The proof described in the Guardian piece is that the petroleum industry was warned as early as 1954 of CO2 risks vis-a-vis Climate Change. From the article:

The proof described in the Guardian piece is that the petroleum industry was warned as early as 1954 of CO2 risks vis-a-vis Climate Change. From the article: “The possible consequences of a changing concentration of the CO2 in the atmosphere with reference to climate, rates of photosynthesis, and rates of equilibration with carbonate of the oceans may ultimately prove of considerable significance to civilization,” Epstein, a researcher at the California Institute of Technology (or Caltech), wrote to the group in November 1954.