Data for period 800.000 years ago to 1.000 years ago: CDIAC
Loulergue, L., A. Schilt, R. Spahni, V. Masson-Delmotte, T. Blunier, B. Lemieux, J.-M. Barnola, D. Raynaud, T.F. Stocker, and J. Chappellaz. 2008. Orbital and millennial-scale features of atmospheric CH4 over the past 800,000 years. Nature, Vol. 453, pp. 383-386, 15 May 2008. doi:10.1038/nature06950

Data for period 1000 years ago to 1980: NOAA
Etheridge, D. M., Steele, L. P., Francey, R. J., and Langenfelds, R. L., 1998, Atmospheric methane between 1000 A.D. and present: Evidence of anthropogenic emissions and climatic variability J. Geophys. Res. Vol. 103, No. D13, p. 15,979 (98JD00923)/div>

Data for period 1980 to present: NOAA