A two-dimensional (latitude-altitude) model of atmospheric CO2 and Δ13C was constructed to simulate some features of seasonal carbon cycle fluctuations The model simulates air-sea exchange, atmospheric diffusion, and fossil fuel carbon sources, which are functions of time and latitude. In addition, it uses biosphere-atmosphere fluxes of carbon that are based on global-scale biological models of vegetation growth and decay. Results of the model show fair agreement with observational results for CO2 and Δ13C seasonal fluctuations. Our model results have far northern fluctuations with smaller amplitudes than observed. Analysis of sources of CO2 change at given latitudes shows that, for far southern latitudes, southern hemisphere biospheric fluxes are dominant in affecting the seasonal CO2 fluctuations. Long-term decrease of Δ13C for the model is larger than for observations. This may be due to errors in the formulation for oceanic fluxes for 13C in the model or to a net uptake of carbon by the biosphere.