“Pteropods in Peril” is not the stuff of headlines, nor have Fabry’s findings grabbed our attention like the plight of the polar bears. Yet the loss of pteropods would impact our lives much more directly. Puny though they are, pteropods are a major food source of some of the biggest cash cows in the sea—salmon, herring, cod, and pollack. A significant decline in their population, Fabry says, could have grave economic consequences.
What do pteropods eat? Put a drop of seawater on a slide under a microscope and you will see: amoebas, tiny crustaceans, and plankton, many of which also sport shells. A major thrust of current research is to understand how creatures like these at the bottom of the food chain respond to ocean acidification. Toward that goal, scientists have scooped samples of seawater from a variety of latitudes and studied the rich broth of microorganisms they contain in simulator tanks built into the decks of ships. “The idea is to keep the specimens as fresh as possible in their natural habitat,” explains David Hutchins, a biological oceanographer at the University of Southern California in Los Angeles. Then the simulation trials begin: The temperature, pH, and CO2 levels of the tank are adjusted to mimic conditions expected a century from now.
What can Hutchins discern about the future from these simulations? There will be winners and losers, but the overall picture is, he says, “frightening.”
As the temperature and acidity of a test tank climb, diatoms that dominate the cold northern oceans fall off steeply in number—an ominous sign, given that they currently support by far the richest fisheries in the world. The Bering Sea alone generates about 30 percent of the global harvest of seafood. In the frigid southern oceans, plankton species are different, but some have shells, and the trend is the same: Their populations rapidly decline. At both poles, organisms in decline are being replaced by plankton called flagellates. According to Hutchins, flagellates are not nearly as good at passing their stored energy up the food chain to fish and other higher life forms. “That’s going to disrupt food chains that sustain the kinds of creatures we’re used to seeing at the poles—sea lions, penguins, and whales—and instead promote a microbe-dominated community,” he says.
The Great Belch of Destruction
The anticipated impact on wildlife resembles a game of dominoes: After acidification has destabilized one species or ecosystem, the damage could ripple up and down the food chain. Especially worrisome is the fact that the shelled plankton under threat are efficient at storing CO2. When the creatures that eat the plankton die, their shells and organic remains fall to the ocean floor, sequestering carbon in the deep water and sediments. “Cold-water planktons are powerful allies in preventing atmospheric CO2 from climbing higher than it already is,” Hutchins says.
Therefore, their rapid decline could quickly turn the planet hotter. “Currently the ocean is a sink for CO2—that is, it takes in more CO2 from the atmosphere than it releases,” Hutchins explains. “But a warming and acidifying ocean could become a net source of CO2.” In other words, the world’s seas could begin belching the gas into the atmosphere, just as our cars and factories do. In his opinion, that could unfold within a few centuries. “It’s hard not to be negative about this,” Hutchins says. “Frankly, ocean acidification is apocalyptic in its impact.”
Robert Cowen, chairman of the division of marine biology and fisheries at the Rosenstiel School of Marine and Atmospheric Science, agrees, but for a different reason. His chief concern is fish populations, which were in steep decline even before ocean acidification was recognized. In just the past 40 years, overfishing, destructive trawling, and poor management of the seas have depleted 75 percent of our commercially important fish stocks, with almost one-third of them—including tuna, marlin, and shark—under particular threat. “We’re hammering fish from the top down and now from the bottom up as more acidic oceans erode the base of the food chain,” Cowen says.
It was at a conference two years ago, Cowen adds, that the scale of the disaster unfolding at sea really hit him. Deeply disturbed, he and his wife, Su Sponaugle, also a marine biologist at Rosenstiel, soon realized they would have to tone down how they talked about the research in front of their adolescent twins. “They overheard one of our conversations and started asking questions like ‘What’s going to happen?’?” Sponaugle recalls. “We could see their distress and hear the agitation in their voices, and then they wanted to know, ‘Is it too late?’ and we’re like, ‘Hmm…well…’?”
What Sponaugle and Cowen didn’t want to say—or couldn’t find the way to say—was yes, it might be too late. You can’t turn an ocean liner on a dime, and in their view, it will take a complete about-face in society’s profligate use of fossil fuels to avert a catastrophe. Nor are they alone in that opinion. “If we were to begin to reduce man-made emissions this year,” NOAA’s Feely says, “it would take decades before we’d see CO2 levels and acidity start to go down instead of up and hundreds or thousands of years to return to preindustrial levels.”
Very simply, the process by which the ocean normally maintains its chemical equilibrium is glacially slow, severely limiting its capacity to adjust to an extreme shock. And make no mistake: The massive influx of industrial emissions is just that.
Over the history of the planet, there have been many sudden peaks in CO2 related to volcanic eruptions, releases from hydrothermal vents, and other natural events. When the pH of the ocean dips as a result of absorbing this excess gas, bottom sediments rich in calcium carbonate begin to dissolve, countering the increase in acidity. This buffering process occurs over 20,000 years, roughly the time it takes for water to circulate along the bottom from the Atlantic to the Pacific and back up to the surface several times. Currently, however, we are pouring man-made CO2 into the atmosphere at 50 times the natural rate. “That overwhelms the natural buffering system for maintaining balance in ocean chemistry,” the Carnegie Institution’s Caldeira says. “To find any parallel in the earth’s history you would have to look to a sudden violent shock to the system far in the geologic past.”
One such event occurred 55 million years ago at the so-called Paleocene-Eocene Thermal Maximum (PETM), when 4.5 million tons of greenhouse gases were released into the atmosphere. Just what triggered this enormous emission is not known, but scientists suspect volcanic activity may have begun the process. That may in turn have caused the planet to heat up enough to melt deposits of methane frozen in sediments on the ocean floor (something, incidentally, that could happen again), discharging even more potent greenhouse gases into the atmosphere and further heating the planet in an escalating feedback loop.
Whatever the exact cause of the CO2 release at the PETM, the earth warmed faster than at almost any other time in its history. The average temperature soared 9 degrees Fahrenheit, entire ecosystems shifted to higher latitudes, and massive extinctions occurred on land and, most telling, at sea. The abrupt rise of CO2 acidified the oceans. James Zachos, a paleo-oceanographer from the University of California at Santa Cruz, analyzed sediment cores obtained from deep drilling in the ocean and discovered that bottom-dwelling creatures with shells disappeared from the fossil record for a period of more than 40,000 years corresponding to the PETM. And once the oceans turned more acidic, Zachos says, they did not recover quickly: It took another 60,000 years before sediments again began to show a thick white streak indicative of fossilized shells.
Drastic as the PETM was, the event is tame compared with acidification today. “Back then,” Zachos says, “4.5 million tons of CO2 were released over a period of 1,000 to 10,000 years. Industrial activities will release the same amount in a mere 300 years—so quickly that the ocean’s buffering system doesn’t even come into play.”
This is not to imply that current CO2 emissions are likely to kill off all life in the sea. Microbes, with their rapid generation times, should evolve and ultimately persist in altered seas. But slower-to-reproduce creatures such as fish and other higher organisms will struggle to survive. “The marine ecosystem will adapt,” USC’s Hutchins believes. Life may be different, but it will go on.
Kleypas of NCAR stands out among marine biologists in her optimism that we will be able to stop the output of man-made CO2 in time to prevent irreparable harm to the marine ecosystem. To do that, she acknowledges, will take incredible sacrifice and an overhaul of infrastructure on an unprecedented scale. “I know people think I’m crazy,” she says, “but we’re the only species that can change our behavior overnight.”