The atmosphere and the oceans carry on an exchange of carbon dioxide (CO2), a major greenhouse gas. This is particularly significant in the equatorial Pacific Ocean because it is one of the most important yet highly variable natural source areas for the emission of CO2 to the atmosphere.
A recent study conducted by oceanographers Taro Takahashi and Stewart Sutherland from Columbia University’s Lamont-Doherty Earth Observatory (LDEO) and Richard Feely and Cathy Cosca from the NOAA Pacific Marine Environmental Laboratory (PMEL) indicates the partial pressure of CO2 (pCO2) measured in surface waters dramatically changed after the Pacific Decadal Oscillation (PDO) phase shift in the Pacific Ocean that occurred around 1990.
“The results of our study show that the intensity of CO2 release from the western equatorial Pacific has increased during the past decade. By 2001, this reduced the global ocean uptake – about 2 billion tons of carbon a year – by about 2.5 percent,” said Takahashi who directed the study that provides a clearer picture of the importance of PDO events on the Earth’s carbon cycle. “This is on top of the CO2 emission and absorption fluctuations seen between El Niño and La Niña years, which occur on shorter timescales.”
Takahashi said that until more is known about the causes for decadal fluctuations, it’s not possible to predict the future trend. He said, “the major question remains—would this trend of increasing oceanic CO2 release rates continue or will it switch back to the decreasing trend of 1977-1990?”
The findings, to be published in the Oct. 31 issue of Science, suggest that natural shifts in the properties of the ocean, observed to occur approximately every 10 to 20 years, may affect the ocean’s absorption and emission rates of CO2.
“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 decadal-scale changes in ocean circulation in the tropical and subtropical Pacific cause changes in oceanic upwelling and possibly the biology which are a major cause of the long-term changes of pCO2 in this region.”
The North and Equatorial Pacific Ocean is known to undergo, over decadal time scales, major physical and biological changes commonly called the Pacific Decadal Oscillation (PDO). The most recent and well-documented major shifts occurred in 1977 and around 1990. While causes and effects of PDO have been investigated extensively in recent years, its effects on CO2 chemistry have not yet been identified.
Measuring the pCO2 for the past two or more decades through each of these shifts, the researchers found that the 1977 PDO shift was followed by a decrease in emission of CO2 from the equatorial Pacific ocean to the atmosphere , while the 1990 shift was followed by an increase in emission. This increase in carbon dioxide release from the sea to the air is large enough to affect the CO2 concentration in the atmosphere.
The equatorial oceans are the dominant oceanic source of carbon dioxide to the atmosphere, whereas colder waters in higher latitudes are sinks of atmospheric CO2. In balance, the global oceans annually take up about 2 billion tons of carbon through sea-air exchange of CO2 gas. This uptake rate corresponds to about 25 percent of carbon emitted to the atmosphere by the combustion of fossil fuels and other human activities. The equatorial Pacific is characterized by high seawater carbon dioxide and nutrient concentrations provided by upwelling, or the bringing up of CO2-rich deep waters to the surface.
As a result, the region is a major site for release of carbon dioxide from the ocean interior to the atmosphere, and the intensity of the release depends on how rapidly the ocean waters circulate vertically. During decades dominated by stronger overall circulation, more carbon dioxide is released to the atmosphere, further exacerbating the global warming impacts of CO2. Thus, decadal changes in ocean circulation in the equatorial Pacific may have a profound impact on the CO2-induced global warming.
NOAA, NASA, and the Ford Motor Company funded this research.
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