Abstract. The ¹³C isotopic ratio of methane, δ¹³C of CH₄, provides additional constraints on the CH₄ budget to complement the constraints from CH₄ observations. The interpretation of δ¹³C observations is complicated, however, by uncertainties in the methane sink. The reaction of CH₄ with Cl is highly fractionating, increasing the relative abundance of ¹³CH₄, but there is currently no consensus on the strength of the tropospheric Cl sink. We use a set of GEOS global model simulations with different predicted Cl fields to test the sensitivity of the δ¹³C of CH₄ to the diversity of Cl output from chemical transport models. We find that δ¹³C is highly sensitive to both the amount and geographic distribution of Cl. Simulatlons with Cl providing 0.28 % or 0.66 % of the total CH₄ loss bracket the δ¹³C observations for a fixed set of emissions. Thus, even when Cl provides only a small fraction of the total CH₄ loss and has a small impact on total CH₄, it provides a strong lever on δ¹³C. The geographic distribution and seasonal cycle of Cl also impacts the hemispheric gradient and seasonal cycle of δ¹³C. The large effect of Cl on δ¹³C compared to total CH₄ broadens the range of CH₄ source mixtures that can be reconciled with δ¹³C observations. Stronger constraints on tropospheric Cl are necessary to improve estimates of CH₄ sources from δ¹³C observations.