Ocean Losing Its Appetite for Carbon

Christine Dell'Amore
National Geographic News
November 18, 2009
The world's oceans, which normally gobble up carbon dioxide, are getting stuffed to the gills, according to the most thorough study to date of human-made carbon in the seas.

Between 2000 and 2007, as emissions of the potent greenhouse gas carbon dioxide skyrocketed, the amount of human-made carbon absorbed by the oceans fell from 27 to 24 percent.

In terms of ocean processes, "that's a pretty large drop, and the trend is pretty clear: The ocean can't keep up with [human-made carbon]," said study leader Samar Khatiwala, an oceanographer at Columbia University's Lamont-Doherty Earth Observatory.

(Related: Find out how Earth's "carbon bathtub" is in danger of overflowing.)

Khatiwala is careful to point out that the total uptake of carbon is not declining—the rate is just not growing as fast as it used to.

But if the oceans continue to be overwhelmed by carbon, more of the gas will remain in the already warming atmosphere, the authors say. (See global warming fast facts.)

"Ultimately the ocean is what's controlling what's going on here," said Chris Sabine, a supervisory oceanographer at NOAA's Pacific Marine Environmental Laboratory in Seattle, Washington, who was not involved in the research.

"It's a big deal that it's becoming less efficient in taking up CO2."

Working Backward

Carbon dioxide easily dissolves in seawater, so the oceans act as gigantic carbon sinks.

The blue part of our planet currently stockpiles about 150 billion tons of carbon. In 2008 the oceans sucked up 2.3 billion tons of carbon—about six years' worth of U.S. gasoline consumption, Khatiwala said.

For their study, Khatiwala and colleagues collected data on seawater temperature and salinity recorded from 1765 to 2008.

The team also gathered data on amounts of ocean pollutants called chlorofluorocarbons. These chemicals act as "tracers," allowing the scientists to figure out the time it takes for a substance to go from the surface of the ocean to the interior.

Based on this data, the team created a mathematical technique that allowed them to "work backward" to determine how much human-made carbon has entered the ocean over the years.

The researchers found that when human-made carbon dioxide began increasing dramatically in the 1950s, the oceans began absorbing more of that carbon.

But in recent decades the rate of absorption has declined, and the reasons for the slowdown are still unclear.

It might have something to do with increased carbon dioxide emissions making seawater more acidic, the authors say. That's because more acidic waters are less able to dissolve carbon dioxide.

Likewise, carbon dioxide can't dissolve as easily in warmer water—which is why about 40 percent of past carbon emissions were absorbed into the chilly oceans off Antarctica, according to the study, published tomorrow in the journal Nature.

(Related: "Antarctic Oceans Absorbing Less CO2, Experts Say.")

Tremendous Service

The new results complement earlier observational studies of carbon dioxide in the oceans, including a 2004 Science paper by NOAA's Sabine and colleagues.

But Sabine cautioned that the new study doesn't take into account biological processes.

For instance, tiny algae called phytoplankton take up carbon dioxide through photosynthesis. When phytoplankton die, their remains drift to the depths and decompose—a natural cycle that keeps carbon trapped on the seafloor for centuries.

So far, scientists have assumed that this process hasn't really changed due to global warming, said study-co author Timothy Hall, a senior scientist at NASA's Goddard Institute for Space Studies.

But it's possible that warming seawater could set off a chain of effects that might damage the natural cycle, Hall said.

The oceans circulate water globally via a series of "pumps" that cause cold, dense waters to sink and nutrient-rich waters to rise.

Some of the pumps have not been functioning as well in recent years, leading scientists to speculate that warming surface temperatures may be to blame.

Less ocean mixing could mean that fewer nutrients from the deep ocean are rising up to sustain phytoplankton, Hall said. Fewer phytoplankton mean less photosynthesis, which could lead to more carbon dioxide in the atmosphere.

"The oceans are performing a tremendous service for humankind," NOAA's Sabine said.

"If we throw [the oceans' carbon uptake] out of whack … the potential is there to completely overwhelm what we're trying to do with limiting our fossil fuel emissions."

© 1996-2008 National Geographic Society. All rights reserved.