U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications

Uptake and Storage of Carbon Dioxide in the Ocean: The Global CO2 Survey

Richard A. Feely1, Christopher L. Sabine2, Taro Takahashi3, and Rik Wanninkhof4

1Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, Seattle, Washington, 98115
2Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, Washington, 98195
3Lamont-Doherty Earth Observatory, Palisades, New York
4Atlantic Oceanographic and Meteorological Laboratory National Oceanic and Atmospheric Administration, Miami, Florida

Oceanography, 14(4), 18–32 (2001).
Copyright ©2001 by The Oceanography Society. Further electronic distribution is not allowed.

Distribution of the Climatological Mean Net Sea-air CO2 Flux

To illustrate the sensitivity of the gas transfer velocity and thus the sea-air CO2 flux to wind speed, we have estimated the regional and global net sea-air CO2 fluxes using two different formulations for the CO2 gas transfer coefficient across the sea-air interface: the quadratic U2 dependence of W-92 and the cubic U3 dependence of W&M-99. In addition, we have demonstrated the effects of wind-speed fields on the computed sea-air CO2 flux using the National Center for Environmental Prediction (NCEP)-41 mean monthly wind speed and the NCEP-1995 mean monthly wind speed distributions over 4° × 5° pixel areas.

In Table 2 the fluxes computed using the W-92 and the NCEP/National Center for Atmospheric Research (NCAR) 41-year mean wind are listed in the first row for each grouping in column one (for latitudinal bands, oceanic regions and regional flux). The column "Errors in Flux" located at the extreme right of Table 2 lists the deviations from the mean flux that have been determined by adding or subtracting one standard deviation of the wind speed (about ±2 m secto the minus 1 on the global average) from the mean monthly wind speed in each pixel area. These changes in wind speeds affect the regional and global flux values by about ±25%. The fluxes computed using the single year mean wind speed data for 1995 are listed in the second line in each column one grouping in the table.

Table 2

The global ocean uptake estimated using the W-92 and the NCEP 41-yr mean wind speeds is –2.2 ± 0.4 Pg C yrto the minus 1. This is consistent with the ocean uptake flux of –2.0 ± 0.6 Pg C yrto the minus 1 during the 1990s (Keeling et al., 1996; Battle et al., 2000) estimated from observed changes in the atmospheric CO2 and oxygen variations.

The wind speeds for 1995 are much lower than the 41-year mean in the northern hemisphere and higher over the Southern Ocean. Accordingly, the northern ocean uptake of CO2 is weaker than the climatological mean, and the Southern Ocean uptake is stronger. The global mean ocean uptake flux of –1.8 Pg C yrto the minus 1 using the NCEP-1995 winds is about 18% below the climatological mean of –2.2 Pg C yrto the minus 1, but it is within the ±25% error estimated from the standard deviation of the 41-yr mean wind speed data.

When the cubic wind speed dependence (W&M-99) is used, the CO2 fluxes in higher latitude areas with strong winds are increased by about 50%, as are the errors associated with wind speed variability. The global ocean uptake flux computed with the 41-year mean wind speed data and the NCEP-1995 wind data is –3.7 Pg C yrto the minus 1 and –3.0 Pg C yrto the minus 1 respectively, an increase of about 70% over the fluxes computed from the W-92 dependence. These flux values are significantly greater than the flux based on atmospheric CO2 and oxygen data (Keeling et al., 1996; Battle et al., 2000). However, the relative magnitudes of CO2 uptake by ocean basins (shown in % in the regional flux grouping in the last four rows of Table 2) remain nearly unaffected by the choice of the wind-speed dependence of the gas transfer velocity.

The distribution of winds can also influence the calculated gas transfer velocity. This is because of the nonlinear dependence of gas exchange with wind speed; long-term average winds underestimate flux especially for strongly non-linear dependencies. To avoid this bias, the relationships are adjusted by assuming that the global average wind speed is well represented by a Rayleigh distribution function. As noted by Wanninkhof et al. (2001), this overestimates the flux. A more appropriate way to deal with the issue of wind speed variability is to use short-term winds. If the NCEP 6-hour wind products are used, the global flux computed using the W&M-99 cubic wind-speed formulation decreases from –3.7 to –3.0 Pg C yrto the minus 1 for the NCEP 41-year winds and from –3.0 to –2.3 Pg C yrto the minus 1 for the NCEP 1995 wind data.

The relative importance of the major ocean basins in the ocean uptake of CO2 may be assessed on the basis of the CO2 fluxes obtained from our pCO2 data and W-92 gas transfer velocity (Table 2 and Figure 6). The Atlantic Ocean as a whole, which has 23.5% of the global ocean area, is the region with the strongest net CO2 uptake (41%). The high-latitude northern North Atlantic, including the Greenland, Iceland and Norwegian seas, is responsible for a substantial amount of this CO2 uptake while representing only 5% of the global ocean in area. This reflects a combination of two factors: the intense summertime primary production and the low CO2 concentrations in subsurface waters associated with recent ventilation of North Atlantic subsurface waters. The Pacific Ocean as a whole takes up the smallest amount of CO2 (18% of the total) in spite of its size (49% of the total ocean area). This is because mid-latitude uptake (about 1.1 Pg C yrto the minus 1) is almost compensated for by the large equatorial release of about 0.7 Pg C yrto the minus 1. If the equatorial flux were totally eliminated, as during very strong El Niño conditions, the Pacific would take up CO2 to an extent comparable to the entire North and South Atlantic Ocean. The southern Indian Ocean is a region of strong uptake in spite of its small area (15% of the total). This may be attributed primarily to the cooling of tropical waters flowing southward in the western South Indian Ocean.

Figure 6

Figure 6. Distribution of the climatological mean annual sea-air CO2 flux (moles CO2 mto the minus 2 yrto the minus 1) for the reference year 1995 representing non-El Niño conditions. This has been computed using the mean monthly distribution of sea-air pCO2 difference, the climatological NCEP 41-year mean wind speed and the wind-speed dependence of the CO2 gas transfer velocity of Wanninkhof (1992). The yellow-red colors indicate a region characterized by a net release of CO2 to the atmosphere, and the blue-purple colors indicate a region with a net uptake of CO2 from the atmosphere. This map yields an annual oceanic uptake flux for CO2 of 2.2 ± 0.4 Pg C yrto the minus 1.

Return to previous section or go to next section

PMEL Outstanding Papers

PMEL Publications Search

PMEL Homepage