Thomson [1995> argues that an enhanced greenhouse effect may be altering the seasonal cycle in temperature. We compare trends in the amplitude and phase of the seasonal cycle in observational temperature data in the northern hemisphere with the response of two general circulation models to increased CO2 concentrations. Sizeable amplitude decreases are observed in both models and observations. Significant phase delays (ie, later seasonal transitions) are found in the simulations, opposite to the phase advances isolated in the observations. The retreat of winter sea ice in high-latitude regions appears to explain the model's response to CO2 increase. Much of the variability in the observational data is not predicted by the models. ¿ American Geophysical Union 1996