Data from ice cores show that CO2 and air temperature are highly correlated over the last 157 000 yr. Although this correlation can be taken as evidence that CO2 amplifies orbital forcing of temperature and is thus a strong greenhouse gas, this paper argues that estimating the strength of the CO2 warming effect from statistical evaluations of past climates based on CO2 and orbital forcing is hampered by strong multiple correlations between CH4, CO2, ocean currents, ice volume (and therefore albedo), dust, and nonseasalt sulfate. To estimate the strength of the greenhouse warming effect of CO2 from historical data, these correlations and multiple forcings should be taken into account. ¿ American Geophysical Union 1994