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Uptake and Storage of Carbon Dioxide in the Ocean: the Global C02 Survey Richard A

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Uptake and Storage of Carbon Dioxide in the Ocean: the Global C02 Survey Richard A Uptake and Storage of Carbon Dioxide in the Ocean: The Global C02 Survey Richard A. Feely Pacific Marine and Environmental Laboratory National Oceanic and Atmospheric Administration • Seattle, Washington USA Christopher L. Sabine University of Washington • Seattle, Washington USA Taro Takahashi Lamont-Doherty Earth Observatory. Palisades, New York USA Rik Wanninkhof Atlantic Oceanographic and Meteorological Laboratory National Oceanic and Atmospheric Administration. Miami, Florida USA Introduction Human activity is rapidly changing the composi- atmosphere. A significant impetus for carbon cycle tion of the earth's atmosphere, contributing to warm- research over the past several decades has been to ing from excess carbon dioxide (CO2) along with other achieve a better understanding of the ocean's role as a trace gases such as water vapor, chlorofluorocarbons, sink for anthropogenic CO2. There are only three global methane and nitrous oxide. These anthropogenic reservoirs with exchange rates fast enough to vary sig- "greenhouse gases" play a critical role in controlling nificantly on the scale of decades to centuries: the the earth's climate because they increase the infrared atmosphere, the terrestrial biosphere and the ocean. opacity of the atmosphere, causing the surface of the Approximately 93% of the carbon is located in the planet to warm. The release of CO2 from fossil fuel con- ocean, which is able to hold much more carbon than the sumption or the burning of forests for farming or pas- other reservoirs because most of the CO2 that diffuses hare contributes approximately 7 petagrams of carbon into the oceans reacts with seawater to form carbonic (1 Pg C = 1 x 10 '5 g C) to the atmosphere each year. acid and its dissociation products, bicarbonate and car- Approximately 3 Pg C of this "anthropogenic COd' bonate ions (Figure 1). accumulates in the atmosphere annually, and the Our present understanding of the temporal and remaining 4 Pg C is stored in the terrestrial biosphere spatial distribution of net CO2 flux into or out of the and the ocean. ocean is derived from a combination of field data, Where and how land and ocean regions vary in which is limited by sparse temporal and spatial cover- their uptake of CO2 from year to year is the subject of age, and model results, which are validated by com- much scientific research and debate. Future decisions parisons with the observed distributions of tracers, on regulating emissions of greenhouse gases should be including natural carbon-14 (14C), and anthropogenic based on more accurate models of the global cycling of chlorofluorocarbons, tritium (3H) and bomb 14C. The lat- carbon and the regional sources and sinks for anthro- ter two radioactive tracers were introduced into the pogenic CO2, models that have been adequately tested atmosphere-ocean system by atomic testing in the mid against a well-designed system of measurements. The 20~ century. With additional data from the recent glob- construction of a believable present-day carbon budget al survey of CO2 in the ocean (1991-1998), carried out is essential for the reliable prediction of changes in cooperatively as part of the Joint Global Ocean Flux atmospheric CO2 and global temperatures from avail- Study (JGOFS) and the World Ocean Circulation able emissions scenarios. Experiment (WOCE) Hydrographic Program, it is now The ocean plays a critical role in the global carbon possible to characterize in a quantitative way the cycle as a vast reservoir that exchanges carbon rapidly regional uptake and release of CO2 and its transport in with the atmosphere, and takes up a substantial por- the ocean. In this paper, we summarize our present tion of anthropogenically-released carbon from the understanding of the exchange of CO2 across the air-sea Oceanography • VoL 14 • No. 4/2001 18 Atmosphere CO2(g ) 1XCO2 2XCO2 280 560 Gas E c hange Surface ocean CO2(aq)+ H20 ~ H2CO3 8 15 Carbonicacid H2CO3 H++HCO3" 1617 1850 Bicarbonate HCO3" ~ H++CO32- 268 176 Carbonate 1893 2040 DIC R1; 7 CJl nH Figure 1. Schematic diagram of the carbon dioxide (C02) system in seawater. The 1 x C02 concentrations are for a surface ocean in equilibrium with a pre-industrial atmospheric C02 level of 280 ppm. The 2 x C02 concentrations are for a surface ocean in equilibrium with an atmospheric C02 level of 560 ppm. Current model projections indicate that this level could be reached sometime in the second half of this century. The atmospheric values are in units of ppm. The oceanic concentrations, which are for the surface mixed layer, are in units of #mol kg -1. interface and the storage of natural and anthropogenic These data were collected at a time when no com- CO2 in the ocean's interior. mon reference materials or standards were available. As a result, analytical differences between measurement Background groups were as large as 29 ~lrnol kg-1 for both DIC and The history of large-scale CO2 observations in the Av which corresponds to more than 1% of the ambient ocean date back to the 1970s and 1980s. Measurements values. Large adjustments had to be made for each of of the partial pressure of CO2 (pCO~), total dissolved the data sets based on deepwater comparisons at near- inorganic carbon (DIC) and total alkalinity (AT) were by stations before individual cruise data could be com- made during the global Geochemical Ocean Sections pared. These differences were often nearly as large as (GEOSECS) expeditions between 1972 and 1978, the the anthropogenic CO2 signal that investigators were Transient Tracers in the Oceans (TTO) North Atlantic trying to determine (Gruber et al., 1996). Nevertheless, and Tropical Atlantic Surveys in 1981-83, the South these early data sets made up a component of the sur- Atlantic Ventilation Experiment (SAME) from 1988-1989, face ocean pCO2 measurements for a global climatology the French Southwest Indian Ocean experiment, and and also provided researchers with new insights into numerous other smaller expeditions in the Pacific and the distribution of anthropogenic CO2 in the ocean, par- Indian Oceans in the 1980s. These studies provided ticularly in the Atlantic Ocean. marine chemists with their first view of the carbon sys- At the onset of the Global Survey of CO2 in the tem in the global ocean. Ocean (Figure 2), several events took place in the United Oceanography " Vol. 14 • No. 4/2001 19 40E 80E 120E 160E 160W 120W 80W 40W 0 ° 4( 0 o 0 o 4( 40E 80E 120E 160E 160W120W 80W 40W 0 ° Figure 2. The Global Survey of C02 in the Ocean: cruise tracks and stations occupied between 1991 and 1998. States and in international CO2 measurement communi- discrete pCO2 measurements. An extremely precise ties that significantly improved the overall precision and method for pH measurements based on spectrophotom- accuracy of the large-scale measurements. In the United etry was also developed by Robert Byrne and his col- States, the CO2 measurement program was co-funded leagues at the University of South Florida. These by the Department of Energy (DOE), the National improvements ensured that the internal consistency of Oceanic and Atmospheric Administration (NOAA) and the carbonate system in seawater could be tested in the the National Science Foundation (NSF) under the tech- field whenever more than two components of the car- nical guidance of the U.S. CO2 Survey Science Team. bonate system were measured at the same location and This group of academic and government scientists time. This allowed several investigators to test the over- adopted and perfected the recently developed coulo- all quality of the global CO2 data set based upon CO2 sys- metric titration method for DIC determination that had tem thermodynamics. Laboratories all around the world demonstrated the capability to meet the required goals contributed to a very large and internally consistent glob- for precision and accuracy. They advocated the develop- al ocean CO2 data set determined at roughly 100,000 sam- ment and distribution of certified reference materials ple locations in the Atlantic, Pacific, Indian and Southern (CRMs) for DIC, and later for A~, for international dis- oceans (Figure 2). The data from the CO2 survey are avail- tribution under the direction of Andrew Dickson of able through the Carbon Dioxide Information and Scripps Institution of Oceanography (see sidebar). They Analysis Center (CDIAC) at Oak Ridge National also supported a shore-based intercomparison experi- Laboratory as Numeric Data Packages and on the World ment under the direction of Charles Keeling, also of Wide Web (http://cdiac.esd.ornl.gov/home.html). Taro Scripps. Through international efforts, the development Takahashi and his collaborators have also amassed a of protocols for CO2 analyses were adopted for the CO2 large database of surface ocean pCO2 measurements, survey. The international partnerships fostered by spanning more than 30 years, into a pCO2 climatology for JGOFS resulted in several intercomparison CO2 exercis- the global ocean (Takahashi et al., in press). These data es hosted by France, Japan, Germany and the United have been used to determine the global and regional flux- States. Through these and other international collabora- es for CO2 in the ocean. tive programs, the measurement quality of the CO2 sur- vey data was well within the measurement goals of _+ 3 C02 ExchangeAcross the Air-Sea Interface ~mol kg -1 and + 5 ~rnol kg -1, respectively, for DIC and AT. In seawater, CO2 molecules are present in three Several other developments significantly enhanced major forms: the undissociated species in water, the quality of the CO2 data sets during this period. New [CO2]aq, and two ionic species, [HCO3-] and [COd-] methods were developed for automated underway and (Figure 1). The concentration of [CO~]aq depends upon Oceanography • Vol. 14 • No. 4/2001 2O Andrew G.
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