Temperature-corrected intensities of the J = 1−0 (top) and J = 2−1 (bottom) lines of the main CO isotopologs as a function of H₂ column density in California, Perseus, and Orion A (blue, green, and red circles, respectively). Note the lower level of scatter and the better inter-cloud agreement compared to the uncorrected intensities shown in Fig. 1. The dashed line in the ¹³CO(2−1) panel indicates the slope of a linear trend.