Silently and steadily, a tragedy is unfolding beneath the ocean's waves: Coral reefs around the world are disappearing. According to some projections, there could be few, if any, left by the end of the century.

This dire and credible prediction has shocked many marine scientists, who had not realized how close to the tipping point coral reefs are. The news is especially disheartening because 2008 is the International Year of the Reef.

The culprit here is carbon dioxide, the greenhouse gas that is responsible for global warming and that also is turning our oceans into an acid bath.

Remember your mother's warning that too much Coke would dissolve your teeth? Well, too much acid in the oceans prevents corals from growing their calciferous skeletons. In a December Science magazine article, researchers reported results of models in which they simulated the effects of carbon dioxide emissions over the next century. By 2050, the projections revealed, oceans will be too acidic for coral reefs to grow.

Why should we care if coral reefs continue to grow? After all, they cover only 0.1 percent of the Earth's surface. Unlike rain forests, they are tiny on a global scale.

In terms of biodiversity, however, coral reefs are the rain forests of the ocean. Reefs are home to between 1 million and 9 million species. Nobody knows the exact number, says Nancy Knowlton, a coral reef expert at the Smithsonian Institution in Washington, D.C., because scientists have only just begun to seriously map marine biodiversity. That's one of the goals of the Census of Marine Life being conducted by a network of researchers from more than 50 nations. If reefs disappear, at least half the species that live on them also might go extinct, according to the Science article.

Here's the problem. When carbon dioxide enters the ocean, it reacts with water to form carbonic acid. A few other chemical steps ensue, with the outcome that fewer carbonate ions are available for biological systems. Corals are not the only organisms that suffer. All shell-forming marine creatures are adversely affected.

Taking a human analogy, it would be as if your bones could no longer keep growing.

We are seeing the effects of ocean acidification. Today, the concentration of carbon dioxide in the Earth's atmosphere is more than 380 parts per million. That's more than at any time during the last 20 million years.

About 25 percent of this carbon dioxide ends up being absorbed by the oceans. As carbon dioxide levels have risen during the industrial era, the average pH level in the ocean, an indicator of acidity, has dropped by 0.1 pH unit. (On the pH scale, a lower number means more acidic.) That might not sound like much, but evidence from Antarctic ice cores shows that the global average is lower than at any time over almost half a million years. As the Science article notes, changes in atmospheric carbon dioxide over the last century "are two or three orders of magnitude higher than most of the changes seen in the past 420,000 years."

Until recently, many ocean scientists had imagined that as global temperatures rise, corals might begin to adapt. But acidification is a far more serious problem to these inherently delicate organisms. Knowlton says that "it's just not possible for organisms to adapt rapidly to such fundamental chemical changes in their environment." Imagine, by way of comparison, that you were suddenly told that instead of drinking water, you'd have to settle for Coke all the time.

The corrosive effects of acidification are evident in the Great Barrier Reef in the Coral Sea off Queensland, Australia. Here, massive Porites coral have experienced a 20 percent drop in growth in the last 16 years.

The best-case scenario from the Intergovernmental Panel on Climate Change, which tracks global warming, predicts that carbon dioxide concentrations in the atmosphere will rise to 450 ppm this century unless we change our consumption of fossil fuels quickly. Most models predict a rise to at least 500 ppm if we don't change our consumption habits. That will spell disaster for coral reefs.

Besides being enclaves of biodiversity, coral reefs perform other important functions. In Asia, coral reef-based fisheries provide one-quarter of the fish that help to feed a billion people.

They are also critical mechanisms for protection throughout the Earth's tropical regions. Without coral reef barriers, coastal areas will become more vulnerable to the kind of devastation caused by hurricanes like Katrina.

Finally, many developing nations — especially in the Caribbean — rely on reef tourism as a crucial part of their economies.

Brainless, immobile and with only the most primitive nervous systems, coral polyps have built some of the most magnificent structures on our planet. They protect us, feed us and astound us with their beauty. Now they need our help — and time is running out.

Margaret Wertheim is the co-creator, with her sister Christine, of the Crochet Coral Reef Project, now showing in New York. She wrote this commentary for the Los Angeles Times.