The Vostok ice cores produced data demonstrating 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 present on the left, i.e., time running right to left).

From J.M. Barnola et al., Nature 329 (1987): 408-414, p. 410.