Discussion of Foster & Brown's Time and Tide: Analysis of Sea Level Time Series

The recognition of the non-accelerating, periodic pattern of sea levels as described by the tide gauges measurements does not require any special mathematical tool. Providing enough data of sufficient quality have been recorded, If the classical linear fitting is used to compute the rate of rise at any time, then the acceleration is simply the time rate of change of this velocity. By using this technique, the lack of any acceleration over the last few decades is evident in the naturally oscillating, slow rising, tide gauges of appropriate quality and length. Contrary to what is claimed in the commented paper, the accuracy in the computation of the periodicities of the natural oscillations does not impact at all on this result. As the measured sea level oscillations are not perfectly sinusoidal, clearly different assumptions about the shape of the periodic functions, their number and the definition of the algorithm used to compute the parameters of the selected periodic functions all affect the determination of the periodicities. This has however very little influence on the present sea level debate questioning the presence or the absence of acceleration at the tide gauges.