The seasonal cycle amplitude of atmospheric CO<inf>2</inf> (SCA) has enhanced largely in the northern high latitudes over the past six decades. However, it remains unclear whether this SCA enhancement occurs consistently across the globe during different historical periods and what the underlying mechanisms are. This study used 68-site CO<inf>2</inf> observations and 27 model data sets from inversion products and process-based models to estimate global trends in SCA and the seasonal amplitude of terrestrial and oceanic carbon fluxes across six latitudinal bands. We found that the northern high-latitude SCA was enhanced significantly and this enhancement has accelerated by 74.24%. In contrast, significant SCA increases were rare in historical periods of the tropics (38.30%) and the southern mid-high latitudes (22.43%), where SCA trends exhibited a decadal transition from negative to positive. Through the statistical decomposition and GEOS-Chem factorial simulations, we attributed that the acceleration of the northern high-latitude SCA enhancement was driven by combined effects of terrestrial carbon fluxes (45.60%) and atmospheric transport (40.91%). Meanwhile, the decadal transition of SCA trends in the tropics and the southern temperate region was dominated by atmospheric transport. Overall, these results provide a global perspective on the variability of SCA trends and advance our understanding of the seasonal variation of global carbon processes.