What exactly is the short-time rate of change (growth rate) in the trend of [Formula: see text] data such as the Keeling curve? The answer to this question will obviously depend very much on the duration in time over which the trend has been defined, as well as the smoothing technique that has been used. As an estimate of the short-time rate of change we propose to employ a very simple and robust definition of the trend based on a centered 1-year sliding data window for averaging and a corresponding centered 1-year difference (2-year data window) to estimate its rate of change. In this paper, we show that this simple strategy applied to weekly data of the Keeling curve (1974-2020) gives an estimated rate of change which is perfectly consistent with a more sophisticated regression analysis technique based on Taylor and Fourier series expansions. From a statistical analysis of the regression model and by using the Cramér-Rao lower bound, it is demonstrated that the relative error in the estimated rate of change is less than 5 [Formula: see text]. As an illustration, the estimates are finally compared to some other publicly available data regarding anthropogenic [Formula: see text] emissions and natural phenomena such as the El Niño.