Figure 3a. Concentrations of atmospheric methane from the 18th century to the present. Top: Multi-year mean concentrations from ice core data at Siple Station in West Antarctica for the period 1771-1956 [Source: B. Stauffer et al.] supplemented by annual mean concentrations at the Mauna Loa Observatory for the period 1984-93 [Source: CMDL/NOAA]. Bottom: Monthly mean concentrations at Mauna Loa for the period 1983-93 [Source: As above].