The impulse response of CO2 concentration due to variation in CO2 emissions was estimated from observed time series using the ARX method augmented by testing residuals for self-correlation. It was found to be a simple exponential with a half-time of 43 years with no remnant component. The longer half times and large remnant fraction of the impulse response derived from ocean circulation models is attributed to the failure of these models to account for turbulent mixing in the deep ocean.