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Not Just a Drop in the Ocean

At first scientists considered it a good thing—the oceans of the world absorbing between 25% to 30% of the carbon dioxide produced by our consumption of coal, oil, and natural gas, thus mitigating the various consequences of climate change. Instead of the atmosphere trapping 100% of the released carbon dioxide, the world’s forests and Ocean absorbed more than half of it, and as a result, global warming, in particular, could proceed at a slower (if nonetheless inexorable) pace.

But as they analyzed more and more data, scientists realized that decades of carbon-dioxide absorption was actually “changing the chemistry of the seawater, [in] a process called Ocean Acidification,” a chemical reaction linked to climate change and posing a danger as potentially calamitous as global warming.

To appreciate the ramifications of ocean acidification, it’s important to consider that:

• the Ocean covers more than 70% of the planet’s surface and is the most complex ecosystem on Earth
• all life began in the Ocean as a result of the not-yet-completely-understood chemical processes that took place at ocean depths some 38 billion years ago
• the complex food web in the Ocean, one that sustains not only us but more than half the world’s plant and animal life, has evolved and thrived in an environment with a pH level that has averaged 8.2 for the last 300 million years
• in just the last 200 years, the pH level of the Ocean has dropped from 8.2 to 8.1

Now, a pH level change of just 0.1 might not seem like a particularly significant drop, but as Elizabeth Kolbert explains in her pulitzer-prize-winning book, The Sixth Extinction: An Unnatural History,

Like the Richter scale, the pH scale is logarithmic, so even such a small numerical difference represents a very large real-world change. A decline of .1 means that the oceans are now thirty percent more acidic than they were in 1800. Assuming that humans continue to burn fossil fuels, the oceans will continue to absorb carbon dioxide and will become increasingly acidified. Under what’s known as a “business as usual” emissions scenario, surface ocean pH will fall to 8.0 by the middle of this century, and it will drop to 7.8 by the century’s end. At that point, the oceans will be 150 percent more acidic than they were at the start of the industrial revolution.

While it appears that some marine organisms will actually benefit from the Ocean’s increasing acidity levels—sea grasses, for example—many others may suffer significant losses.

Among the first victims of decreasing pH levels are likely to be those that depend on calcium carbonate: Coral Reefs and many shelled organisms, including oysters, mussels, urchins, and starfish. “Reef-building corals,” we learn from the Smithsonian’s Ocean Portal, “craft their own homes from calcium carbonate, forming complex reefs that house the coral animals themselves and provide habitat for many other organisms.” But scientists have found that increasing Ocean acidity levels not only weaken existing coral skeletons but also slows the growth of new skeletal material, making the reef more susceptible to erosion and predation.

Like corals, such marine animals as mussels, oysters, urchins, and starfish also depend on calcium carbonate to create their skeletons, and increased acidity levels will not only weaken their shells (their primary protection against predation) but also expose them to other risks. For example:

Mussels’ byssal threads, with which they famously cling to rocks in the pounding surf, can’t hold on as well in acidic water. Meanwhile, oyster larvae fail to even begin growing their shells. In their first 48 hours of life, oyster larvae undergo a massive growth spurt, building their shells quickly so they can start feeding. But the more acidic seawater eats away at their shells before they can form; this has already caused massive oyster die-offs in the U.S. Pacific Northwest.

Such die-offs not only affect the health of the oyster population itself (an important cash crop in Washington and other coastal communities) but also the health of the creatures further up the food chain—comb jellies, fish, birds, blue crabs, snails, sea stars, and sea otters—that rely on oysters as a food source.

Ocean acidification also disrupts the web of life in polar regions, causing shell dissolution in another important prey species. According to the National Oceanic and Atmospheric Administration (NOAA),

The pteropod, or “sea butterfly,” is a tiny sea creature about the size of a small pea. Pteropods are eaten by organisms ranging in size from tiny krill to whales and are a major food source for North Pacific juvenile salmon. When pteropod shells were placed in sea water with pH and carbonate levels projected for the year 2100, the shells slowly dissolved after 45 days. Researchers have already discovered severe levels of pteropod shell dissolution in the Southern Ocean, which encircles Antarctica. Pteropods are small organisms, but imagine the impact if they were to disappear from the marine ecosystem!

Besides disrupting skeletal growth in shelled sea organisms, increasing Ocean acidity has affected some fish species, as well. Scientists have found that changes in pH levels disrupt “the ability of larval clownfish to locate suitable habitat. When subjected to lower pH levels, the larval clownfish lost their chemosensory ability to distinguish between their favored and protective anemone habitat among the reefs and unfavorable habitats like mangroves.” The fish also find it more difficult to tell the difference between predators and other clownfish, putting the youngsters at an increased danger of predation. Rising ocean acidity has a similar effect on Pollock, an enormously important species to commercial fisheries in the Pacific Northwest. By reducing the Polock’s ability to detect predators, it puts not only the fish but an entire industry at risk.

On the East Coast of the United States, meanwhile, scientists have identified an entirely different consequence of Ocean Acidification that affects silver baitfish, a species that surfaces to “gulp at oxygen-rich waters.” Climate Central’s John Upton explains that: “As carbon dioxide dissolves into oceans and makes them more acidic, less oxygen is dissolving into warming waters. The research showed that these two chemical consequences of climate change could conspire to affect the surface-breathing behavior of some fish.” Silver baitfish

“are superabundant — everything else eats them,” said Seth Miller, an ecologist at the Smithsonian Environmental Research Institute who led the research, published Tuesday in the journal Marine Ecology Progress Series. “What happens to them really affects what happens to the rest of the food web.”

With an ecosystem as large, complex, and as little understood as the Ocean, few (if any) solutions to the problem of ocean acidification are likely to surface any time soon. But we can’t afford to underestimate the danger it poses to life on this planet.

From the bottom of the food chain—tiny Pteropods and larval oysters—to the apex predators—sharks, Orca, whales, and humans—Ocean Acidification poses a threat as potentially lethal as the anoxia that spread throughout the Ocean during the end-Permian extinction.

You can learn more about Ocean Acidification and its consequences for the world’s food supply by reading some of the following articles: