Virtual Lab · Marine Biology

Ocean Acidification and Coral Reefs

Explore how rising carbon dioxide is changing ocean chemistry — and what that means for one of Earth’s most important ecosystems.

What Is Ocean Acidification?

When we burn fossil fuels — coal, oil, and natural gas — we release carbon dioxide (CO₂) into the air. You probably already know that CO₂ is a greenhouse gas that warms the planet. But here’s something you might not know: the ocean absorbs about 30% of all the CO₂ we release. That might sound helpful, but it comes at a serious cost.

The Chemical Reaction:
When CO₂ dissolves in seawater, it forms carbonic acid(H₂CO₃) – A weak acid formed when carbon dioxide dissolves in water; plays a key role in blood. That acidA substance that releases hydrogen ions (H⁺) in solution. releases hydrogen ionsCharged atoms or molecules. (H⁺), which lower the water’s pHA measure of hydrogen ion concentration in a solution.. Lower pH = more acidicA solution with a pH below 7, having a higher concentration of H⁺ ions.. This process is called ocean acidification.

Since the Industrial Revolution (around 1850), ocean pH has already dropped from 8.2 to 8.1. That sounds tiny — but the pH scale is logarithmic, meaning that drop represents a 26% increase in acidity. Scientists predict that if CO₂ emissions continue at the currentThe flow of electrical charge, as in ions moving across a neuron’s membrane. rate, ocean pH could reach 7.95 by 2100.

Why Do Coral Reefs Care?

The problem with ocean acidification is that it directly attacks aragonite. As seawater absorbs CO₂ and becomes more acidic, the concentration of carbonate ions drops — because those ions react with the extra hydrogen ions flooding the system (this is exactly what the bicarbonate(HCO₃⁻) – A crucial buffer in blood that helps maintain pH balance; formed when carbon dioxide buffer slider shows). When carbonate ions become scarce, corals struggle to build new skeleton. Even more alarming is a concept called the aragonite saturation state — a measure of how “comfortable” the water chemistry is for aragonite to exist. When the saturation state drops below 1.0, aragonite doesn’t just stop forming; it actually begins to dissolve. Scientists are watching portions of the polar oceans approach this thresholdThe minimum voltage needed to trigger an action potential. already, and tropical reef zones are expected to follow later this century under high-emission scenarios.

This is why ocean acidification is sometimes called “the other CO₂ problem.” Even if water temperatures stayed perfectly stable — no bleaching, no warming — acidification alone could undermine reef structure by slowing skeletal growthAn increase in size and number of cells., weakening existing skeletons, and making reefs erode faster than they can rebuild. Some researchers describe it as slowly dissolving the foundation of the house while the coral is still trying to live in it. Aragonite is a powerful concrete example of how a chemistry equation on a whiteboard — the bicarbonate buffer systemThe main buffer in ECF, involving HCO₃⁻ and H₂CO₃. — translates directly into the collapse of an ecosystem that a quarter of all marine species call home.

Scale of the Crisis: Coral reefs cover less than 1% of the ocean floor, yet they support about 25% of all marine species. They protect coastlines from storm waves, provide food for over 500 million people, and generate billions of dollars in tourism each year. Their loss would be catastrophic.

How Scientists Test This

Scientists can’t wait 80 years to see what happens at pH 7.92 — so they run experiments in the lab and in special outdoor tanks called mesocosms. Here’s how they test the effect of acidification on coral:

Example Experiment: Researchers placed coral fragments into tanks of seawater at five different pH levels: 8.20, 8.10, 8.00, 7.90, and 7.80. After 90 days, they measured how much calcium carbonate (skeleton material) each coral had added. They kept all other conditions — temperature, light, nutrients — exactly the same in every tank.

Real World Connections

Who Is Most at Risk?

Reef damage isn’t just an environmental issue — it’s a human one. Millions of people around the world depend directly on coral reefs for their food and livelihood.

• Southeast Asia & Pacific Islands: Over 275 million people live within 30 km of a reef and rely on reef fish as a primary protein source.
• Florida & the Caribbean: Reef-related tourism generates over $8 billion per year. Florida’s reef tract is the only living barrier coral reef in the continental U.S.
• Coastal protection: Reefs absorb 70–97% of wave energyThe capacity to do work or cause change. from storms. Without them, coastal flooding and erosion would increase dramatically.

Aragonite Saturation and pH
Satellite Data

1. Click on the + on the left-side menu.  Click Climate, Click on Simulations, Click on Aragonite Saturation. 
2. Zoom out to see the globe using the zoom tools on the right side of the screen.
3. On the left side of the screen there is a camera icon.  Click this icon and a map of aragonite saturation in 1861 downloads to your computer.  Below the menu with the camera icon is a menu with a slider for time.  Slide the slider to 2025 and download a picture.  Slide the slider to 2100 and download a picture.
4. Now, use the Back arrow at the top of the right side menu to go back and choose pH instead of aragonite saturation.  Take the same pictures at the same three time intervals.

Global Data

Is it too Late? Not necessarily — but the window to act is narrow. Research shows that if global CO₂ emissions are dramatically reduced by 2040, some reefs could partially recover. Some scientists are also studying assisted evolution — breeding heat- and acid-resistant corals — as a possible tool. But prevention is far more effective than any cure we currently have.

Coral Degree Heating Weeks