The Vostok demonstration of a close link between carbon dioxide and temperature through the last glacial cycle. Upper curve: CO2 concentration, parts per million. Lower curve: reconstruction of atmospheric temperature from measurements of the isotope Deuterium. Both are plotted against age (thousands of years Before Present, with the present at the far left).
From J.M. Barnola et al., Nature 329 (1987): 408-414, p. 410.