Fig. S6.

Simulation of the change in global ${\delta }^{13}$C-CH₄ relative to 2006 based on varying OH concentrations from the AGAGE and NOAA inversions (blue and red, respectively), with total CH₄ emissions and its source ¹³C/¹²C ratio held fixed and varying CH₄ emissions from the “constant OH” inversion with the source ¹³C/¹²C ratio held constant (dashed line). The black points indicate the observed global mean ${\delta }^{13}$C-CH₄. Each of these signals is a transient response in the model. Over timescales on the order of decades, ${\delta }^{13}$C-CH₄ returns to the values before any perturbations in either the OH concentration or the bulk emissions [similar behavior and response timescales were noted previously (29, 61)].