Long-term observations are critical records for distinguishing natural cycles from climate change. We present a data set of 40 surface ocean CO₂ and pH time series that suggests the time length necessary to detect a trend in seawater CO₂ due to uptake of atmospheric CO₂ varies from 8 years in the least variable ocean regions to 41 years in the most variable coastal regions. This data set provides a tool to evaluate natural cycles of ocean CO₂, with long-term trends emerging as records lengthen.