Fig. 4: Observed and simulated atmospheric CH₄ and δ¹³C-CH₄ from TM5 atmospheric modeling.

a, b Model-data comparison of long-term trend of (a) atmospheric CH₄ from 1985 to 2016 (in ppb) and (b) δ¹³C-CH₄ from 1999 to 2016 (in ‰) by observation (gray) and simulations from Scenario A (yellow), B (red), C (blue), and D (skyblue). c Model-data comparison of normalized north–south gradient of atmospheric δ¹³C-CH₄ for Scenario A (yellow), B (red), and C (blue) in 2012. The north–south δ¹³C-CH₄ was calculated by zonally-averaging the surface δ¹³C-CH₄ and normalized based on the mean δ¹³C-CH₄ at 60–90 °S. The normalized north–south δ¹³C-CH₄ for other years is in Supplementary Fig. 20 and Supplementary Table 9. d Histogram of the difference between simulated and observed δ¹³C-CH₄ for Scenario A (yellow), B (red), and C (blue) for 6 measurement sites located in the northern hemisphere. The histogram plots for all measurement sites are in Supplementary Fig. 22. Information about Scenarios A–D is in Table 1.