developed shells that were 15-25% weaker than those raised in normal conditions. Given that sea urchins play a critical role in controlling algae growth on reefs, their decline could trigger algal overgrowth that smothers coral.

Foraminifera - tiny shelled organisms that form a significant component of marine sediments and the base of many ocean food webs - are showing measurable shell thinning in response to acidification. A 2025 study from the Australian National University found that foraminifera shell mass in the Southern Ocean has decreased by approximately 30% since the pre-industrial era.

What Can Be Done

Addressing ocean acidification fundamentally requires reducing CO2 emissions. There’s no other way to stop the chemistry. As long as atmospheric CO2 concentrations rise, the ocean will continue absorbing CO2 and becoming more acidic.

Some researchers are exploring localised interventions. Adding alkaline minerals (such as olivine or limestone) to seawater can locally increase pH and carbonate ion availability. Several pilot projects are underway globally, including one in the Great Barrier Reef Marine Park that’s testing whether adding dissolved alkalinity to reef waters can improve coral calcification rates.

These interventions are experimental and face significant scaling challenges. The ocean is enormous. Treating localised areas might help protect specific high-value ecosystems (like oyster beds or reef sections), but it’s not a solution to the global problem.

Other approaches include breeding or selecting for acidification-tolerant strains of commercial shellfish species, developing early warning systems for acidification events in coastal waters, and incorporating pH monitoring into marine protected area management.

The Monitoring Gap

One significant challenge in understanding ocean acidification is the lack of comprehensive monitoring. We have far fewer pH measurement stations than temperature stations, and long-term time series of ocean pH data are rare outside a few well-studied locations.

In Australia, IMOS operates a network of ocean monitoring stations, but pH measurements are not consistently included at all sites. Expanding this monitoring network is critical for tracking acidification trends in Australian waters and providing data for management decisions.

Some research groups are working with AI consultants in Sydney and other technology partners to develop lower-cost pH sensors that could be deployed more widely, and machine learning models that can predict local pH conditions from satellite-observable variables like sea surface temperature and chlorophyll concentrations. This kind of predictive modelling could fill gaps in the monitoring network, though it’s still in early stages.

Looking Ahead

Ocean acidification is often called “the other CO2 problem,” and that framing captures both its importance and its neglect. While climate warming dominates public discussion and policy attention, acidification is quietly changing the fundamental chemistry of the ocean at a rate that has no precedent in the geological record.

The organisms most affected - corals, shellfish, pteropods, foraminifera - aren’t just ecologically important. They’re economically important. Global shellfish aquaculture is a $30 billion industry. Coral reef tourism generates billions more. The economic case for addressing acidification is strong, even before you account for the ecological value of healthy ocean ecosystems.

The science is clear. The trajectory is clear. What’s less clear is whether the pace of emissions reduction will be fast enough to prevent the worst outcomes. Current emissions pathways suggest that ocean pH could drop another 0.1-0.3 units by 2100, depending on the scenario. The lower end of that range is challenging but potentially manageable for many species. The upper end would fundamentally alter marine ecosystems in ways we’re only beginning to understand.