Measurements of the vertical distribution of atmospheric sodium, made at S¿o Jos¿ dos Campos (23¿S, 46¿W) from 1972 to 2001 show a negligible net long-term trend in the layer over this time period. In view of the fact that cooling of the upper atmosphere, as indicated by some experimental studies and predicted by models, would be expected to influence the vertical distribution of sodium, this result can be used to estimate the limits of such cooling. In order to investigate the sensitivity of the sodium layer to changes in the atmospheric temperature profile, and thus estimate an upper limit to the changes it should be possible to detect, we have used a comprehensive model for the mesosphere and lower thermosphere (65--110 km) that includes all species and reactions thought to be relevant to the sodium chemistry. On the basis of this study we find that the sodium layer observations are consistent with model calculations for the global effects of increased CO2 but are not consistent with many of the experimental studies, which show cooling trends much larger than expected from theoretical studies.