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Dissolved CO₂ in karst water is the key driving force of karstification. Replenishment of CO₂ concentrations in karst water occurs by meteoric water that percolates through the vadose zone, where CO₂ produced from microbial activity is dissolved. CO₂ can thus be transported with the percolating water or in the gas phase due to ventilation in karst systems. We measured seasonally fluctuating CO₂ concentrations in the air of a karst cave and their influence on aqueous CO₂ concentrations in different depths of a stagnant water column. The observed data were compared to numerical simulations. The data give evidence that density-driven enhanced dissolution of gaseous CO₂ at the karst water table is the driving force for a fast increase of aqueous CO₂ during periods of high gaseous concentrations in the cave, whereas during periods of lower gaseous concentrations, the decline of aqueous CO₂ is limited to shallow water depths in the order of 1 m. This is significant because density-driven CO₂ dissolution has not been previously considered relevant for karst hydrology in the literature. Attempts at reproducing the measured aqueous CO₂ concentrations with numerical modeling revealed challenges related to computational demands, discretization, and the high sensitivity of the processes to tiny density gradients.