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.
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."
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