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Contact: Jana Goldman
The atmosphere and the oceans usually carry on a natural exchange of carbon dioxide (CO2), a major greenhouse gas. However, during an El Niño event, the oceans, especially in the equatorial Pacific, hold more of the CO2 than normal, according to a study conducted by the Commerce Department's National Oceanic and Atmospheric Administration.
The NOAA scientists and their colleagues
have determined that El Niños are a
Writing in the April 15 issue of Nature, Feely and his colleagues suggest that the findings, based on direct measurements in the equatorial Pacific from 1992-1996, indicate that during the strong El Niño events, the release rate of CO2 from the ocean to the atmosphere was reduced to 30-80% of that of normal non-El Niño periods. This decline in carbon dioxide release from the sea to the air is large enough to be seen as a CO2 anomaly in the atmosphere.
"Since CO2 is one of the primary greenhouse gases, scientists are interested in determining what are the causes of its variability in nature," said Feely. "The implication of this study is that El Niño-induced decreases in oceanic upwelling are a major cause of the interannual variability of the air-sea exchange of CO2.."
The equatorial oceans are the dominant oceanic source of carbon dioxide to the atmosphere, annually amounting to a net sea-to-air exchange of 0.7-1.5 Pg (1015) of carbon as CO2. One Pg (petagram) of carbon equals one billion metric tons of carbon. One metric ton is about 2,205 pounds, or the weight of a small car. The equatorial Pacific is characterized by high seawater carbon dioxide and nutrient concentrations provided by upwelling, or the bringing up, of CO2-rich water to the surface. As a result, the region is a major site for release of carbon dioxide from the ocean interior to the atmosphere.
From studies conducted in the central and eastern equatorial Pacific, the scientists observed as much as a four-fold reduction in the carbon dioxide exchange from the ocean to the atmosphere caused by the decline of upwelling during the 1991-94 El Niño event. The decline in the upwelling is caused by a relaxation of the normal easterly winds. The total reduction of the sea-to-air carbon dioxide exchange during the 1991-94 El Niño is estimated to be 0.8-1.2 PgC, which is equivalent to 16-36% of the decline of the growth rate of CO2 in the atmosphere observed over the same period.
During decades dominated by strong El Niño events, such as the present one, more carbon dioxide is retained by the oceans compared with normal decades. Thus, changes in the frequency of El Niño events may have a profound impact on the sea-to- air exchange of carbon dioxide.
Dr. Feely is an oceanographer at NOAA's Pacific Marine Environmental Laboratory in Seattle, Wash. Dr. Wanninkhof is an oceanographer at NOAA's Atlantic Oceanographic and Meteorological Laboratory in Miami, Fla. Dr. Tans is an atmospheric chemist at NOAA's Climate Monitoring and Diagnostic Laboratory in Boulder, Colo. Dr. Takahashi is an oceanographer at Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y.
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