by Margaret C. Bowles and Hugh D. Livingston
The growing concern about the effects on global climate of carbon dioxide (CO2) and other gases released into the atmosphere through human activity has sharpened scientific interest in the role of the ocean in the global carbon cycle. The ocean holds some 95% of the carbon that circulates actively in the biosphere. Over the long term, the ocean carbon cycle plays the dominant part in the natural regulation of CO2 levels in the atmosphere and their contribution to global temperature.
Unlike most gases in the atmosphere, CO2 reacts readily with seawater, dissociating to form bicarbonate and carbonate ions. But the capacity of the ocean to take up CO2 is not infinite. Researchers in the 1950's discovered two important limiting factors. Because of the long time scale of ocean circulation, the ocean takes up CO2 slowly, too slowly to match the rate at which CO2 from anthropogenic sources is accumulating in the atmosphere. And the chemical capacity of seawater to take up CO2 goes down as the amount of CO2 added increases.
A number of physical, chemical and biological processes govern the transport of carbon in the ocean from the surface waters to the deep waters and sediments of the ocean floor as well as its cycling among various organic and inorganic forms. Carbon dioxide is more soluble in the cold surface waters near the polar regions than it is in the warmer regions of the ocean; these denser waters take up CO2 and sink to form deep waters that circulate slowly through the ocean. This "solubility pump" helps to keep the surface waters of the ocean lower in CO2 than the deep water, a condition that promotes the flux of the gas from the atmosphere into the ocean.
Phytoplankton take up nutrients and CO2 from the water in the process of photosynthesis. They create organic matter, some of which is cycled through the food web in the water column and some of which sinks to the bottom in the form of particles or is mixed into the deeper waters as dissolved organic or inorganic carbon. Even more than the solubility pump, this "biological pump" serves to maintain the gradient in CO2 concentration between the surface and deep waters.
Tracing the relatively small signal of carbon from anthropogenic sources against the vast natural flux of carbon through the ocean requires better knowledge of the variation in time and space in the functioning of the processes that regulate the ocean carbon cycle. It also requires the development of more precise and accurate methods of measuring very small differences in biogeochemical variables. The multi-national Joint Global Ocean Flux Study (JGOFS) is undertaking a decade-long investigation designed to address the need for a better understanding of biogeochemical processes and patterns in the ocean and their response to environmental change.
The U.S. JGOFS program, a component of the U.S Global Change Research Program, grew out of the recommendations of a National Academy of Sciences workshop in 1984. The international program, which has more than 30 participating nations, began three years later under the auspices of the Scientific Committee on Oceanic Research (SCOR). In 1989, JGOFS became a core program of the International Geosphere-Biosphere Programme (IGBP).
JGOFS has two primary goals:
The strategy for addressing these goals has five major components:
U.S. JGOFS researchers participating in a global survey aboard WOCE cruises are amassing a large-scale data set on the distribution and variation in CO2 levels in surface waters throughout the ocean and thus the patterns of exchange between the ocean and atmosphere. Bio-optical measurements are providing data on pigments in the surface ocean that can be used to verify satellite measurements of ocean color.
Late in 1997, the U.S. JGOFS program is expected to start receiving data from SeaWiFS, an instrument designed to observe ocean color from a satellite. This information, coupled with bio- optical measurements from ships and buoys on the ocean's surface, will enable investigators to begin to put together a global-scale picture of phytoplankton stocks and to derive estimates of biological production in the ocean.
In 1988, U.S. JGOFS established long-term time-series programs at Bermuda and Hawaii. After nine years of monthly measurements, the Hawaiian Ocean Time-series (HOT) program and the Bermuda Atlantic Time-series Study (BATS) have amassed substantial data sets. Observers at both sites are noting surprising levels of seasonal and interannual variability in key biological and chemical processes, demonstrating the need to develop continuous monitoring instruments that can capture short- term variations over time.
JGOFS process studies began in 1989 with a five-nation pilot project in the North Atlantic, a study of the spring phytoplankton bloom that was designed to test plans for the full- scale process studies to follow. The observations in the North Atlantic established that variability in the partial pressure of CO2 in the surface waters of temperate ocean regions is strongly tied to the biological dynamics of the phytoplankton bloom. Another key finding was the unexpected importance of microbial activities in recycling carbon and nitrogen in sub-polar regions.
The first full-scale U.S. JGOFS process study was carried out in 1992 in the Equatorial Pacific Ocean, a region regarded as a major source of atmospheric CO2 because of high upwelling. Extraordinary good luck favored the study with unusually warm (El Nino) conditions in the spring and a return to more normal patterns in the fall. Preliminary results suggest that physical rather than biological processes control the flux of CO2 in the Equatorial Pacific. Biological cycling in the upper ocean is highly efficient, and relatively little carbon is exported to the depths, compared to areas characterized by phytoplankton blooms.
Process studies have also taken place in the Arabian Sea and the Southern Ocean. The monsoons of the Arabian Sea drive a uniquely intense carbon cycling system. Although poorly understood, the Southern Ocean could be a major sink for atmospheric CO2, and the large pool of unused nutrients in its surface waters offers great potential for changes in carbon storage.
U.S. JGOFS completed a major process study in the Arabian Sea for about sixteen months from late 1994 until early 1996. These cruises provided seasonal coverage of the annual monsoon and inter-monsoon cycles in the area south east of Oman - from where the cruises were staged. In addition to the NSF supported program in the Arabian Sea, both ONR and NASA had programs which were well integrated with the NSF plans. These plans were made in the wider context of international JGOFS plans for this region and in collaboration with WOCE cruises scheduled for this region. The first Arabian Sea Data Workshop was held in July 1996 at the University of New Hampshire and a further large data synthesis meeting is planned there in July 1997. Many preliminary data presentations have been made in special sessions at recent national meetings.
The U.S. JGOFS Antarctic Environment and Southern Ocean Process Study (AESOPS) began field work on August 29, 1996 and will continue through April of 1998. This study has been jointly funded by the Division of Polar Programs and the Division of Ocean Sciences at NSF. It is a two ship campaign, with the ice edge work being handled by the R/V Nathanial B. Palmer and the frontal zone work planned for the 1997-98 austral summer on R/V Roger Revelle. It was planned that the U.S. JGOFS Southern Ocean program would include a strong and active modeling component that allows for the development and implementation of many levels of modeling effort. Some pre-fieldwork modeling studies were initiated following a joint announcement of opportunity from NSF to support US JGOFS and US GLOBEC Southern Ocean modeling studies. This study completes the US JGOFS program of Process Studies.
On a smaller scale, U.S. JGOFS scientists interested in the role of iron on oceanic productivity conducted a small-scale experiment in the ocean southwest of the Galapagos in 1993. It was designed to test the hypothesis that availability of iron limits the growth of phytoplankton in areas of the ocean that are rich in nutrients such as nitrogen and phosphorus. The underlying question is whether an increase in oceanic phytoplankton production would affect the flux of CO2 from the atmosphere into the ocean.
Recent advances in modeling oceanic processes are assisting the U.S. JGOFS field programs. Modelers have succeeded in linking physical circulation models for the North Atlantic, Equatorial Pacific and Arabian Sea with ecosystem models to reproduce some of the most significant biogeochemical and physical characteristics of these regions. Others are modeling the processes that govern the transformation of CO2 into organic matter in the upper ocean and its remineralization in the deep ocean in order to improve our understanding of the way the biological pump moves carbon through the ocean.
Modeling represents the synthesis of our process understanding as well as an approach for testing our current understanding of various biogeochemical cycles. With sufficient development, models can be used to examine sampling strategies for process studies as well as large scale observations. In the near term, models also suggest possible linkages where improved understanding will provide the greatest advantage. U.S. JGOFS views models in all these ways, however, our objective is to provide a useful synthesis of our understanding which can be used for diagnosis of the current ocean role in the carbon system, as well as for future forecasts of the ocean state.
Modeling efforts supported directly through the U.S. JGOFS program have been rather limited. Some modest model studies were made in advance of all the major process studies to help define issues and priorities for the field programs. Until FY 1996, only about 3% of U.S. JGOFS funds were used for model studies. Now that the field programs are winding down, this situation has begun to change dramatically. A major new initiative, the U.S. JGOFS Synthesis and Modeling (SMP) project has been started. Its central objective is to synthesize knowledge gained from U.S. JGOFS and related studies into a set of models that reflect our current understanding of the ocean carbon cycle and its associated uncertainties. Emphasis will be given to processes that control partitioning of carbon among oceanic reservoirs and the implications of this partitioning for exchange between the ocean and atmosphere. The first of several Announcement of Opportunities for support of SMP projects was issued by NSF in early 1997 and the level of support is expected to ramp up into a major ongoing effort to crystallize the advances in understanding of the ocean carbon cycle through US JGOFS. In anticipation of this, support of synthesis activities began through the US JGOFS Planning Office during the last grant cycle. Support was provided for major data and synthesis workshops for the Equatorial Pacific and Arabian Sea Process Studies - and will be continued for the Southern Ocean Study. Production and dissemination of Special Issues of Deep-Sea Research was supported for Time-Series and Equatorial Pacific research and synthesis papers. The first annual major SMP workshop was held in August 1996 in New Hampshire and a second one is planned for August 1997 (emphasizing Time-Series Synthesis). A Synthesis and Modeling Steering Committee, headed by Jorge Sarmiento and Scott Doney, was set up by the US JGOFS Steering Committee to provide the necessary oversight and guidance to the SMP program. They will act in a manner analogous to the Process Studies coordinators who provided the leadership for each study.
This text was originally prepared for an article on JGOFS for McGraw-Hill's Yearbook of Science and Technology entitled: Assessing The Ocean's Role In The Global Carbon Cycle: The Joint Global Ocean Flux Study by Margaret C. Bowles and Hugh D. Livingston. It has been updated to reflect developments in the program as of 1997.