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Global Climate Change Digest
A Guide to Information on Greenhouse Gases and Ozone Depletion
Published July 1988 through June 1999

FROM VOLUME 12, NUMBER 5, MAY 1999

OCEANS

Item #d99may13

“Oceanic Minerals: Their Origin, Nature of Their Environment, and Significance,” M. Kastner, Proc. Natl. Acad. Sci. USA, 96, 3380-3387 (March 1999).

Dense cold water plunges beneath the surface in the North Atlantic, spreads along the bottoms of the world’s oceans, and is replaced by the northward transport of warm water. The circulation patterns of the oceans have changed over geologic time, and indicators of global environmental changes are stored in deposits of marine minerals, such as nodules and crusts that scavenge and incorporate trace elements. The work here demonstrates that variations in these depositions allow measurement of changes, such as those in ocean circulation and in climate, even though these nodules grow at only 1 to 10 mm per million years.

Item #d99may14

“Global Warming and Marine Carbon Cycle Feedbacks on Future Atmospheric CO₂,” F. Joos et al., Science, 284 (5413), 464-467 (1999).

A simple model that represents the oceans, the atmosphere, and the marine and terrestrial biospheres was run with a large variety of CO₂ atmospheric concentrations. It indicated that, at high CO₂ concentrations, the North Atlantic circulation collapses; the resulting decrease in mixing reduces CO₂ uptake in that region significantly. At lesser increases in CO₂ concentration, sea-surface warming occurs, which also decreases CO₂ uptake. But, it also modifies the marine biological cycle slightly, allowing that cycle to compensate for the decreased mixing brought on by the slowing of the oceans’ circulation.

Item #d99may15

“Influence of El Niño on the Equatorial Pacific Contribution to Atmospheric CO₂ Accumulation,” R. A. Feely et al., Nature 398, 597-601 (1999).

The equatorial oceans are the dominant oceanic source of carbon dioxide to the atmosphere, annually releasing 0.7 to 1.5 Pg of carbon as CO₂. The equatorial Pacific has high seawater carbon dioxide and nutrient concentrations provided by the upwelling 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. Direct measurements in the equatorial Pacific from 1992 to 1996 indicate that during strong El Niño events, the release rate of CO₂ from the ocean to the atmosphere was reduced to 30 to 80% of that of non-El Niño periods. This decline in the upwelling is caused by a relaxation of the normal easterly winds, and the resulting decrease in carbon dioxide released from the sea into the air is large enough to be seen as an atmospheric CO₂ anomaly. The total reduction of the sea-to-air carbon dioxide exchange during the 1991-94 El Niño is estimated to be 0.8 to 1.2 PgC, which is equivalent to 16 to 36% of the decline of the growth rate of CO₂ in the atmosphere observed over the same period. Because more carbon dioxide is retained by the oceans during strong El Niño events, changes in the frequency of El Niño events may have a profound impact on the sea-to-air exchange of carbon dioxide.

Item #d99may16

“Variability of Inorganic and Organic Phosphorus Turnover Rates in the Coastal Ocean,” C. R. Benitez-Nelson and K. O. Buesseler, Nature, 398, 502-505 (1999).

The activities of two cosmogenic phosphorus nuclides were measured in situ in a coastal marine environment to assess phosphorus turnover rates. Dissolved inorganic, dissolved organic, and total phosphorus were measured for a full seasonal cycle. The study showed that turnover rates are rapid and vary seasonally, implying that low phosphorus concentrations can support high rates of primary production. Picoplankton were found to preferentially use dissolved organic phosphorus compounds to obtain carbon and nitrogen. These results indicate that the roles of phosphorus in supporting primary production have been underestimated.

Item #d99may17

“Increased El Niño Frequency in a Climate Model Forced by Future Greenhouse Warming,” A. Timmermann et al., Nature, 398, 694-697 (1999).

A model was developed that had sufficiently high resolution in the tropics to represent the equatorial upwelling and low-frequency waves. When an enhanced-CO₂ atmosphere is simulated, the model results indicate (1) more frequent El Niño events and (2) stronger cold events in the tropical Pacific Ocean.

Item #d99may18

“Continental-Shelf Sediment as a Primary Source of Iron for Coastal Phytoplankton,” K. S. Johnson, F. P. Chavez, and G. E. Friederich, Nature, 398, 697-700 (1999).

Phytoplankton primary productivity is frequently limited by iron availability in the equatorial Pacific Ocean. Measurements of soluble iron across the California Current System revealed high iron concentrations during periods of strong upwelling and low concentrations during El Niños. The low-iron periods in the study corresponded to periods of high efflux from rivers and low offshore salinity. These results indicate that the primary source of iron in the upper ocean is resuspension of benthic-layer particles that are carried to the surface by upwelling.