Using a 3-dimensional atmospheric transport model driven by the ECMWF winds and the non-interannually varying seasonal biospheric flux from the CASA global ecosystem model, we show that a significant change in the downward-zero crossing day (DZCD) of the atmospheric CO2 seasonal cycle in the Northern Hemisphere above 300N latitude can be obtained by year-to-year changes in the atmospheric transport alone. The transport model is able to reproduce both the trend and standard deviation of DZCD observed at Pt. Barrow. With only atmospheric transport changing from year to year, eastern North Atlantic, Europe, southern Eurasia, southeastern United States and western North America show DZCD starting earlier by 5--10 days after 21 years of model integration. If DZCD obtained from CO2 monitoring stations is going to be used as a proxy for determining the growing season length, then it is important to take into account of that portion caused by the atmospheric transport.