The exchange of carbon dioxide (CO₂) between the ocean and atmosphere plays a crucial role in regulating Earth’s climate and ocean chemistry. In this study, we examined how accurately this exchange is captured along the California coast, where seasonal wind‐driven upwelling brings CO2‐rich waters to the surface. We compared CO₂ flux estimates using data collected every 3 hours for 10 years with those derived from monthly averages. We found that monthly averages often miss important short‐term variability, particularly during upwelling seasons. These seasonal changes cause the region to emit more CO₂ into the atmosphere than previously thought, challenging the perception that the coastal inshore areas of the California Current are a CO₂ sink. High‐frequency calculations revealed that winds and ocean conditions are closely linked, driving this variability in CO₂ exchange. Our findings highlight the need for continuous and high‐resolution measurements to better quantify sea‐air CO₂ exchange.

Key message: These findings imply that the exchange of CO₂ between the ocean and atmosphere computed at monthly-resolution may miss important shorter-term variability that contributes to a net release of CO₂ from the ocean – making these waters a net CO₂ source rather than a sink.