EXPLAINER - What is ocean acidification and why does it matter?

Ocean acidification, caused by rising CO₂, is silently reshaping marine life, threatening shellfish, coral reefs, and coastal communities. Scientists warn its impacts could ripple through food chains, economies, and human livelihoods unless urgent action is taken.

Ocean acidification is sometimes called “climate change’s invisible twin”, a hidden crisis happening beneath the waves. Human activities release carbon dioxide (CO₂) into the atmosphere. Around a third to a half of this CO₂ is absorbed by the oceans. While this slows atmospheric warming, it also chemically alters seawater, making it more acidic. Since the industrial era, ocean water has become 30% more acidic, faster than at any time in the last 50 million years.

“We’re watching the ocean’s foundation dissolve under our eyes,” says Dr. Jane Smith, a marine biologist at NOAA.

The increased acidity reduces carbonate availability, a critical building block that corals, oysters, mussels, and other marine organisms need to form shells and skeletons. Some species, such as pteropods (tiny sea snails), are already dissolving in Arctic waters. Even organisms that survive must expend extra energy to maintain their health, often at the cost of growth, reproduction, or survival.

“Mussels and sea urchins are literally dissolving their shells to survive the water’s acidity,” explains Dr. Robert Johnson, NOAA Ocean Acidification Program.

The struggle for marine life

Many ocean plants and animals are calcifying organisms, they rely on calcium carbonate to build shells and skeletons. Increased acidity slows this process, and under severe conditions, can dissolve shells faster than they form. Larvae are particularly vulnerable: sea urchin and oyster larvae may fail to develop properly, while fish larvae can lose their sense of smell, making them unable to avoid predators.

“It’s not just the animals, they’re part of a food chain that humans rely on,” Dr. Johnson adds. “If shellfish populations collapse, coastal communities lose both income and food security.”

Why it matters to humans

Ocean acidification threatens fisheries, aquaculture, tourism, and coastal protection. In the mid-2000s, the $117 million West Coast shellfish industry nearly collapsed due to acidified waters. Coral reefs weakened by acidity provide less storm protection and recover more slowly from damage. Acidification could also reduce other benefits that are difficult to value, from tourism opportunities to natural ecosystem services.

What’s causing this and what can be done

The rapid pace of change is alarming. NOAA’s global monitoring shows CO₂ levels rising faster than ever over the past three years, accelerating ocean acidification.

“Science isn’t just observing the problem, it’s helping communities survive it,” says Dr. Smith.

In response, ambitious measures have been taken: the United States has pledged to reduce emissions from 1,000 power plants by 32% by 2030, equal to removing the annual output of 150 million cars. Monitoring is crucial. NOAA co-leads the Global Ocean Acidification Observing Network, a 66-nation partnership tracking the ocean’s changing chemistry and identifying areas at greatest risk. Real-time data helps shellfish hatcheries schedule production safely and informs adaptation strategies in communities in Alaska and Maine.

Yale University

What we can expect in the future

If CO₂ emissions continue on the current trajectory, oceans could become 40% more acidic by 2100, with polar seas and upwelling regions acidifying fastest. While some algae and seagrass may benefit from higher CO₂, many species—including molluscs, corals, and plankton, will struggle to survive.

“Ocean acidification may be silent, but its effects will be loud—and felt by everyone,” warns Dr. Johnson.

Taking action

Smart investments in monitoring and observing are essential. We can’t manage what we don’t measure. NOAA’s efforts, including forecasting and early-warning systems for acidified water, are helping coastal industries and communities adapt. Public–private partnerships, robust forecasting capabilities, and ongoing research into biological and economic impacts are critical.

“The health of our oceans is directly tied to the health of our communities and economies,” says Dr. Smith. “Every ton of CO₂ we reduce helps protect marine life and human livelihoods.”

Ocean acidification is a global problem, but targeted action, research, and collaboration can make a difference. Understanding and acting on it now is key to ensuring oceans continue to provide food, protection, and livelihoods for generations to come.

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