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Abstract:
The spatial and temporal distribution of atmospheric carbon dioxide and its 13C/12C ratio have been simulated with a three-dimensional transport model which predicts variations on time scales from synoptic to annual. This paper describes the results of model simulations of the mean annual fields of CO2, on the assumption that the carbon cycle has no time variations longer than 1 year. Interannual variations are addressed, however, by defining a series of four quasi-stationary states which characterize the average nonseasonal cycle for four historical periods: 1962, 1968, 1980, and 1984. Meridional variations predicted by the model for these periods are compared with observational data obtained from an array of stations extending from the Arctic to the South Pole. An adjustable parameter in the model controls the predicted average release of CO2 by the tropical oceans, and another the uptake of CO2 by the north Atlantic Ocean. These two parameters are globally adjusted to achieve an optimal fit of the model predictions to the observations of CO2 concentration. Good fits are realized under a variety of assumptions about the strengths and locations of sources and sinks of the terrestrial biosphere, indicating that the model is not closely constrained on the basis of CO2 concentration data alone. The model's prediction of the 13C/12C ratio of atmospheric CO2 is acceptable, however, only if the prescribed net terrestrial biospheric exchange at high northern latitudes is overshadowed by the postulated oceanic sink in the North Atlantic. By far the largest contributor to meridional variations in both concentration and 13C/12C ratio is the combustion of fossil fuels, 95 percent of which are injected into the northern hemisphere. The build-up of CO2 in the northern hemisphere in response to this injection is clearly seen in the comparison of the historical profiles which cover a period in which the injection doubled from 2.6 to 5.3 × 1012 kgC yr−1. In contrast, the model predictions of the adjustable oceanic sources and sinks vary by less than 0.4×1012 kgC yr−1 from their mean value from one historical period to another, suggesting only relatively minor changes in these natural exchanges of CO2 from 1962 to 1984.
