Methyl bromide (CH3Br) is an important ozone-depleting gas for which 20th century trends of the atmospheric concentration have recently been derived from air trapped in Antarctic firn. In this paper, a two-dimensional (2-D) global model, with a coupled atmosphere and ocean, is used to examine this historical trend, analyzing its implication for the various source strengths and the lifetime of CH3Br. The results illustrate that not only is the current understanding of the present-day atmospheric budget of CH3Br incomplete but so too is our understanding of the budget of CH3Br prior to major industrial emissions and anthropogenic changes to it. This difference cannot be explained by the overall error in the model results as determined from the uncertainties in the major source and sink terms. Either the estimate of the sink strength is too large or there is an underestimate of a known nonindustrial source or an unaccounted nonindustrial source, or some combination of these. Further, the results imply that the increase in the anthropogenically influenced sources during the 20th century is underestimated or that the sink strength is too strong. Since this applies to both before and after 1950, this suggests that the increase in nonfumigant sources and possibly fumigant sources is underestimated, assuming the sinks not to be overestimated. A longer lifetime has the effect of not only increasing the absolute concentrations but also increasing the rate of growth in concentrations and thus improving the agreement with the firn data. The sensitivity of the results to the uncertainties in the individual source and sink terms is assessed. The budget is also examined in terms of the properties of an artificial source necessary to provide balance. Other data sets of observed CH3Br concentration are also used to evaluate the modeled source and sink scenarios in terms of seasonal and latitudinal variations.