The increase of atmospheric CO2 (ref. (1)) has been predicted to impact the seasonal cycle of inorganic carbon in the global ocean(2,3), yet the observational evidence to verify this prediction has been missing. Here, using an observation-based product of the oceanic partial pressure of CO2 (p(CO2)) covering the past 34 years, we find that the winter-to-summer difference of the p(CO2) has increased on average by 2.2 +/- 0.4 mu atm per decade from 1982 to 2015 poleward of 10 degrees latitude. This is largely in agreement with the trend expected from thermodynamic considerations. Most of the increase stems from the seasonality of the drivers acting on an increasing oceanic p(CO2) caused by the uptake of anthropogenic CO2 from the atmosphere. In the high latitudes, the concurrent ocean-acidification-induced changes in the buffer capacity of the ocean enhance this effect. This strengthening of the seasonal winter-to-summer difference pushes the global ocean towards critical thresholds earlier, inducing stress to ocean ecosystems and fisheries(4). Our study provides observational evidence for this strengthening seasonal difference in the oceanic carbon cycle on a global scale, illustrating the inevitable consequences of anthropogenic CO2 emissions.