Atmospheric CO2 accumulates near the Earth's surface because of relatively deeper vertical mixing when photosynthesis is active than when it is not. Some models simulate an excess of more than 2.5 ppmv CO2 in the remote Northern Hemisphere due to this ‘‘rectification’′ of an annually balanced terrestrial carbon cycle. The covariance between CO2 flux and vertical mixing, and the resulting vertical structure of CO2 are generally consistent with field data at local scales, but it is difficult to reconcile such a strong rectifier signal with current ideas about the global carbon budget. A rectifier effect of 2.5 ppmv at northern flask sampling stations implies an unreasonably strong northern sink of atmospheric CO2, and a corresponding source in the tropics or Southern Hemisphere.