Significance The increasing atmospheric CO ₂ seasonal cycle amplitude in the Northern Hemisphere suggests northern ecosystems are responding strongly to global change. Considerable uncertainties remain regarding drivers of these changes and their locations. We find that seasonal flux changes in Siberian and temperate ecosystems together shape the increasing CO ₂ amplitude. Particularly, enhanced seasonal carbon exchange in Siberia drives most of observed amplitude increases at the surface of northern high latitudes. Arctic-boreal North America shows much smaller changes in flux seasonality and has only localized impacts, consistent with more browning in this region from field and satellite observations. Our findings highlight divergent ecosystem responses among regions, countering the simple narrative that CO ₂ fertilization and climate warming are the dominant drivers.