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This article is from the In-Depth Report Copenhagen and Climate Change

Sinking Global Warming: Is There a Reliable Way to Track Atmospheric Carbon Dioxide Levels?

CO2 emissions rise as natural sinks slow, but how can scientists precisely track this greenhouse gas, especially in advance of a potential global treaty to reduce its emissions?
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The planet soaks up excess carbon dioxide via oceans, plants and soils, among other natural systems, locking away some of the greenhouse gases emitted by burning fossil fuels. In fact, every year these natural "sinks" absorb a larger and larger tonnage of emissions—but thanks to the increasing amount of CO2 and other greenhouse gases dumped in the atmosphere by human activity, the proportion that is reabsorbed is beginning to dwindle, according to new studies.

As efforts get underway to craft a global treaty that begins to reduce man-made greenhouse gas emissions, precise and accurate measurements of the emanations, along with their sources and sinks are increasingly important. Novel technologies as well as new scientific efforts are contributing to that project—although the recent loss of NASA's Orbiting Carbon Observatory caused a setback.

First and foremost, greenhouse gas emissions worldwide continue to grow, according to a new report from the Global Carbon Project (GCP) published online by Nature Geoscience on November 17. (Scientific American is part of the Nature Publishing Group.) From 2000 to 2008, such discharges jumped by 29 percent and, in spite of the onset of the Great Recession, still managed to rise 2 percent in 2008 alone (although they are expected to fall back to 2007 levels this year), according to oceanographer Corinne Le Quere of the University of East Anglia in England. In fact, global emissions from fossil-fuel burning and deforestation are now roughly 37 billion metric tons of CO2 per year—or 41 percent higher than in 1990.

Le Quere and her colleagues' research also hints that the portion of CO2 remaining in the atmosphere may be on the rise, as well—creeping from around 40 percent in 1959 to roughly 45 percent in 2008. That translates to roughly 14 billion metric tons of CO2 a year, or a 1.8-part-per-million increase in atmospheric concentration annually.

Part of the reason for that rise may be explained by a slowing in the amount of CO2 absorbed by the world's oceans, particularly the Southern Ocean surrounding Antarctica, whose frigid waters absorbed 40 percent of the nearly nine billion metric tons of CO2 sequestered by the seas last year. Reconstructing the record back to 1765, oceanographer Samar Khatiwala of Columbia University's Lamont–Doherty Earth Observatory and his colleagues estimate that the proportion of fossil-fuel emissions absorbed by the sea has declined 5 percent already in the first years of the 21st century, according to a new paper published November 19 in Nature.

This might indicate that the natural ocean sink—whose carbon dioxide uptake increased in the 1950s—is unable to cope with increasing greenhouse discharges produced by human activity. "The more carbon dioxide you put in, the more acidic the ocean becomes, reducing its ability to hold CO2," Khatiwala said in a prepared statement. Seawater's average pH—a measure of acidity—has dropped to 8.1 from roughly 8.2 in the 19th century.

That is balanced, in some part, by an apparent rise of roughly four billion metric tons of CO2 absorbed by land-based sinks, possibly due to CO2 being used by growing plants. Then again, the researchers' estimate for land uptake of carbon is suspect, given that it is based on what has not been absorbed by the oceans or remains in the atmosphere.

This kind of estimate is endemic to climate research and is perhaps best exemplified by the national emission inventories, which are often based on emissions factors—mathematical formulas for the amount of CO2 produced by burning a particular type of coal, for example. Simply multiplying consumption figures for such coal by these factors is the primary way the U.S. and other nations estimate the greenhouse gas load they contribute to the problem.

"Those emission estimates are not very accurate compared to high-calibration devices," says Michael Woelk, CEO of Picarro, a California-based manufacturer of such a device for measuring CO2 emissions. "No country today is using scientific instruments to monitor and calibrate the effect of policy," not even the European Union, which has an emissions trading scheme for greenhouse gases.

The U.S. uses similar technology to precisely monitor emissions of acid rain–causing sulfur dioxide or smog-forming nitrogen oxides as part of its trading programs for those pollutants. And the World Meteorological Organization has opted to employ Picarro's "cavity ring-down spectroscopy" technology—a computerlike device that measures isotopes to determine both amount and source, whether man-made or natural, of greenhouse gases—to ensure that its global measures are accurate.

The devices depend on the fact that carbon dioxide generated from fossil-fuel consumption has less of the isotope known as carbon 13 than other forms of the gas, thanks to plants preferentially absorbing the lighter version, known as carbon 12. By measuring this ratio, scientists might be able to determine more accurately the proportion of human contribution to climate-warming gases.

But a lot of variability in such isotopes remains; for example, the ratio of these isotopes can vary significantly in natural gas alone, potentially throwing such measurements into dispute. And a paper in the November 7 issue of Geophysical Research Letters from earth scientist Wolfgang Knorr of the University of Bristol in England argues that the ocean's uptake of carbon dioxide has not slowed at all in the last 150 years, making it possible that natural systems could compensate for human emissions.

Ultimately, what will matter most is how much the oceans and other natural sinks can buffer the human contribution from fossil-fuel burning. If greenhouse gas emissions continue on their current trajectory, global average temperatures could be 6 degrees Celsius warmer by the end of the century, Le Quere says.

As oceanographer Richard Feely of the National Oceanic & Atmospheric Administration's Pacific Marine Environmental Laboratory, a contributor to the GCP, noted: "We're concerned that if the natural sinks can't keep pace with the increased CO2 emissions, then the physical and biological impacts of global warming will accelerate over the next century."

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