We then recorded the interpolated concentrations at the depths of 100, 300, 500, 1,000, 2,000, 3,000, and 4,000 m for each coordinate. Next, we went through each cast of the GO-SHIP observations and recorded the interpolated concentrations at these same depths. For each cast, we found the difference of the observed concentrations and the mapped climatology at these specified depths. We then fitted linear regressions to the differences at each coordinate. If improved technology imposed a measurement bias, then the differences at deeper depths may show a negative trend with time.