Sindya N. Bhanoo, The New York Times 18 Nov 09;
The Earth’s oceans, which have absorbed carbon dioxide from fuel emissions since the dawn of the industrial era, have recently grown less efficient at sopping it up, new research suggests.
Emissions from the burning of fossil fuels began soaring in the 1950s, and oceans largely kept up, scientists say. But the growth in the intake rate has slowed since the 1980s, and markedly so since 2000, the authors of a study write in a report in Thursday’s issue of Nature.
The research suggests that the seas cannot indefinitely be considered a reliable “carbon sink” as humans generate heat-trapping gases linked to global warming.
The slowdown in the rise of the absorption rate resulted from a gradual change in the oceans’ chemistry, the study found. “The more carbon dioxide the ocean absorbs, the more acidic it becomes and the less carbon dioxide it can absorb,” said the study’s lead author, Samar Khatiwala, a research scientist at the Lamont-Doherty Earth Observatory of Columbia University and a professor at the Georgia Institute of Technology.
“It’s a small change in absolute terms,” Dr. Khatiwala said. “What I think is fairly clear and important in the long term is the trend toward lower values, which implies that more of the emissions will remain in the atmosphere.”
To calculate the slowdown, Dr. Khatiwala and his collaborators created a mathematical model using tens of thousands of measurements of seawater collected over the past 20 years, including temperature, salinity and the presence of manufactured chlorofluorocarbons as a reflection of industrial activity.
They then worked backward with the data to create a formula that estimated the accumulation of human-generated carbon dioxide in the oceans from 1765, the opening of the industrial era, to 2008.
Even as human-generated emissions of carbon dioxide increase, the oceans’ uptake rate growth appears to have dropped by 10 percent from 2000 to 2007, Dr. Khatiwala said.
The last major research effort to measure industrial carbon uptake in the oceans was published in a 2004 Science study led by Christopher Sabine.
His methodology was different but arrived at similar conclusions.
Dr. Sabine used carbon dioxide measurements taken by more than 100 cruise ships to come up with a single figure: the oceans’ total industrial carbon uptake until 1994.
Dr. Khatiwala’s approach provides estimates of ocean carbon storage for every year from 1765 to 2008.
“Sabine’s estimate was like a single fuzzy snapshot,” Dr. Khatiwala said. “We’ve gone from that to having a relatively short movie of what happened from the start of the industrial era.”
Dr. Sabine said he agreed with the analogy, pointing out that his estimate for uptake up to 1994 was very close to Dr. Khatiwala’s for that period.
“Even though the techniques are completely different, they are in consensus at the one point that we can compare them,” Dr. Sabine said.
Yet much work remains to be done to confirm the results and to expand upon them, Dr. Khatiwala said.
Ocean Losing Its Appetite for Carbon
Christine Dell'Amore, National Geographic News 18 Nov 09;
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.
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.
"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 absorbtion 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.
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 means 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."
