and model the oceanic carbon system, then a fundamental improvement in the accuracy of the thermodynamic constants is one of the required steps. This is an exceptionally challenging experimental task.
The calculation of a signal which appears to be very closely related to a direct observation of the accumulated ocean fossil fuel CO2 invasion term, such as the profile given in Fig. 3, seems to be readily achievable. The principal debate is about the levels of accuracy acceptable for such a calculation, and the ultimate goal of integrating the signal on an ocean basin scale and relating the quantities to the global fossil fuel CO2 budget. The subsurface signals revealed are integrals mixed from several water mass sources, and divining the unique surface signature of any one oceanic region will be difficult. Moreover in practice there are strong regional and seasonal sources and sinks of CO2 in the surface ocean, and until we find strategies for dealing with these there will always be room for debate.
Here there is room for optimism; the ability to gain seasonal CO2 data at a remote site has been greatly aided by the recent development of accurate and reliable new sensors that can be mated with buoy technology (36). The ability to predict in real time the mixed layer depth of the ocean has also undergone a revolution through advanced remote observational and computing techniques (37), and the results are now being made available to the civilian community. A specimen global map of calculated mixed layer depths produced by us from data supplied by the Fleet Numerical and Oceanography Center for the month of March 1995, close to the northern hemisphere vernal equinox and probably representing maximum values, is shown in Fig. 5.
The problem of detecting the fossil fuel CO2 signal in sea water is relatively easy. Modifications of the calculation of Brewer (8) or of Chen and Millero (11) all yield a robust signal of about 45 µmol·kg-1 in surface ocean waters today. In the North Pacific Ocean example given here the signal decays with depth, corresponding to the ventilation age of the water masses, and may be traced to about 400-m depth, consistent with deep winter time mixed layer formation in the northwest sector.
The most consistent results are presently obtained by using the oxidative decomposition ratios of Anderson and Sarmiento (25) and the thermodynamic constants of either Goyet and Poisson (30) or Roy et al. (33).
Refining the calculation will require increased knowledge of the Redfield ratio, and of surface winter time total alkalinity values. New sensing and sampling technologies now developed offer every prospect of yielding this information, and of improving estimates of long term ocean CO2 uptake.
The very careful work of the teams of ocean scientists now producing data on the ocean carbon system as part of a coordinated global survey is gratefully acknowledged. This work was supported by a grant to the Monterey Bay Aquarium Research Institute from the David and Lucile Packard Foundation, and by National Aeronautics and Space Administration Grant Earth Observing System (EOS) NAG-232431 to the Woods Hole Oceanographic Institution.
1. Ciais, P. , Tans, P. P. , Trolier, M. , White, J. W. C. & Francey, R. J. ( 1995 ) Science 269 , 1098–1102 .
2. Tans, P. P. , Fung, I. Y. & Takahashi, T. ( 1990 ) Science 247 , 1431–1438 .
3. Tsunogai, S. , Ono, T. & Watanabe, S. ( 1993 ) J. Oceanogr. 49 , 305–315 .
4. Siegenthaler, U. & Sarmiento, J. L. ( 1993 ) Nature (London) 365 , 119–125 .
5. Callendar, G. S. ( 1938 ) Q. J. R. Meteorol. Soc. 64 , 223–240 .
6. Revelle, R. & Suess, H. E. ( 1957 ) Tellus 9 , 18–27 .
7. Oeschger, H. , Siegenthaler, U. , Schatterer, U. & Gugelmann, A. ( 1975 ) Tellus 27 , 168–191 .
8. Brewer, P. G. , ( 1978 ) Geophys. Res. Lett. 5 , 997–1000 .
9. Goyet, C. & Brewer, P. G. ( 1993 ) in Modeling Oceanic Climate Interactions , NATO Series I11 , eds. Willebrand, J. & Anderson, D. L. T. ( Springer , Berlin ), pp. 271–297 .
10. Jones, E. P. & Levy, E. M. ( 1981 ) J. Mar. Res. 39 , 405–416 .
11. Chen, C.-T. & Millero, F. J. ( 1979 ) Nature (London) 277 , 205–206 .
12. Chen, C.-T. ( 1982 ) Deep Sea Res. 29 , 563–580 .
13. Chen, C.-T. ( 1993 ) J. Oceanogr. 18 , 257–270 .
14. Broecker, W. S. , Takahashi, T. & Peng, T.-H. ( 1985 ) Reconstruction of the Past Atmospheric CO2Contents of the Contemporary Ocean: An Evaluation ( U.S. Department of Energy , Washington, DC ), Rep. DOE/OR 857 .
15. Wallace D. W. R. ( 1995 ) Monitoring Global Ocean Carbon Inventories, Ocean Observing System Background Report 5 ( Texas A&M Univ. , College Station ).
16. Bates, N. R. , Michaels, A. F. & Knap, A. H. ( 1996 ) Deep Sea Res. 43 , 347–383 .
17. Redfield, A. C. ( 1934 ) James Johnstone Memorial Volume ( Liverpool Univ. Press , Liverpool, U.K. ), 176–192 .
18. Redfield, A. C. , Ketchum, B. H. & Richards, F. A. ( 1963 ) in The Seas , ed. Hill, M. N. ( Wiley-Interscience , New York ), Vol. 2 , pp. 26–77 .
19. Brewer, P. G. , Wong, G. T. F. , Bacon, M. P. & Spencer, D. W. ( 1975 ) Earth Planet. Sci. Lett. 26 , 81–87 .
20. Brewer, P. G. & Goldman, J. C. ( 1976 ) Limnol. Oceanogr. 21 , 108–117 .
21. Goldman, J. C. & Brewer, P. G. ( 1980 ) Limnol. Oceanogr. 25 , 352–357 .
22. Bradshaw, A. L. , Brewer, P. G. , Shafer, D. K. & Williams, R. T. ( 1981 ) Earth Planet. Sci. Lett. 55 , 99–115 .
23. Takahashi, T. , Broecker, W. S. & Langer, S. ( 1985 ) J. Geophys. Res. 90 , 6907–6924 .
24. Boulahid, M. & Minster, J.-F. ( 1989 ) Mar. Chem. 26 , 133–153 .
25. Anderson, L. A. & Sarmiento, J. L. ( 1994 ) Global Biogeochem. Cycles 8 , 65–80 .
26. Martin, J. H. , Knauer, G. A. , Karl, D. M. & Broenkow, W. W. ( 1987 ) Deep Sea Res. 34 , 267–285 .
27. Brewer, P. G. , Bradshaw, A. L. , Shafer, D. K. & Williams, R. T. ( 1986 ) in The Changing Carbon Cycle: A Global Analysis , eds. Trabalka, J. R. & Reichle, D. E. ( Springer , New York ), pp. 348–370 .
28. Broecker, W. S. ( 1974 ) Earth Planet. Sci. Lett. 23 , 100–107 .
29. Glover, D. M. & Brewer, P. G. ( 1988 ) Deep Sea Res. 35 , 1525–1546.
30. Goyet, C. & Poisson, A. ( 1989 ) Deep Sea Res. 36 , 1635–1654 .
31. Takahashi, T. , Olafsson, J. , Goddard, J. G. , Chipman, D. W. & Sutherland, S. C. ( 1993 ) Global Biogeochem. Cycles 7 , 843–878 .
32. Millero, F. J. ( 1995 ) Geochim. Cosmochim. Acta 59 , 661–677 .
33. Roy, R. N. , Roy, L. N. , Vogel, K. M. , Moore, C. P. , Pearson, T. , Good, C. E. , Millero, F. J. & Campbell, D. M. ( 1993 ) Mar. Chem. 44 , 249–268 .
34. Hansson, I. ( 1973 ) Deep Sea Res. 20 , 461–478 .
35. Mehrbach, C. , Culberson, C. H. , Hawley, J. E. & Pytkowicz, R. M. ( 1973 ) Limnol. Oceanogr. 18 , 897–907 .
36. Friederich, G. E. , Brewer, P. G. , Herlien, R. & Chavez, F. ( 1995 ) Deep Sea Res. 42 , 1175–1186 .
37. Clancy, R. M. & Sadler, W. D. ( 1992 ) Weather Forecasting 7 , 307–327 .