Figure 2a. Concentrations of atmospheric carbon dioxide from the 18th century to the present. Top: Multi-year average concentrations from ice core data at Siple Station in West Antarctica for the period 1744-1953 [Source: A. Neftel et al.] supplemented by annual mean concentrations at the Mauna Loa Observatory for the period 1959-93 [Source: Scripps Institution of Oceanography for 1959-74 data and the Carbon Cycle Group of CMDL/NOAA for 1974-92 data]. Bottom: Monthly mean concentrations at Mauna Loa for the period 1958-92 [Source: As above].