Quantitative information about sources and sinks on large spatial scales is extracted from small but persistent concentration differences. Major uncertainties in the global budgets of all three gases still exist. Increasing use is being made of isotopic ratio measurements to untangle the separate contributions from different processes. For instance, carbon in plants has a different isotopic ratio than oceanic carbon. The oxygen isotopic ratio of CO 2 partially reflects oxygen fractionation occurring in the hydrological cycle.
Fossil methane possesses different isotopic ratios than methane produced by bacteria.
Another area under rapid development is the direct measurement of the air-sea exchange of gases, which has not been done successfully until now. Such measurements would provide much tighter coupling between atmospheric and oceanic observations, improving our grasp of the gas budgets. Selected Publications R.J. Francey and P.P. Tans. P.P. Tans, T.J. Conway, and T. Nakazawa. P.P. Tans, I.Y. Fung, and T. Takahashi.