The air-sea CO<inf>2</inf> flux FCO<inf>2</inf> is an important component of the global carbon cycle and understanding its response to climate change is crucial to adjust mitigation pathways. Multi-linear regression supports the expectation that the balance between the CO<inf>2</inf> partial pressures of air and the sea surface (pCO<inf>2</inf>) is the most important driver of temporal FCO<inf>2</inf> variability. Discrepancies in FCO<inf>2</inf> variability between state-of-the-art Earth System Models (ESMs) and gap-filled pCO<inf>2</inf>-products suggest that systematic biases exist across an ensemble of ESMs. In the high latitudes, the climate change induced trend towards lighter seawater is overestimated in ESMs, which yields - in contrast to observations - shallower mixed layers over the contemporary period and hence a suppressed carbon supply from depth. While mixed layer depth variability and trends appear biased throughout the global ocean, this is not a determining factor for pCO<inf>2</inf> variability in the strongly stratified subtropical gyres. The results highlight the importance of accurately modeling circulation and hydrographic properties to obtain robust estimates of FCO<inf>2</inf> and its variability.