The New York Times, Nov. 19, 2009
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.
Science Daily, Nov. 19, 2009
The oceans play a key role in regulating climate, absorbing more than a quarter of the carbon dioxide that humans put into the air. Now, the first year-by-year accounting of this mechanism during the industrial era suggests the oceans are struggling to keep up with rising emissions -- a finding with potentially wide implications for future climate. The study appears in the November 19 issue of the journal Nature.
The researchers estimate that the oceans last year took up a record 2.3 billion tons of CO2 produced from burning of fossil fuels. But with overall emissions growing rapidly, the proportion of fossil-fuel emissions absorbed by the oceans since 2000 may have declined by as much as 10%.
Some climate models have already predicted such a slowdown in the oceans' ability to soak up excess carbon from the atmosphere, but this is the first time scientists have actually measured it. Models attribute the change to depletion of ozone in the stratosphere and global warming-induced shifts in winds and ocean circulation. But the new study suggests the slowdown is due to natural chemical and physical limits on the oceans' ability to absorb carbon -- an idea that is now the subject of widespread research by other scientists.
"The more carbon dioxide you put in, the more acidic the ocean becomes, reducing its ability to hold CO2" said the study's lead author, Samar Khatiwala, an oceanographer at Columbia University's Lamont-Doherty Earth Observatory. "Because of this chemical effect, over time, the ocean is expected to become a less efficient sink of manmade carbon. The surprise is that we may already be seeing evidence for this, perhaps compounded by the ocean's slow circulation in the face of accelerating emissions."
The study reconstructs the accumulation of industrial carbon in the oceans year by year, from 1765 to 2008. Khatiwala and his colleagues found that uptake rose sharply in the 1950s, as the oceans tried to keep pace with the growth of carbon dioxide emissions worldwide. Emissions continued to grow, and by 2000, reached such a pitch that the oceans have since absorbed a declining overall percentage, even though they absorb more each year in absolute tonnage. Today, the oceans hold about 150 billion tons of industrial carbon, the researchers estimate--a third more than in the mid-1990s.
For decades, scientists have tried to estimate the amount of manmade carbon absorbed by the ocean by teasing out the small amount of industrial carbon -- less than 1 percent -- from the enormous background levels of natural carbon. Because of the difficulties of this approach, only one attempt has been made to come up with a global estimate of how much industrial carbon the oceans held -- for a single year, 1994.
Khatiwala and his colleagues came up with another method. Using some of the same data as their predecessors -- seawater temperatures, salinity, manmade chlorofluorocarbons and other measures -- they developed a mathematical technique to work backward from the measurements to infer the concentration of industrial carbon in surface waters, and its transport to deep water through ocean circulation. This allowed them to reconstruct the uptake and distribution of industrial carbon in the oceans over time.
Their estimate of industrial carbon in the oceans in 1994 -- 114 billion tons -- nearly matched the earlier 118 billion-ton estimate, made by Chris Sabine, a marine chemist at the National Oceanic and Atmospheric Organization in a 2004 paper in the journal Science.
Sabine, who was not involved in the new study, said he saw some limitations. For one, he said, the study assumes circulation has remained steady, along with the amount of organic matter in the oceans. "That being said, I still think this is the best estimate of the time variance of anthropogenic CO2 in the ocean available," said Sabine. "Our previous attempts to quantify anthropogenic CO2 using ocean data have only been able to provide single snapshots in time."
About 40 percent of the carbon entered the oceans through the frigid waters of the Southern Ocean, around Antarctica, because carbon dioxide dissolves more readily in cold, dense seawater than in warmer waters. From there, currents transport the carbon north. "We've suspected for some time that the Southern Ocean plays a critical role in soaking up fossil fuel CO2," said Khatiwala. "But our study is the first to quantify the importance of this region with actual data."
The researchers also estimated carbon uptake on land, by taking the known amount of fossil-fuel emissions and subtracting the oceans' uptake and the carbon left in the air. They were surprised to learn that the land may now be absorbing more than it is giving off.
They say that until the 1940s, the landscape produced excess carbon dioxide, possibly due to logging and the clearing and burning of forests for farming. Deforestation and other land-use changes continue at a rapid pace today -- but now, each year the land appears to be absorbing 1.1 billion tons more carbon than it is giving off.
One possible reason for the reversal, say the researchers, is that now, some of the extra atmospheric carbon -- raw material for photosynthesis--may be feeding back into living plants and making them grow faster. "The extra carbon dioxide in the atmosphere may be providing a fertilizing effect," said study coauthor Timothy Hall, a senior scientist at NASA's Goddard Institute for Space Studies. Many other scientists are now working to determine the possible effects of increased carbon dioxide on plant growth, and incorporate these into models of past and future climates.
Khatiwala says there are still large uncertainties, but in any case, natural mechanisms cannot be depended upon to mitigate increasing human-produced emissions. "What our ocean study and other recent land studies suggest is that we cannot count on these sinks operating in the future as they have in the past, and keep on subsidizing our ever-growing appetite for fossil fuels," he said.