The ocean is becoming more acidic at a rate unprecedented in at least 300 million years, driven by the absorption of excess carbon dioxide from the atmosphere. Since the Industrial Revolution, ocean pH has dropped by approximately 0.1 units, a seemingly small number that actually represents a 26% increase in hydrogen ion concentration. This chemical shift, known as ocean acidification, threatens marine ecosystems from coral reefs to polar waters, disrupting the biological processes that sustain fisheries, coastal communities, and the ocean’s critical role in regulating Earth’s climate.
The Chemistry of Ocean Acidification
When carbon dioxide dissolves in seawater, it undergoes a series of chemical reactions that increase the water’s acidity. CO2 reacts with water to form carbonic acid (H2CO3), which rapidly dissociates into bicarbonate ions (HCO3⁻) and hydrogen ions (H⁺). The increased hydrogen ion concentration lowers pH, the measure of acidity. On top of that, the excess hydrogen ions react with carbonate ions (CO3²⁻) to form more bicarbonate, reducing the availability of carbonate ions that marine organisms need to build shells and skeletons.
The ocean absorbs approximately 25% of anthropogenic CO2 emissions, roughly 22 million tonnes per day. While this absorption slows the rate of atmospheric CO2 increase (and therefore the pace of global warming), it comes at a steep ecological cost. Current atmospheric CO2 concentrations of approximately 425 parts per million have already pushed ocean pH to 8.1, down from the pre-industrial value of 8.2. Under continued high-emission scenarios, ocean pH could drop to 7.7-7.8 by 2100, a level not experienced in tens of millions of years.
The saturation state of calcium carbonate minerals, particularly aragonite and calcite, the building blocks of shells and coral skeletons, decreases as pH falls. When seawater becomes undersaturated with respect to these minerals, calcium carbonate structures begin to dissolve. Polar and deep waters, which are naturally colder and hold more CO2, are approaching or have already crossed undersaturation thresholds, creating “corrosive” conditions for shell-building organisms.
Impacts on Marine Organisms
Calcifying organisms, those that build shells or skeletons from calcium carbonate, are most directly threatened by ocean acidification. Corals, the architects of the most biodiverse marine ecosystems on Earth, face a double threat: acidification reduces their ability to build limestone skeletons, while warming causes bleaching events that expel their symbiotic algae. Together, these stresses have caused widespread coral decline across the tropics, with the Great Barrier Reef experiencing unprecedented mass bleaching events.
Shellfish, including oysters, mussels, clams, and scallops, show reduced calcification rates, thinner shells, and increased mortality under acidified conditions. Oyster hatcheries on the US Pacific coast experienced catastrophic larvae die-offs in the late 2000s that were directly linked to upwelling of acidified deep water. The billion-dollar shellfish industry across North America, including significant Canadian operations in British Columbia, New Brunswick, and Prince Edward Island, faces growing climate risk.
Pteropods, tiny swimming sea snails called “sea butterflies”, are among the most vulnerable organisms. These important food web organisms show shell dissolution in waters that are already undersaturated with aragonite in the Southern Ocean and Arctic. Their decline would cascade through marine food webs, affecting fish, seabirds, and marine mammals that depend on them as prey.
Fish are affected through multiple pathways. Elevated CO2 impairs sensory function and decision-making in fish larvae and juveniles, potentially affecting predator avoidance, homing behavior, and habitat selection. Acidification can alter the acoustic properties of seawater, affecting fish communication and navigation. Metabolic costs increase as fish expend energy maintaining internal acid-base balance, reducing energy available for growth and reproduction.
Ecosystem-Scale Consequences
Coral reef ecosystems face existential threat from the combination of acidification and warming. Reefs support approximately 25% of all marine species despite covering less than 1% of the ocean floor. At atmospheric CO2 concentrations above 450 ppm, most tropical reefs are projected to shift from net carbonate accretion (growing) to net dissolution (eroding), a tipping point that could be reached within decades under current emission trajectories.
Cold-water coral ecosystems in deep waters off Canada’s Atlantic coast are particularly vulnerable because deep waters acidify faster than surface waters. These ecosystems, which provide habitat for commercially important species including redfish, Greenland halibut, and deep-sea shrimp, may experience corrosive conditions decades before tropical systems.
Marine food webs face reorganization as acidification-sensitive species decline and more tolerant species expand. Some studies suggest that certain algae, seagrasses, and jellyfish may benefit from elevated CO2, potentially creating ecosystems dominated by these groups at the expense of calcifiers. Such shifts would profoundly alter fisheries productivity, coastal protection (reefs buffer wave energy), and ocean carbon cycling.
Canadian Waters at Risk
Canada’s three ocean coastlines, Pacific, Atlantic, and Arctic, each face distinct acidification challenges. The Arctic Ocean is acidifying fastest because cold water absorbs more CO2 and because sea ice loss is exposing previously protected waters to atmospheric CO2 absorption. Some Arctic waters are already undersaturated with aragonite, threatening the base of Arctic food webs that support Indigenous harvesting of fish, shellfish, and marine mammals.
British Columbia’s Pacific coast experiences seasonal upwelling that brings naturally CO2-rich deep water to the surface. Climate change is intensifying this upwelling, delivering increasingly acidified water to productive coastal ecosystems. The Fraser River estuary, one of the most productive salmon habitats in the world, is particularly vulnerable because freshwater input further reduces buffering capacity.
Atlantic Canada’s productive coastal waters support major fisheries for lobster, crab, scallops, and groundfish, all potentially affected by acidification. The Gulf of St. Lawrence, a semi-enclosed sea with limited deep-water circulation, is accumulating CO2 and showing accelerating acidification trends that concern fisheries scientists.
Monitoring and Research
Canada’s Ocean Acidification Research program monitors pH, carbonate chemistry, and biological indicators across all three oceans. The Ocean Networks Canada observatory maintains continuous monitoring stations providing real-time ocean chemistry data. Fisheries and Oceans Canada conducts regular surveys that now include ocean acidification parameters alongside traditional biological and physical measurements.
Laboratory and field experiments are improving our understanding of species-specific and ecosystem-level responses to acidification. Mesocosm experiments, large enclosed water volumes where CO2 levels can be manipulated, provide insight into community-level responses. Free-Ocean CO2 Enrichment (FOCE) systems bring experimental manipulation to natural seafloor environments. AI-driven analysis of the enormous datasets generated by monitoring networks is accelerating discovery of acidification trends and biological responses.
Solutions and Mitigation
The only effective long-term solution to ocean acidification is reducing CO2 emissions. Every tonne of CO2 not emitted is a tonne that does not acidify the ocean. The transition to renewable energy, electrified transportation, and carbon capture all directly benefit ocean chemistry by reducing atmospheric CO2 accumulation.
Local mitigation strategies can help protect vulnerable ecosystems while global emissions are reduced. Reducing nutrient pollution (which exacerbates coastal acidification through eutrophication), protecting seagrass beds and kelp forests (which locally reduce CO2 through photosynthesis), and managing marine protected areas to maintain ecosystem resilience all provide near-term benefits. Ocean alkalinity enhancement, adding alkaline minerals to seawater to increase its capacity to absorb CO2 without pH decline, is being researched as a potential larger-scale intervention, though ecological risks require careful assessment.
Ocean acidification is often called climate change’s “evil twin”, less visible than warming but potentially just as devastating. The chemistry is unambiguous, the biological evidence is mounting, and the trajectory is clear. Protecting the ocean’s chemistry is inseparable from protecting the climate, and both depend on the same fundamental action: rapidly reducing humanity’s carbon dioxide emissions.
