Regular carbonate-system observations from more than 9,000 seawater samples collected around the Korean Peninsula between 2015 and 2025 reveal a sustained rise in surface seawater pCO₂, suggesting a weakening regional CO₂ sink and a possible shift toward CO₂ source conditions. Using seasonal measurements of temperature, salinity, total alkalinity, dissolved inorganic carbon, and atmospheric CO₂, we examine how thermal and non-thermal changes shape the observed pCO₂ increase and its future trajectory. Observation-constrained projections are used to estimate when seawater pCO₂ may exceed atmospheric CO₂ under different emission scenarios. The projected timing of this sink-to-source transition is later under high-emission scenarios, mainly because atmospheric CO₂ also rises more rapidly. Thus, a delayed transition should not be interpreted by itself as a more favorable carbon-cycle outcome. This study provides an observation-based framework for assessing emerging shifts in air-sea CO₂ exchange around rapidly warming Korean coastal seas.