Gaze down from the edge of a robust coral reef. The hues are truly striking: sea fans floating in a slow current, schools of fish passing through them like ribbons of light, and branching structures in orange and purple hues. A healthy coral reef has a higher density of life per square meter than practically any terrestrial ecosystem. The focus is on Amazon.
For the majority of people who haven’t seen one, reefs continue to be an abstract concept—something from a nature documentary, something distant and vaguely tropical. Because of this distance, it has become easier to ignore what is happening to them, slowly and continuously, in a process that doesn’t result in any noticeable smoke, audible crack, or single dramatic event that makes the evening news.
| Field | Details |
|---|---|
| Issue | Ocean Acidification — chemical change in seawater driven by CO2 absorption |
| CO2 Absorbed Since Industrial Era | ~525 billion tons; currently ~22 million tons per day |
| Ocean pH Change | Dropped from 8.2 to 8.1 since industrial revolution — 30% more acidic |
| Projected pH Drop by 2100 | Additional 0.3–0.4 units; ocean more acidic than any time in 20 million years |
| Term First Coined | 2003 — when rapid chemical shift drew sustained scientific attention |
| Primary Victims | Coral reefs, oysters, mussels, pteropods, certain plankton species |
| Coral Reef Coverage | Less than 1% of ocean floor; supports ~25% of all marine species |
| Scale of CO2 Dissolution | ~30% of all human-produced CO2 absorbed by the ocean |
| Economic Risk | Fisheries, aquaculture, coastal tourism — food security for hundreds of millions |
| Key Monitoring Body | NOAA — National Oceanic and Atmospheric Administration |
| Reference Website | Smithsonian Ocean – Ocean Acidification |
The claim that ocean acidification is the equally evil twin of climate change is true, but it may understate how very different the two issues are from one another. People can sense the effects of climate change through heat waves, changing seasons, and storms that arrive with unusual intensity. Only in 2003 did researchers studying seawater pH realize that the rate of change had accelerated far beyond what the natural buffering system could handle.
Since then, ocean acidification has been occurring almost entirely out of sight, in water chemistry, at a molecular level. The ocean has taken in about 525 billion tons of carbon dioxide from the atmosphere since the start of the industrial era, or about 22 million tons every day. Because CO2 remained in the ocean rather than warming the atmosphere, scientists initially viewed this absorption as a sort of unintentional benefit. What the ocean does with all of it was something they failed to take into consideration.
When carbon dioxide dissolves in seawater, it reacts with water molecules to produce carbonic acid, which is weaker than most acids but acts on a scale and over a period of time that makes its weakness irrelevant. The pH is lowered by the hydrogen ions that are released by the carbonic acid. Ocean surface pH has decreased from 8.2 to 8.1 since the industrial revolution.
On a logarithmic scale, a drop of 0.1 denotes a thirty percent increase in acidity. According to projections, if current emission trajectories continue, pH could drop by an additional 0.3 to 0.4 units by the end of this century, making ocean chemistry more acidic than it has been in the previous 20 million years. This would happen in about 200 years. Evolution is a slow process. There is no way for the organisms that have spent millions of years adjusting their biology to a stable ocean chemistry to keep up.
The chemistry issue with coral reefs is straightforward and structural. Calcification, the process by which corals construct their skeletons from calcium carbonate, is dependent on the presence of carbonate ions in the surrounding water in sufficient amounts. When pH falls, hydrogen ions form bonds with carbonate ions to transform them into bicarbonate, thereby eliminating them from the pool that corals require for growth.
Slower growth, weaker structures, and in certain documented instances, actual dissolution of existing reef material are the outcomes. Coral skeletons are growing more brittle, more prone to disease, and more susceptible to storm damage. Globally, the incidence of coral disease has increased eightfold over the last 20 years. This statistic, along with the more well-known bleaching events, is proof that reefs are under simultaneous pressure that they were never intended to withstand.
The same chemicals that affect corals are also affecting other calcifying organisms in ways that spread throughout the food chain and have less obvious but no less significant effects. Calcium carbonate is used by oysters, mussels, and some species of plankton known as pteropods, which are tiny, free-swimming sea snails that serve as the foundation of many marine food webs. Pteropod shells have already been seen to dissolve in the wild in waters off the Pacific Northwest coast.
Under a microscope, researchers have discovered shells that appear to have been etched from the outside—pitted, corroded, and structurally compromised—after removing these microscopic organisms from the water. Millions of people rely on commercially vital fish populations for protein, but these animals are at the base of a food chain that sustains everything above them. Since serious research on how marine life reacts to acidification only started in earnest around 2003, it’s still unclear how far the cascade extends. However, preliminary data suggests the effects don’t end at the shell.
The lack of public awareness of ocean acidification in relation to its magnitude is almost paradoxical. Wildfires, glaciers, and flooding coastlines are examples of the visual vocabulary of climate change. Acidification doesn’t. The color of the water remains unchanged. An acidifying ocean appears to be the same as a healthy one from the surface.
Damage is building up in the chemistry and biology, as evidenced by the progressively thinner shells and slower coral growth that only show up when a once-spectacular reef is reduced to a field of bleached rubble that scientists are photographing to document what once existed. Measurable effects are being seen in the Great Barrier Reef, the Caribbean reefs, and the cold-water reefs off the coasts of Norway and Scotland. Because all of Earth’s oceans absorb the same atmosphere, the issue is dispersed throughout them all.
Certain species may be able to adapt; organisms that live in naturally variable environments, such as estuaries, have demonstrated greater tolerance for pH shifts, and scientists are looking into the possibility of producing more acid-resistant varieties of commercially significant shellfish through selective breeding. In certain places, coral gardening initiatives—which involve growing heat-and acid-tolerant coral fragments in underwater nurseries and transplanting them to damaged reefs—are yielding truly encouraging outcomes.
However, the researchers in charge of these initiatives are generally honest about what they are: actions rather than solutions. As long as the atmosphere is changing, the ocean’s chemistry will also be changing. Time is bought by restoration. The underlying process is not reversed. There is a sense that the field is working harder and more quickly than the policy response it is attempting to inform as the science advances in tandem with the accumulation of damage.
There is no public relations department in the ocean. It doesn’t submit reports, host press conferences, or provide outward clues about what’s going on beneath the surface. The issue is that it simply continues to absorb 22 million tons of carbon dioxide every day, changing the chemistry and causing the reefs to thin.
