Rotational diagrams of CO υ = 1 − 0 P-branch transitions produced using our LTE slab model for four different column densities at a temperature of T = 1000 K; the colour of each symbol indicates the line optical depth. The dotted line at E_up = 4600 K is the energy above which transitions fall shortwards of the MIRI/MRS blue wavelength limit of 4.9 µm. For each model, the rotational temperature derived from a linear fit to transitions in the MIRI range (i.e. above E_up = 4600 K) is shown by the black lines.